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DEC-08-G61B-D

December 1966

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OCR Text

DEC—OB-G6lB-D

DIGITAL EQUIPMENT CORPORATION

MAYNARD, MASSACHUSETTS

DEC-O8-G6lB—D

PROGRAMMED BUFFERED DISPLAY 338
PDP-8 PROGRAMMING MANUAL

OiGITAL EOLJIPMENT CORPORATION

MAYNARD. MASSACHUSETTS

PROGRAMMED BUFFERED DISPLAY 338

CONTENTS

Page

Chapter
SYSTEM INTRODUCTION

.....

Functional Description

l .i
‘

u—ul

.........

Intensity

.........................................

..........................................

.................................................

....................................................

Mode

....................................................

Subroutining
Light Pen

Flags

..............................................

.................................................

Pushbuttons

.......................

State

................

Timing

..................................................

Parameter

Jump

Pop

..............................

...................................................

Control State Commands

Mode

....................................................

DATA FORMATS

NNNNN

....................................

Display Parameters (Coordinate System)
Scale

lo'o'xr'oin'ix'oio

memhwwN—J
\O

........................................

................................................

.....................

..............................

....................................................

..............................................

.......

Conditional Skip (Bank l)

...................................

Conditional Skip (Bank 2)

..........................

Miscellaneous

........

.......................................

Data State Commands

Increment Mode

.....

....................................

...........................................

Vector Mode (Two Words)

...................................

Vector Continue Mode (Two Words)

..........................

i2
i3

15
16
i7

21
2]
22
24

........

.................................

...........................................

.....................................

Character Mode (Optional)

Graphplot Mode

..........................................

Point Mode (Two Words)

Short Vector

PROGRAMMED BUFFERED DISPLA‘Y' 338

(continued)

CO NTE NTS

Chapter

Page

Display Oriented Computer Instructions

2.3

From the Display

...........................

2.3.l

Group I

2.3.2

Group 2.

To the Display

2.3.3

Group 3.

IOT Skip on Display Flags

...............................

...................................

29
32

.........................

.....................................

Appendix
l

VC38 CHARACTER GENERATOR

PROGRAMMING EXAMPLES

REFERENCETABLES................................................

.........................

............

45
5i

PROGRAMMED BUFFERED DISPLAY 338

PARTI

SYSTEM INTRODUCTION

The Type 338 Programmed Buffered Display is a precision incremental display system, consisting
of a small scale,
is the Digital

high speed computer and a display subsystem for control of the CRT. The computer used

Equipment PDP-8 (for Programmed Data Processor).

It is a single address, fixed word length

(twleve bits) machine. The complete cycle time for its random access magnetic core memory is l .5 psec.
All arithmetic operations are performed in 25 complement notation.

This guide enables the experienced user to adapt the powerful capabilities of the Type 338 to
his application

This information is readily available in the PDP-8 User Handbook, F—85.

instruction set.

1.1

The contents assume that the reader is familiar with the operation of the PDP-8 and its

FUNCTIONAL DESCRIPTION
A self-contained unit with built-in control and power provisions, the 338 Display may be inter-

faced to an existing system or it may stand alone as a powerful computer-driven display system.
is a l6-inch tube with 9—3/8 inch

by 9-3/8 inch usable display area. Magnetic deflection and focusing

techniques re5ult in uniform resolution over this area
to

the 338 Display.

The CRT

Up to eight display CRTs may be remotely slaved

All can receive identical information, or all can receive different information, or

any combination can be established.

The routing of information to the slave display is a function of

display file instructions in the control state.
The display logic can be thought of as a special purpose computer which stores its instructions

(display commands) in the memory of the PDP-8, and interacts with the computer through a series of in—
struction interrupts and data transfers.

The display is an output device with respect to the computer for

the following reasons:
a.

The PDP-8 has a series of instructions which start, stop, and load and interrogate the

registers of the display.
b.

The PDP-8 can modify the data commands which are interpreted by the display be-

cause

the commands are stored in the PDP-8 memory.

The commands are transferred to the display control via the PDP-8 single cycle data break system.

The display file words are loaded into a table or block of successive memory locations; and the

beginning location of this table is loaded into a special register called the display address counter (DAC)
The output of the DAC is applied to the inputs of the memory address (MA) register.

A data break is then

initiated by either the display or the computer, and this address is read into the MA.

The computer then

PROGRAMMED BUFFERED DISPLAY 338

goes through a break cycle in which it fetches the word from memory and places it into its memory buffer

(MB) register from where it is transferred to the buffer register (DX) in the display. During this time, the
display starts its operation and the DAC is incremented by one.
not

incremented during the break cycle.

The computer program counter (PC) is

At the end of the break cycle the PDP-8 continues its main pro-

gram until the display requires another data break.

]

l .l

DiSplay Parameters (Coordinate System)
The display screen which is 9-3/8 inches square, has l0 bits of resolution; in other words there

are

l,024 points in the x and y directions or about a million points in all. The x and y position registers

are

l3 bits long,

however, and therefore the screen represents only l/64 the total addressable area (paper).

The paper is broken up into 64 sectors corresponding to the upper 3 bits of x and y, with sector 0 defined
as

the lower left sector.

Only information in sector 0 is intensified so that translation is accomplished by

moving the paper in relation to sector 0.

The lower left corner is point (0,0), and the coordinates in-

crease

to the

right and up, and decrease to the left and down.

drawn

across

the boundary of the paper.

An edge violation occurs when a line is

This is a warning that an overflow condition has just occurred

in the x or y position register.

A vertical edge flag indicates the y position register went from all is to

all Os, or from all Us to all ls.

The horizontal edge flag indicates overflow in the x register.

flow can be set to occur after the

The over-

10th, llth, 12th or 13th bit in x and y. The virtual paper size can

therefore be changed under program control.

l.l.2

Scale

The scale setting determines the number of positions each succeeding spot is moved before it
is intensified.

It effects both the size and appearance of lines or symbols drawn in the vector, vector

continue, short vector, increment, or character modes. At scale setting “2’ each point can be clearly

distinguished.

At scale setting

002, lines and symbols appear to be continuous. The point spacing is illus-

trated in the following table.

Scale

Point Spacing

Intensify

002

Every

0l2

2nd

l02

4th

8th

PlROGRAMMED BUFFERED DISPLAY 338

1.1.3

Intensity
There are eight intensity levels available on the display, ranging From

visible, to ill 2 which is very bright.
,

0002, which is barely

Note that scale and intensity settings are interrelated.

For ex-

ample, iF characters are drawn (with the character generator) at the lowest scale setting, and too high
an

intensity is used, they will be badly blurred. On the other hand, iF many characters are to be displayed

simultaneously or iF the light pen is to be used, it is best to use as high an intensity level as possible.
1.1.4

State

The display logic is broken into two states, data state and control state.
mands are interpreted as instructions to the display logic to change parameters,

Control state com-

iump, skip, etc. The

data state commands are instructions to move the beam via the x and y position registers.

When the display

is initialized, the commands are accepted in control state until an "enter data state'l command is given.

The display returns to control state From data state by escaping.
In control state, the First three bits (op code) designate the operation to be performed by the

remaining nine bits. Seven oF the eight op codes are used:
0

Parameter

Mode

Jump

Pop

Conditional skip l

Conditional skip 2

Miscellaneous (microprogrammed)

Arithmetic compare 1

Arithmetic compare 2

Skip on Flags

Count

4—7

Set slaves (optional)

Spare

The details are discussed in part 2 oF this manual.

1.1.5

Mode
Data state words are accepted in one oF seven Formats according to the contents oF the mode

PROGRAMMED BUFFERED DISPLAY 338

Point

Increment
Vector
Vector continue

\lo~U1-t>om

Short vector
Character (optional)

Graphplot
Spare

All modes are entered from control state by the "enter data state" command.

Each mode,

however, has its own way of escaping back to control state. The mode register is cleared by power clear

and initialization of the display (IOT l65)
l. l .6

Subroutining
This enables unconditional

The display has control state commands which will modify the DAC.

display iumps (iump),iump to subroutine (push jump) and the return from subroutines (pop).
is specified by 15 bits allowing direct addressing of 32K of core.

The new address

The jump and push iump commands are

specified by two conseCUtive l2-bit words. Push iump stores the return address, mode, intensity, scale,
and

light pen on a push-down pointer list which resides in the first 4K of PDP—8 core. This information is

automatically written into two locations in the format shown below:

Lighl

Break Field
0

Scale

Pen

Mode

Intensity
8

First Word

Low Order l2-Bits of Memory Address
0

‘1

Second Word

The information is placed in the address indicated by the push-down pointer,
a

l2—bit register in the display logic.

When a push

(PDP) which is

jump is executed, the PDP is incremented twice,

adding a new entry to the PDP list. This allows multi-level and recursive subroutines in the display.
The pop command takes the last entry on the PDP list from core and gates it back to the
proper registers.

The display status, however, can be inhibited from being restored.

maticaily decremented by two, making the PDP list a last in first out stack.
4

The PDP is auto-

PROGRAMMED BUFFERED DISPLAY 338

l .l .7

Light Pen
The light pen is an input device which generates a signal

preted by the computer.

(flag) that can be sensed and interleaving the contents of all display

A light pen interruption steps the display,

registers intact, and signals the computer that an interruption has occurred.
programmer can examine the contents

of the display registers to determine the location (on the display)

of the point of light that was sensed by the light pen
word specifying that point.

l 8
.

When this happens, the

and/or determine the memory location of the data

The light pen detects light in the range 4300 to 5600 angstroms.

Push buttons
The 338 is equipped with a bank of twelve pushbuttons.

clear button to reset that group.

They are placed six in a row with a

The buttons in each group are interlocked, but two buttons in different

groups can be pressed simultaneously.

Pressing a button complements an associated flip-flop.

the pushbutton is lit when its flip-flop is in the 1 state.

For reference,

The state of the pushbuttons can be sensed both by

the display, using the control state skip instructions, and the PDP-8, which can read the state of the push-

buttons into the acoumulator.

The PDP-8 and display can also clear and set the pushbuttons.

three—way communication between the operator, display logic, and PDP-8.

This enables

The buttons are labeled 0

through ii and are packaged in a compact, portable box. The box is connected to the display by a 20-ft
cable.

l.l.9

There is also a special computer interrupt button on the box.

Flags
There are a number of special conditions that can arise in the display which require the atten—

tion of the PDP-8 processor.

These conditions are indicated by display flags which can interrupt the

computer and be sensed by IOT skip instructions.
a.

Internal stop

External stop

Edge

Light pen find

Push button hit

Manual interrupt

The flags are:

The flags can be separated into two groups;

which do not stop the display.

a-d are flags which stop the display; e-f are flags

Group 1 flags are cleared in one of three ways: initialization of a display

sequence (6165); resuming from the point the display stopped

flag if the display is no longer needed (éléls)

(6W48

6l648); and

Power clear clears all display flags.

pulse to clear the

PROGRAMMED BUFFERED DISPLAY 338

l. I .9.l

Internal Stop

Internal stop is a control state “mode” command

(14008). When the display

stops, the DAC has already been incremented and points to the location after the stop code.

skips if the internal stop flag is on and IOT l7l is given.
resume

IOT I64 with the AC

The PDP—8

0 causes the display to

from this point.

1.] .9.2

External Stop

The PDP-8 IOT

l54, with AC bits 0 and 4 at 0, causes the display to stop.

When the display stops, the external stop flag is set and interrupts the computer if the interrupt system
is on

The time lapse between the IOT and the display stop is dependent upon the display command

being executed when IOT l54 is given. The rule is that the entire command will be exeCUted before the
If the command is a two-word instruction like iump,

display is stopped.

pimp, or vector, or point mode

command, both words are pulled out of memory and exeCUted before the display stops.

This allows simple

resuming of the sequence even if the display is used in the interim.

The external stop flag causes the

PDP-8 to skip when IOT 151 is given (with AC bits 0 and 4 at O).

The display sequence can be resumed

by giving IOT 174.

I. I .9.3

FEE

—

The edge is defined as the point at which the x or y position registers overflow.

initial conditions of the display can be set up so that all edges are ignored.

The

In the normal case, the edge

flag stops the display; when the edges are ignored, however, the display waits 35 psec and then resumes

automatically.
causes

The IOT T74 will restart the display from the edge if the display has stopped.

the PDP-8 to skip if either the horizontal or vertical edge flag is on.

LI .9.4

Light Pen Find

The light pen find flag always stops the display as soon as it senses light.

This occurs about 6 psec after the initial flash.
more

IOT l52

The display logic therefore has time to execute several

points in the vector before it stops. This must be taken into consideration when reading back the

and y coordinates after the display has stopped.

The PDP-8 can skip on the light pen flag using IOT l32.

The vector or increment sequence can be resumed by giving IOT I74.

1.1.9.5

Pushbutton Hit Flag

If any of the twelve pushbuttons are hit, the pushbutton hit flag is raised.

This flag will rat stop the display, but can cause a computer interrupt.
which reads back the status of the flag into the AC.
the accumulator.

I .I .9.6

box.

This flag will

IOT l7l reads the state of the twelve pushbuttons into

There is no skip IOT for the pushbutton hit flag.

Manual Interrupt

It has a

The flag is cleared by IOT 062,

The manual interrupt flag is set by the interrupt button on the pushbutton

light associated with the flag in the button. Whenever the flag is set, the button is lit.

n_ot stop the display.

gram counter to skip if the

The flag can be cleared by IOT

flag is set.

I72, which will also cause the pro-

PROGRAMMED BUFFERED DISPLAY 338

Four of the display flags must be gated onto the interrupt line by the initial condition IOT T45.
These flags are edge,

light pen find, pushbutton, and internal stop.

The external stop and manual inter~

rupt flags always cause an interrupt if the interrupt system is on.

l.l.l0

Timing
The display can take a break cycle a maximum of one out of three machine cycles.

effective cycle time of the display is 4.5 usec if single cycle instructions are being executed.*

control state instructions except iump,

push iump, and pop, are executed in one display cycle.

instruction takes two cycles because it is a two-word instruction.

The push

The
All

The iump

iump is a four-cycle instruc-

tion, two to get the instruction out, and two to push the two status words away on the push down list.
pop instruction. is three cycles,

one

to obtain the instruction and two to

The

pull the status words from the push

down list.

In data state, the mode and the number of intensified points determine execution time.

In the

incremental modes, increment, vector, short vector, and vector continue, points are plotted at a rate of
one

every I .l psec for intensified lines and 300 nsecs for nonintensified lines.

Point and Graphplot

mode words are given a 35-psec delay to allow the beam to settle, whether the point is intensified or not.
If points are plotted in the same general area, as in a continuous curve, the delay for settling is only
6 psec.

The small delay is given if, and only if, the two points plotted have the same high order 6 bits

(of l3) in both x and y position. The time needed to fetch the information must then be added (one or
two

display cycles) to get the total execution time.
The VC38 Character Generator (optional) plots at about 37 psec per character.

variable depending on the number of intensified points in the character.

This time is

Control characters are executed

in two display cycles except for carriage return which takes an additional 35 psec.

To estimate the time to display a 6-bit character, add up the time required for drawing out the

character, then add in 2-l/2 display cycles needed for the character dispatch

*See PDP-8 Users Manual for details on the data break system.

PROGRAMMED BUFFERED DISPLAY 338

PARTZ

DATA FORMATS

Listed below are the data Formats for the control and data state commands.

short general description, and a bit-by-bit explanation are given.

common

The bit structure,

The control state also has a list of

mnemonics.

Control state is broken into the seven op codes with the miscellaneous command broken down
further to Five microprogrammed commands.

2 .i

In data state, each of the seven modes is discussed separately.

CONTROL STATE COMMANDS

Control state commands are instructions to the display logic to change a register, such as scale,

DAC, mode, etc. Some of the commands change more than one register, so an enable bit is provided
It the enable bit is a l, the associated register is reset to the appropriate value.

For each function.

the enable bit is a O, the associated register is unchanged independent of contents of the bit(s) following
the enable bit.

2 l
.

There is no restriction on the number at registers that can be changed with one instruction

Parameter

Op Code:
O
0

l
'

Light

Scale

Parameter
4

Intensity

Pen
5

l l

Parameter command is an instruction to change the scale,

light pen, and intensity registers.

When the light pen reigster is a l, a light pen hit stops the display.

It the light pen register is a 0, all

light pen hits-are ignored.

PROGRAMMED BUFFERED DISPLAY” 338

Bit(s)

Interpretation

0-2

Op code: parameter

Enables scale change.

4,5

Determines one of four possible scale settings if bit 3 is a l.

Enables light pen change.

Turns the light pen on when it is a

Enables intensity change.

9, l0, ll

Determines one of eight possible intensity settings if bit 8 is a l.

l, or off when it is 0, provided bit is a l.

ASSOCIATED MNEMONICS AND VALUES
Mnemonic

Octal

Symbol

Code

Operation

LPOF

0040

Light pen off.

LPON

0060

Light pen on.

SCl

0400

Set scale to Xl

SC2

0500

Set scale to X2.

SC4

0600

Set scale to X4

SC8

0700

Set scale to X8.

INT*

00l0

Set the intensity.

*INT only enables the change of intensity; 0 space then a number between 0 and 7 to indicate the de-

sired intensity should follow.
a

Any of the mnemonics of the same op code may be concatenated to form

A typical parameter command would be:

compound command.

SC2

LPOF

INT

which would have an octal value of 554.

Mode

2 .l .2

Clear

Stop

Op Code:

Code

Mode

Clear

PushButton

Mode

Sector
Bll'S
.

Flag
0

l
0

Clear
Coord.

mate
.

BI‘l’S
l0

Enter
Data

State
l l

PROGRAMMED BUFFERED DISPLAY 338

The mode command has six separate Functions:

It can cause the display to stop and set the

internal stop flag, clear push-button and manual interrupt Flags, set the mode register, clear sector bits,

clear coordinate bits, or enter data state.

Only the mode change has an enable bit to prevent its being

changed.

Bits

Interpretation

0,1,2

Op code:

Stops the display and sets the internal stop flag when bit is 1

Clears the push-button and manual interrupt Flags when bit is a 1.

Enables mode change

6,7,8

Determines one of eight possible mode settings if bit 5 is a 1.

Clears only the high order three bits in both the x and y position registers when

bit is a 1

mode.
.

Clears only the low order ten bits in the x and y position registers when bit is a 1

When bit is 1, the next instruction is accepted as a data state word rather than con—
trol state.

The display remains in data state until an escape is executed.

ASSOCIATED MNEMONICS AND VALUES
Mnemonic

Octal

Symbol

Code

Operation

EDS

1001

Enter data state.

CCB

1002

Clear coordinate bits.

CSB

1004

Clear sector bits.

POINT

1100

Set mode to 0.

INCR

1110

Set mode to 1

VEC

1120

Set mode to 2.

VECON

1130

Set mode to 3.

SVEC

1140

Set mode to 4.

CHAR

1150

Set mode to 5

GRAPH

1160

Set mode to 6.

CLDF

1200

Clear flag.

STOP

1400

Stop display.

PROGRAMMED BUFFERED DISPLAY 338

2 l
.

Jump

0%
0

Code:

Break Field

Push

Pen

Light

Scale

ump

First Word

Low Order
0

12 Bits of Address
4

Second Word

The iump command is a 2-word (24-bit) instruction, l5 bits of which specify the new address.
The T5 bits used are the last 3 bits in the First word, which specifies the memory field, and the entire
second word, which specifies the address in one of the 4K memory banks.
causes

see

two words to be entered on

the end of the PDP list.

It bit 8 is a

l, the command

For full explanation of the push

iump command,

"Subroutining section." The iump command can also change the scale and light pen registers.

Bits

Interpretation

0, l ,2

Op code: iump.

Enables scale change.

4,5

Determines one of four possible scale settings if bit 3 is a l.

Enables light pen change.

Turns the light pen on when it is a

When bit is a O, the command is a simple iump, the scale and light pen are changed,
and the new l5-bit address is inserted in the DAC and the display continues From
there.

l, or off when it is a 0, provided bit 6 is a l.

When bit is a l, the command is a push iump.

The old address and the status

of the display are stored on the PDP list; then the new address and scale or light pen

change is inserted.
9, 10, ll

Specifies the high order

3 (of ‘5) address bits For the lump or push

lump command.

Second

VVOrCl
n}
ll
“I
i

Specifies the low order l2 (01c l5) address bits For the iump or push iump command.

PROGRAMMED BUFFERED DISPLAY 338

ASSOCIATED MNEMONICS AND VALUES
Mnemonic

Octal

Symbol

Code

JUMP

2000

O pera f.Ion

Jump to 15-bit address
contained in last digit
and the next word addressed.

Jump to subroutine addressed

2010

PJMP

the same as JUMP.
‘

LPOF

0040

Light pen off.

LPON

0060

Light pen on.

SCl

0400

Set scale to Xl

SC2

0500

Set scale to X2.

5C4

0600

Set scale to X4.

SC8

0700

Set scale to X8

2 l.4

flop
Inhibit Restoring

OpPCode.

Scale

Light Pen

The pop command is the last word in a subroutine file.
_

old address and old status From the PDP list.
bits 8, 9, and/or 10.

‘

Pen and Intensity

Data

State
10

status is restored.

Mode

Scale
2

Enter

Light

It causes the display to bring out the

The status can be blocked from being reinserted by raising

The new light pen and scale settings in the pop will always be inserted after the

The First instruction after the push iump can be a data state instruction by entering

data state during the pop.

PROGRAMMED BUFFERED DISPLAY 338

Bits

Interpretation

0,1,2

Op code:

Enables scale change.

4,5

Determines one oF Four possible scale settings iF bit 3 is a l.

Enables light pen change.

Turns the light pen on when it is a l, or oFF when it is a 0,

The mode status From the PDP list will not be restored when bit is a l

The light pen and scale status From the PDP list will not be restored when bit is a l

The intensity status From the PDP list will not be restored when bit is a l

The display wili be in data state when the word at the address taken From PDP list
is executed.

pop

provided bit 6 is a l.
.

ASSOCIATED MNEMONICS AND VALUES

Mgemonic
ymbol
POP

Octal

Operation

Code

3000

Exit From subroutine to next
address aFter PJMP.

FEDS

300T

Pop and enter data state.

PNI

3002

Pop and inhibit restoring intensity.

PNLS

3004

Pop and inhibit restoring light
pen and scale.

PNM

30l0

Pop and inhibit restoring mode.

LPOF

0040

Light pen oFF.

LPON

0060

Light pen on.

SCl

0400

Set scale to Xl

SC2

0500

Set scale to X2.

SC4

0600

Set scale to X4.

SC8

0700

Set scale to X8

PROGRAMMED BUFFERED DISPLAY 338

Conditional Skip (Bank l)

2.l.5

Op Code:

Sense

Conditional Skip

(Bank 1)

Test

l
l

Clear

Comple_

3'“

ment '3'”

ATTer

Afl'er

Test

Test
5

Selected Buttons 0-5
PBO
6

PB4

PB3

PBZ

PBl

PB5
H

All display skip commands skip two display words it the condition tested Fails.

The display

skips two instructions so that a JUMP or PJMP command (which are two words long) can be executed or
not executed

properly.

The buttons to be tested should have ls in the proper bits of the skip command.

Using the clear and complement Facilities, the buttons can be set in any desired configuration. The
sense

test bit determines whether

the user is-testing For is or Os.

Bits

Interpretation
conditional skip (bank

l).

0,1,2

Op code:

If bit is 0, the display skips two words it any of the indicated push buttons are 0.
If bit is l,

the display skips two words if any of the indicated push buttons are l.

Sets all the selected push buttons to the 0 state (light off) when it is a

l, inde-

pendent ot the outcome of the test.

Complements all the selected push buttons after the test when it is a l, independent
Since the buttons are cleared before they are complel state by having both bits 4 and 5 at l
can
be
set
to
the
mented, they

of the outcome of the test.

,1]

Selected push buttons of bank 1; e.g., bit 6

push button 0, bit ll

push button 5.

PROGRAMMED BUFFERED DISPLAY 338

ASSOCIATED MNEMONICS AND VALUES
Mnemonic

Octal

Symbol

Value

SKI

4000

l
0 p e rat'on

Skip it any of the selected buttons are

Invert sense of test (skip it any

0400

INV

selected button is l).
CLAT

0200

Clear buttons tested after test.

COAT

0100

Complement buttons tested
after test.

Conditional Skip (Bank 2)

2.l .6

OpCode.
Conditional

Skip

(Bank 2)
0
I

Test
3

Clear

Sense

Comple—

Bits

After
Test

Selected Buttons 6-l l

ments
,

Bll'SAfl"
er

PB?

PB8

PB7

PBé

Test

PBlO
10

PBII
II

This command is identical to conditional skip (bank I) except that it tests push buttons 6—] I

Bits

0, I ,2

Interpretation
Op code: conditional skip (bank 2)

3
4

Same as conditional skip (bank 2).

5
6-H

Selected push buttons of bank 2; e.g.

bit 6

push button 6, bit II

push button ll

PROGRAMMED BUFFERED DISPLAY 338

ASSOCIATED MNEMONICS AND VALUES
Mnemonic

Octal

Symbol

Value

SKZ

5000

O p erat'o
I
n

Skip iF any of the selected buttons are

0400

INV

Invert sense of test (skip it any
of the selected buttons are l)
.

CLAT

0200

Clear buttons tested after test.

COAT

0100

Complement buttons tested after
test.

2 l .7

Miscellaneous

2.1.7.1

Arithmetic Compare Push Buttons (Bank 1)

M'CXP'Egm'IWed‘

Op Code:
Miscellaneous

Push Buttons (0-5)

“f metic

PBO

Compare PB (0-5)

PB4

PB3

PB2

PBl

PB5

l
l

l l

IF all the

Bits 6-ll of this command are compared to the contents of buttons 0-5 (bank

bits and buttons match, the test succeeds and the display Follows a normal sequence.

the test tails,

the display skips two words.

Arithmetic Compare Push Buttons (Bank 2)

2.1.7.2

Microprogrammed:

Op Code.
_

Push Buttons (6 ll)
_

Arithmetic

Miscellaneous

Compare p3 (6-11)

P86

PB9

P38

P87

PBiO

PBH

Same instruction as above, except bits 6-ll are compared to push buttons 6—l l

PROGRAMMED BUFFERED DISPLAY 3

2.l .7.3

Skip on Flags

Op Code:
.

‘3

ditional

Uncon-

Miscellaneous

Ski P

Microprogrammed:
Skip on Flags

Skip

Skip on

if not

Push-Button

Sector

Hll' Flag

0
7

Bank l

Bank 2

(l2008)

Sector 0 is defined as any point where

the x and y position registers have all Os in the high order 3 bits (of l3).
the individual banks (l

All the commands will skip two words if the test fails.

Spare

The push button hit skip skips

2). Both flags are cleared by IOT 062 or by the display command CLDF

Bit(s)

Interpretation

O,l,2

Op code: miscellaneous.

3,4,5

Microprogrammed: skip on flags.

Do not execute the next two display words if bit is l.

Do not execute the next two words unless the high order three bits of both the x and
y position registers are 0; i.e. ,

skip if the beam is not on the screen.

Skip if any push button 0—5 has been pushed.

Skip if any push button 6-ll has been pushed.

l0, ll

Spare.

2. l .7.4

Count

Op Code:
Miscellaneous
0

Count

l
l

Count
Scale

Microprogrammed:
4

5
l

6
l

Count

Intensity
8

On
l0

Blink
Off
l l

PROGRAMMED BUFFERED DISPLAY 338

The scale and intensity registers are also up-down counters.
the scale register stays at

ll2

They cannot overflow; however,

matter how many count scale up commands are given.

The blink Facility

allows the picture or any section of it to Flash on and off at l cps (1/2 sec with the intensity on and 1/2
sec

with the intensity off)

Bit(s)

Interpretation

0,1,2

Op code: miscellaneous.

3,4,5

.Microprogrammed:

count.

Enables count scale logic

count scale up

count scale down

Enables count intensity logic.

count intensity up

count intensity down (unless at

(unless at 1112)

0002)

Turn blink on, all intensified points will be gated through the blink logic.

Turn blink oFt'.

Slave Logic (Optional)

2. l .7.5

Micro-

Op COde:

GrOUP

Miscellaneous
0

(unless at “2'
(unless at 002)

pro.

Uh” O

Number

Slaves
3

Uhl'l' l

The slave logic allows the display to control up to eight CRTs.

The slave logic blanks or

unblanks the intensity and light pen at each CRT For the display File being executed until the slave status
is changed.

IOT 072 and IOT 074 can read back the status of the eight slaves into the PDP-8.

PROGRAMMED BUFFERED DISPLAY 338

Bit(s)

Interpretation

0,1,2

Op code: miscellaneous.

Specifies slave logic when it is a l.

4,5

Choose one of four slave groups, with two units each, to be modified.

Enables change of unit 0 of slave group specified in bits 4,5.

Turns on light pen of unit 0 if bit is a

Turns on the intensity of unit 0 if bit is a

Enables change of unit 1 of slave group specified in bits 4,5.

Turns on

ll.

Turns on the intensity of unit l if bit is a

l; turns it off if bit is a 0.
l; turns it off if bit is a 0.

light pen of unit l if bit is a l; turns it off if bit is a 0.
1; turns it off if bit is a 0.

ASSOCIATED MNEMONICS AND VALUES

Msnemoni
ymbol c

Octal
Value

Operation

SK3

6000

Arithmetically compare pushbuttons
(0—5) with last two digits of instruction; skip if not equal.

SK4

6l00

Same as SK3 butwfor buttons 6-ll

SKIP

6240

Unconditional skip (two locations).

SNSZ

6220

Skip if sector 0 flag is not up.

SPBl

62l0

Skip if push button (0-5) flag is down.

SPB2

6204

Skip if push button (6-l l) flag is down.

SCUP

6340

Count scale up.

SCDN

6360

Count scale down.

INTUP

6310

Count intensity up;

INTDN

6314

Count intensity down

BKON

6302

Blink on.

BKOF

630i

Blink off.

$60

6400

Set slave group 0.

SGl

6500

Set slave group l.

5G2

6600

Set slave group 2.

5G3

6700

Set slave group 3.

SUO

0040

Turn light pen and

LPO

0060

Unit 0 light pen on.

intensity off on unit 0.

PROGRAMMED BUFFERED DISPLAY 338

ASSOCIATED MNEMONICS AND VALUES (continued)

Mnemonic

Octal

Symbol

Value

ITO

0050

SUI

0004

Operation

Unit 0 intensity on.

Turn

light pen and intensity off on

unit I

LPl

0006

Unit I

light pen on.

0005

Unit I

intensity on.

2.2

DATA STATE COMMANDS
All data state commandschange the x and y position registers which are in turn connected

through D to A converters to the CRT deflection system.
format used by the display.

In control state only the mode command can change the mode.

the eight possible modes are used.

Seven of

If the display tries to enter data state in the unused mode,

the display stops but no error flag is raised.
to control state.

The mode register determines the data state

(I l l2),

Each of the seven modes has an escape mechanism to return

Since most of the modes are different, each is described below.

Point, vector, and vector continue modes are two-word commands; a single command is

specified by two consecutive locations in the display list. Both words must be brought out before execution, and therefore there are two input buffer registers. The register DX, which is used for all com-

mands, receives its information directly from the data lines. If the command is two words long, the first
input word is transferred to the DY register while the second input word is brought in to DX from memory.
The only exception to this is data state increment mode words.
executed from the DY register.
wait for input words.

In this case a single word command is

The DX register is used for double buffering virtually eliminating the

Short vector mode uses the DY register in order to appear as a normal vector.

other words, the delta Y portion of the command is transferred to the DY register.

Point Mode (Two Words)

2.2.1

(000)

Point

Intensity Inhibit
0

Y Position
2

First Word (DY)

PROGRAMMED BUFFERED DISPLAY 338

(000)

Point

Escape
‘0

Inhibit

X Position
2

Second Word (DX)

The basic action is to iam bits 2 through ll of the first word (from DY) into the low order ten
bits of the y position register, and the same bits in the second word into the x position register.
order three bits in x and y remain unchanged.

The high

If bit l in either word is up, the contents of the associated

position register are not changed during that command. This is useful if the user does not know the present
beam position and wishes to change either x or y and leave the other at the same value.
first y point word is a

If bit 0 in the

l, the point specified is intensified when the beam reaches the proper position. If

bit 0 in the second word (x point) is a l, it indicates an escape and the next word is interpreted as a control state command.

If the bit is a 0, the next word is interpreted as the first word of another point mode

The scale setting has no effect in point mode.

command.

Bit

Word

2.2.2

If bit is a

Inhibit changing )1 position register.

2-H

New y coordinate (low order ten bits).

Escape to control state.

Inhibit changing x position register.

2-“

New x coordinate (low order ten bits).

l, intensity given point.

increment Mode

Increment

COT

NO: OF MOVES

Intensity
0

Interpretation

Direction (0-7)
3

No. of Moves

Intensity

Direction (0—7)
9

PROGRAMMED BUFFERED DISPLAY 338

Increment mode is a mechanism for moving the beam 0 short distance in a“. efficient manner.

The beam is moved from its previous position to a new position according to a 6-bit increment byte. Each

byte is handled separately and executed independently of the other.
are

Both bytes (first, 0-5, second, 6-l l)

identical; therefore only the first will be discussed.
The byte is broken into three sections:

first, to indicate whether the byte move is to be in-

tensified (bit 0 (6)); second, to give the number of moves to be made (bits l, 2 (7, 8)); and third,

specify the direction in which the move is to be made (bits 3, 4, 5 (9, 10, ll))

The beam is only in-

tensified at discrete points according to the scale setting (see "Scale" section 1

), it is not run with

the beam on during the motion.

The increment can consist of one, two, or three moves per byte, with

each move being one, two, four, or eight points apart on the raster depending on the scale.

bits that control the number of moves are programmed as follows:

Bits l, 2

Interpretation

Move the beam once in the indicated direction and escape.

Move the beam once; do not escape.

Move beam twice; do not escape.

Move beam three times; do not escape.

The three bits for direction indicate one of eight 45 degree directions:

4 <

Starting Position

The two

PROGRAMMED BUFFERED DISPLAY 338

The letter "A" is programmed in Subroutine form as an illustration of the use of increment mode.
0

O
O

starting —9

point

final
Po'nt

The "0” indicates an intensified point, the "x" a nonintensified point, and "E" the point at

which the escape is given.

INCR

ALPHAA,

/octa| (NH)

EDS

3762
270]

/byte 1 —\1' i m, noninten. byte 2 -’i‘ 3 m, inten.
/byte 1 JT‘ 3 m, inten. byte 2 -7i i m, inten.
/bytei——>,2m,inten.byte2-5\,Tm,inten.
/byte 1 -\l/ 3 m, inten. byte 2 <—-, 3 m, inten.
/byte T X 3 m, noninten. byte 2 4} 2 m, inten.
/byte 1 -§\ 2 m, noninten. byte 2 -71 i m, noninten. and escape

POP

/octal (3000)

1672

7251

6057

7674

—

Bit(s)

Interpretation

It bit is a

1 ,2

Number of moves in the byte

3,4,5

Direction in which moves are to be made.

If bit is a

7,8

Same structure as byte 1 bits 1 and 2.

9,10,11

Same structure as byte 1 bits 9,

T, intensify the First byte.

(002 =>

Move once and escape).

T, intensify the second byte.

10, H.

Vector Mode (Two Words)

2.2.3

Vector

(010)
TO—Bit Delta Y

Intensity
0

First Word {DY}

PROGRAMMED BUFFERED DISPLAY 338

lO—Bir Delta X

Esca pe
5

Second Word (DX)

Vector Mode is used to draw long straight lines.

The beam is moved from its present location

in the direction and distance specified by the delta y and delta x.
moves

to make in x and y

before stopping.

The number of raster points between each move is again de—

pendent upon the contents of the scale register.
from one end of the screen to the other.

"paper."

The deltas tell the beam how many

At a scale of one, the 10-bit vector can take the beam

At a scale of eight, the beam can go to any point on the l3-bit

Bit 1 in both words indicates the sign (direction) of the vector.

up for y and to the right for x.

A + is indicated by a

0 and is

Bit 0 in the first word indicates whether the vector is to be intensified.

Bit 0 in the second word is the escape to control state which indicates the end of a vector string.

The following display subroutine program places on the screen a 1—inch square.
is drawn with vectors, it can be put anywhere on the screen

VEC EDS

Beam Movement

4067
0000

/Ay=+55. intensify
/Ax= 0.
/Ay= 0. intensify
/Ax=+55.
/Ay=-55. intensify

4000
0067
/

Starting and
final point

/Control state octal (5l5)
/Control state octal (112])

5C2 INT 5
Direction of

(relocatable)

6067
0000

/Ax=

4000

/Ay=

6067

/Ax=-55. escape
/Control state octal (3000)

POP

intensify

Since the box

PROGRAMMED bUFFERED DISPLAY 338

Interpretation

Bits

Word

intensity the vector.

If bit is a l,

If bit is a 0, the sign of delta y given in bits 2-H is positive; if a

it is negative.

2-H

The 10-bit delta y.

If bit is a

If bit is a 0, the sign of delta x given in bits 2-1] is positive, it a

l, the next word is interpreted in control state.
l,

it is negative.

The lO-bit delta x.

2—H

Vector Continue Mode (Two Words)

2.2.4

(0i l)

Vector Continue

Intensify
0

lO-Bit Delta Y
2

'10

lO-Bit Delta X

Escape
0

Vector continue mode has the same format and action as vector mode, except the vector does
not stop until it reaches the edge

Short Vector

2 .2 .5

(100)

Short Vector

Delta Y

Intensify
O

Delta X

Escape
5

PROGRAMMED BUFFERED DISPLAY 338

Short vector mode is basically the same as vector mode except it is only one word long.
order to fit it into one word, the maximum number of increments has been reduced from
x

command.

Bits 2,

3, 4, and 5 correspond to 8, 9, 10, and ll of the first word; bits 6 and 7 to

bits 0 and l of the second word; and bits 8, 9, l0, and ll
In execution of a

l,024 to l6 in

Bits 0 and l of a short vector word correspond to bits 0 and l of the first word of a

and y directions.

vector

bits 8, 9, 10, and ii of the second word.

short vector mode command, the left half of the input word is transferred to DY and

bits are rearranged to be in the same format as a vector mode command.

Interpretation

Bits

l, intensify the vector.

If bit is a

If bit is a 0, the sign of delta y in bits 2,

3, 4, and 5 is positive; if a I,

it is negative.

2,3,4,5

The 4-bit delta y.

If bit is a

If bit is a 0, the sign of delta x given in bits 8,
if a

l, the next word is interpreted in control state.

The 4-bit delta x.

8,9,l0,ll

Character Mode (Optional)

2 .2.6

(IOl)

Character

Character 2

Character l
O

9, 10, and ii is positive;

it is negative.

Six-bit format

Character

Ignored
0

Seven-bit format

PROGRAMMED BUFFERED DISPLAY 338

The character generator can be run in two different formats:

6-bit format,

packed two to a

word but directly referencing only 64 characters, or 7-bit format, only one character per word but

referencing l28 different characters. The character generator is simply an efficient dispatch system for
instructions stored in core memory.

table that calls them.

The characters, therefore, are programmable as well as the dispatch

The details of programming the character generator are given in appendix l

Special characters available in the character generator include set and count intensity and scale,
set

light pen, change case, change character set, change code size (6-bit versus 7-bit), carriage return

(clear x register), and escape to control state.

2.2.7

Graphplot Mode

Graphplot (HO)

3:: :2

Escape
0

X or Y Coordinate
2

Graphplot mode is a concise way of describing and displaying tabular data.
a

data command is as follows:

escapes to control state

The execution of

First, the x or y coordinate is incremented once; then the other coordinate

is set by bits 2-ll of the graphplot mode command.

cremented and y plotted (bit

Bit 1 of the word specifies whether x is to be in-

l-O), or y incremented and x plotted (bit l-l).

If bit 0 is a

l, the display

The incremented axis moves one, two, four, or eight points depending on the scale setting.
Since one axis is always incremented, the graph is easily translated in this direction by changing the

starting location

Bit(s)

Interpretation

If bit is a

If bit is a 0, increment x coordinate and set the y coordinate with bits 2-l i; if bit
is a

2-l 1

l, escape to control state.

l, increment y and set x.

Ten bits of coordinate information.

PROGRAMMED BUFFERED DISPLAY 338

2.3

DISPLAY ORIENTED COMPUTER INSTRUCTIONS
The PDP-8 has a family of instructions

which it uses to communicate with

A group of these instructions is assigned to the display (IOTs O5,

06, 07, I3, I4, I5, I6, I7, 30).

below are the display IOTs, their mnemonics, and their functions.
the first requests information from the display;
class is the computer skip on display flags.

1/0 equipment.
Given

They are broken into three classes:

the second sends information to the display; and the third

The information transfer is done through the PDP-8 accumu-

later; so if the user expects information, he should clear the AC beforehand, and if sending information,
he should have the data in the AC before giving the IOT.

From the DiSplay

2.3.I

Group I.

2.3. I .I

RPDP 605I Read Push Down Pointer

A Is (inclusive OR) transfer from the push down pointer

(I2 bits) to the AC is done. The PDP should be pointing to the location in which status information will
go on the next push

iump (if it comes before a pop)

Reading the PDP, subtracting the set value, and

dividing by two gives the level of the subroutine.

2.3.I .2

RXP 6052 Read x Position Register

A Is transfer from the x position register to the AC is done.

Only the low order I2 (of I3) bits are transferred; the high order bit must be obtained from the RS2
instruction

2.3. I .3

RYP 6054 Read y Position Register

2.3. I .4

RDAC 606I Read Display Address Counter

transferred from the display to the AC.

Same as RXP, except the y position register is transferred.

The contents of the display address counter are

The DAC will be set at the next command to be executed by the

display.

2.3. I .5

RSI 6062 Read Status I

the break field register.

—

Status I consists of the state of all display flags and the contents of

If the flag is up the associated bit is a I.

*See PDP-8 User Handbook for details.

PROGRAMMED BUFFERED DISPLAY 338

Bit(s)

Interpretation

Light pen hit flag.

Vertical edge flag.

Horizontal edge flag.

Internal stop flag.

Sector 0 flag.

Control state flag.

The y position register has overflowed.
The x position register has overflowed.

If bit is a

l, the display is in sector 0.

If bit is a

display is in data state.

I, the display is in control state, if it isa 0 the

Manual interrupt flag.
7

Push-button hit flag.

Display interrupt flag.
puter will interrupt.
the interrupt line.

9,10,] l

If the interrupt system is turned on and bit is a

l, the com-

It is set by one of the six display flags being on and gated onto

Contents of break field register.

These three bits and the twelve bits from the RDAC

instruction give the full 15-bit memory address.

2.3.l .6

R52 6064 Read Status 2

Status 2 consists of the contents of some of the major registers in the

display; e.g., light pen scale, mode, and intensity.
bit of the x and y position registers.

It also contains byte information and the high order

The byte flip-flop indicates whether the left half or right half byte

in increment mode was being exeCUted when the display stopped.

It does not tell whether the right or

left hand character is being executed; this information is obtained from the RCG (IOT 304) instruction.

The low order twelve bits of the 13-bit x and y position register are obtained by giving RXP or RYP.

Bit(s)

Interpretation

Byte flip-flop. If bit is a 0, the left hand increment is being executed; if bit
is a l, the right hand byte is being executed.

Light pen enable.

High order y position register bit.

High order x position register bit.

4,5

Scale

6,7,8

Mode

9,i0, ll

Intensity

If bit is a

l, the light pen is enabled.

PROGRAMMED BUFFERED DISPLAY 338

RPB 6071 Read Push Buttons

2.3.1 .7

The contents of the twelve push buttons (0—1 1) are transferred

into the corresponding AC bits.

RSGl 6072 Read Slave Group l

2.3.l .8

status forces slaves 0,1,2, and 3.

On this instruction, the light pen,

light pen hit, and intensity

The control state command "set slaves" sets the light pen and intensity

If the slave option is not present, the IOT reads back Os into the accumulator.

status.

Slave 0

Light

Slave l

Slave 3

Slave 2

Light
Inlens'ly Pen Hit
.

Pen

AC Format

Bit(s)

Interpretation

Light pen enable, slave 0.

Intensity status of slave 0.

Light pen hit, status slave 0.

3,4,5

Same Format as above for slave l.

6,7,8

Same format as above for slave 2.

9, 10, ll

Same format as above for slave 3.

2.3. l .9

RSG2 6074 Read Slave Group 2

RSGZ has the same format as RSGl , except it reads status

of slaves 4,5,6, and 7.

2.3.l .10 RCG 6304 Read Character Generator

RCG reads in the five character generator parameters:

character generator active (CHACT), character byte (CB), case, code size (CHSZ), and starting address

register (SAR). The CHACT bit indicates whether the character generator is in use; i.e.
in character mode and data state.

the display is

The CB shows whether the left or right half character (6-bit format) is

being executed. The case bit is used (6-bit format) asa seventh bit to allow referencing either the lower
or

upper set of 64 characters.

used.

The CHSZ indicates whether the 6-bit or 7-bit character format is to be

The SAR is a 6-bit register that indicates the starting address of the character dispatch table (see

appendix).
3]

PROGRAMMED BUFFERED DISPLAY 338

22:;
0

Case

Spare

CHSZ

SAR

Spare
5

Bit(s)

Interpretation

If bit is a

If bit is a 0, left character is being executed.

l, the character generator is active.

If bit is a l, right character is being executed.
2

Spare.
If bit is a l, upper case is in use, characters 65-l28.

If bit is a 0, the character generator is using 6-bit format; if bit is a

l, the CG

is using 7-bit format.

Spare.

6-ll

Contents of the 6-bit SAR.

To the Display

2.3.2

Group 2.

2.3.2.l

SPDP 6l35 Set the Push Down Pointer

Since the PDP is a l2-bit register, the PDP list must reside in the first 4K of memory.

2.3.2.2

SIC 6145 Set Initial Conditions

The contents of the AC are transferred into the PDP

SIC sets up a number of status registers in the display.

instruction enables four display flags onto the interrupt line, sets the paper size to l0,
in x and y and

The

ll, l2, or l3 bits

light pen conditions. There are three options in the event the display is resumed after a

light pen hit. The light pen can be left on, it can be turned off completely, or it can be turned off until the completion of the present command, then automatically turned back on at the next data

request.

There is also a register that tells the display to ignore all edge flags; therefore when the position register

overflows, the edge flag is inhibited and the display continues in a normal fashion.
overrides the intensification bit in data state,

Another register

causing all beam movements to be intensified.

is used principally for diagnostic purposes.

This feature

PROGRAMMED BUFFERED DISPLAY 338

Bit(s)

Interpretation

Enable edge Flag interrupt.

Enable l-ight pen flag interrupt.

If bit is a 0, do not disable light pen after the :resume; it bit is a 1, bit 3 indicates
when to reenable the light pen.
If bit is a 0, reenable light pen on the First data request after the display is resumed.

If bit is a

l, the light pen hit is equivalent to a LPOF command.

Set Y dimension.

4,5

00:

9.375 inches

01:

18.75

inches

l0:

37.5

inches

75.0

inches

(l0 bits)
(ll bits)
(12 bits)
(l3 bits)

6,7

Set X dimension, same as Y.

Intensity all points.

Inhibit edge flags.

Enable interrupt on push-button hit.

Enable interrupt on internal stop flag

LBF 6155 Load Break Field

2.3.2.3

This instruction has two functions.

First, it loads the break field

that one may be executed without the other.

If neither enable bit is up, both IOT pulses have other

meanings (STPD—él54 and SPSI—6l5l).

Break Field
2

Push Buttons
3

PROGRAMMED BUFFERED DISPLAY 338

Bit(s}

Interpretation

Enable change of break field.

l ,2,3

New break field.

Enable change of push buttons.
If bit is a 0, set push buttons 0-5 according to AC bits 6-l l; if bit is a

push buttons 6-H according to AC bits 6—H
6—] l

New push-button states.

2.3.2.4

SCG 6303 Set Character Generator

Case

Spare
O

l, set

SCG sets the SAR case and CHSZ.

CHSZ

Spare

SAR
6

Bit(s)

Interpretation

0, l ,2

Spare

Set case O-lower 64
l-upper 64

Set code size 0-6 bit character format

l-7 bit character format
5

Spare

6—l l

Starting address register

2.3.2.5

INIT 6l65 Initialize the Display

The AC is transferred to the low order twelve bits of the

DAC, and the display is initialized by raising the break request flag. The display will run uninterrupted
until a flag is raised.
The five IOTs given (2.3.2.1—2.3.2.5) comprise a standard startup procedure for the display.
The following program is given as an example.

PROGRAMMED BUFFERED DISPLAY 338

/Display start-up routine for
/338 with VC38 Character Generator.
Initial,

CLA

/Clear AC

TAD XPDP

/Starting address for PDP, into AC

SPDP

/IOT to transfer AC to PDP.

CLA

/Clear AC

TAD XSIC

/Constant to set initial conditions.

SIC

/Transfer AC to initial condition register.

CLA

/Clear AC

TAD XLBF

/Constant for BF and/or PB.

LBF

/Transfer AC to BF.

CLA

/Clear AC

TAD XCG

/Constant for character generator.

SCG

/IOT to transfer AC to CG.

CLA

/Clear AC

TAD XIN

/l2-bit starting address for display

INIT

/IOT to transfer AC to DAC and initialize display.

CLA

/Clear AC

ION

/Turn interrupt on

JMP

/Display is now running.

XPDP,

7000

/Starting address of push down list.

xsu:,

2367

/Enable light pen, push button, and internal stop
/flags, set paper size to 75" by 75" Leave light
.

/pen
/flags.

after light pen hit, and enable all edge

XLBF,

4000

/The display program starts in memory core zero.
/Note the enable bit must be a l to change the
/breal< field register.

XCG,

0016

/Set code size to 6-bit format, start in the lower
/case, and the CG dispatch table starts in location
/l6000 (memory bank l location 6000).

XIN,

DISSTT

/Symbolic address of the first location in the display
/file.

PROGRAMMED BUFFERED DISPLAY 338

Edge, or External Stop Flag

2.3.2.6

RESl 6l74 Resume After Light Pen Hit,

to resume

the sequence of instructions from the point at which it stopped.

This IOT tells the display

In the case of a light pen hit

edge flag, the display completes the vector it stopped on before continuing to the next. One of the

above flags must be up when RESl is given; otherwise, the instruction has no effect.

before the display is reinitialized.

2.3.2.7

2.3.2.8

The contents of the AC have no effect during this instruction.

RES2 6l64 Resume After Stop Code

clears the flag before reSuming.

RESl clears the flag

RES2 restarts the display after an internal stop flag and

—

The AC must be zero before RESZ is given.

CFD élél Clear Display Flags

CFD clears the four flags that stop the display.

is given when the display is not to be used any longer, but has been used in this program.

clear pulse (START key) also clears
IOTs:

CFD-6l6l

This command
The power

All display flags can be cleared by giving three

a_ll display flags.

(internal and external stop, light pen hit, and edge); RSl-6062 (push button); and

SPMI-6l72 (manual interrupt).

2.3.2.9

STPD 6l54 Stop Display (External)

STPD stops the display and sets the external stop flag

(see "Display Flags") when the display has stopped.

This is one of the microprogrammed IOTs and re-

quires bits 0 and 4 of the AC to be 0 when the IOT is given.

IOT Skip on Display Flags

2.3.3

Group 3.

2.3.3.l

SPLP 6l32 Skip on Light Pen Hit Flag

Pertains only to the master scope's light pen.

If the

flag is up, the computer skips one instruction.

2.3.3.2

SPSP 6l42 Skip on Slave Light Pen Hit Flag

the computer skips.

gating the AC

2.3.3.3

If any of the eight slave light pen flags are up,

The particular display can be found by giving the RSGl and RSGZ IOTs and interro-

SPES 6l5l

Skip on External Stop Flag

This is a microprogrammed instruction and requires bits

O and 4 of the AC to be 0 when the instruction is given.

2.3.3.4

SPEF 6l52 Skip on Edge Flag

SPEF causes a computer skip if either the horizontal or vertical

edge has been violated. The edge violated can be found by giving the RSl IOT.
the corner of the paper,—

gore

the horizontal and vertical

edge flags will be up.

If the display runs off

PROGRAMMED BUFFERED DISPLAY 338

2.3.3.5

SPSF 617‘.

Skip on Internal Stop Flag

code and SPSF is given

2.3.3.6

Flag is on.

—

The computer skips if the display has executed a stop

SPMI 6172 Skip on Manual Interrupt

SPMI causes the computer to skip if the manual

It also clears the Flag and the light in the push button if it is up.

interrupt

PROGRAMMED BUFFERED DISPLAY 338

APPENDIXT

VC38 CHARACTER

GENERATOR

The VC38 is a dispatch type generator, with both the dispatch table and the execution routines
stored in core memory.

The 6- or 7-bit character is used to index a special l5-bit register (CHAC) which

contains the starting address of the dispatch table.

The word from the referenced location is then used to

index the CHAC to get to the location of the beginning of the variable length execution routine.

At the

end of the routine, an escape code is given which directs the CG (character generator) to accept the
next character and restart the process.

There are also special dispatch words (control characters) which

do not cause a dispatch but rather are direct commands to the logic

The beginning of the dispatch table is specified by the SAR (starting address register) which is

6-bits long.

The SAR is gated to the upper six bits of the CHAC which in turn is gated onto the MA (mem-

ory address) bus.
core

As an example, if the SAR contains

T68, the dispatch table begins at location 60008 in

memory T. The SAR is set and read by the PDP-8 via IOTs (SCG and RCG respectively).

The characters are interpreted in 6- or 7-bit format depending on the contents of the l-bit
code size register (CHSZ)
to the

If the register is a

l, the low order seven bits of the data word are gated in-

low order seven bits of the CHAC, and the SAR is gated into the CHAC to produce the dispatch

Thus, if the SAR is 048 and the character is ”68, the word in location 4] 168 contains

table address.

the dispatch address.
In the 6-bit format, an identical process takes place except that the leftmost six bits are first

gated onto the CHAC and the right six bits are put in a character save (CS) register. The CS register is
gated onto the low order six bits of the CHAC when the first character is complete. The CB register is
also set to a one, indicating execution of the left character.

Along with the six character bits, the CASE bit is gated into bit 8, allowing reference to 128
characters by a 6-bit character code.

CHAC (Six Bit Code Format)
SAR
0

Set to Zero
4

6 Bit Character Code

Case
8

PROGRAMMED BUFFERED DISPLAY 338

The dispatch word obtained from the location specified by the CHAC is interpreted in the same
way whether the six or seven bit format was used.

In the dispatch word, bit position 0 indicates whether the dispatch table word is a control word

(bit 0=l) or a dispatch word (bit 0:0)
or

Short vector mode.

If it is a dispatch word, the character can be drawn lnel’rherinCrement

Bit 1 in the dispatch word indicates the mode.

in increment mode; if it is
as

If the bit is 0, the data is accepted

l, the data is accepted in short vector mode. The low order ten bits are used

the dispatch address and are gated onto the low order ten bits of the CHAC.

Dispatch Word
Mode
O

Address
2

CHAC During Dispatch
Over-

SAR
0

Dispatch Address

Lap
4

Both the SAR and the dispatch address reference bit 5 in the CHAC.
OR of these two registers.

In other words, if the SAR is odd (i .e., bit 5 is a

The bit is the inclusive

l), the dispatch address can

only reference 5l210 locations, whereas, if the SAR is even, the table can reference l02410 locations.
Al .l

CONTROL CHARACTERS
If the dispatch table word has bit 0=l ,

dispatch does not take place, but rather the word is

accepted in one of three special formats indicated by bits 1 and 2.

After execution of the control char-

acter, the next character is immediately fetched.

Al .l .l

Parameter Control

Parameter control format is specified by 00

in bits l and 2 of the dispatch table word.

Bits

PROGRAMMED BUFFERED DISPLAY 338

Dispatch Table Word (Parameter Control)
Scale
0

Light Pen
5

Intensity
8

l l

Bit(s)

Interpretation

=l Control character.

l ,2

=002 Parameter control

Enables scale change.

4,5

Determine one of Four possible scale settings if bit 3 is a l

Enables light pen change.

Turns the light pen on when it is a

Enables intensity change.

9,l0, ll

Determine one of eight possible intensity settings if bit 8 is a l.

Al .l.2

l, or off when it is 0, provided bit 5 is a l.

Table antrol
The table control character is specified by having

lOl2 in bits 0-2 of the dispatch table word.

This control character has the ability to change the CASE bit and the SAR.

Case

Enable

Bits

6-8

9—l l

SAR (3—5)

SAR (0—2)

PROGRAMMED BUFFERED DISPLAY 338

Bit(s)

Interpretation

=l Control character.

l,2

=0l Table control.

Case bit is set by this bit for referencing, 128 characters using 6-bit code size.

Enable changing SAR bits 0-2 from bits 6-8 of control word.

Enable changing SAR bits 3-5 from bits 9-H of control word.

6—8

Set SAR bits 0-2 if bit 4 is a l.

9-H

Set SAR bits 3-5 if bit 5 is a 1.

Migcellqneogs Control

Al .l .3

The miscellaneous control character is specified by 110

in bits 0-2 of the dispatch table word.

In this format the code size can be changed, the intensity and scale can be counted up and down, and

carriage return (clear low order ten bits of x position register), and escape to control state (end character
string, leave character generator) can be executed.

Dispatch Table Word (Miscellaneous Control)
CHSZ
0

l
l

Cgerllige Escape
5

Count Scale
8

Inf::srilly

Spare

Bit(s)

Interpretation

=l Control character.

l,2

=l0 Miscellaneous control.

Enable change of code size.

Set code size to six bit, if bit is 0; set to seven bit, if bit is a I; if bit 3 is a l.

Clear low order ten bits of x position register (carriage return) if bit is a l

Escape to control state if bit is a l.

Enable count scale logic

count scale up

count scale down

PROGRAMMED BUFFERED DISPLAY 338

Bit(s)

Interpretation
Enable count intensity logic
0:

I:
IT

Count intensity up
Count intensity down

Spare

/Program example of the use of the VC38 Character Generator.
Initial,

CLA

/Display start up

TAD XPDP

/See programming manual

SPDP
CLA

TAD SIC
SIC
CLA

TAD XLBF
LBF

CLA
TAD XCG
SCG
CLA
TAD XIN
INIT

CLA
ION
JMP

XPDP,

7000

XSIC,

2367

XLBF,

4000

XCG,

0006

XIN,

DISSTT

/Beginning of the display File
DISSTT,

POINT EDS CCB CBS

/Enter point mode

1000

/Set point to middle of

5000

/the screen and escape

CHAR EDS

/Enter character mode
43

PROGRAMMED BUFFERED DISPLAY 338

0001

/Call character (00) -set scale and intensity then character
/(Ol) -alpha numeric A.

0203

/Carriage return (02) -line teed (03)

0104

/Alpha numeric A (01) —escape to control state (04)

JUMP

/Display iump to top of display File.

DISSTT
*

6000
4555

/Parameter control:SC2 LPOF, INT5

ALPA—D

/Dispatch in increment mode to alpha A.

6i 00

/Miscellaneous control: carriage return

LIN FD—D+2000

/Dispatch in short vector mode to line feed routine:

6040

/Miscellane0us control:

escape to control state.

/The rest of the dispatch table
*D+200

ALPA,

1672

/Alphanumeric A in

725 l

/Increment mode:

see

programming manual for details.

6057
7674
3762
2701

LINFD,

31 40

[Short vector line teed

/y direction
ALPB,

etc

(910) points in the minus

/The rest at the character routines

PROGRAMMED BUFFERED DISPLAY 338

APPENDIXZ
PROGRAMMING

EXAMPLES

/Interrupt handler, the display is
/placed First, however, a device with critical
/timing should be before the 338, e.g., tape or drum.
CLA

INTER,

SPLP
SKP

/Skip on light pen Flag

JMP LPR

/Jump to light pen routine
/Sl<ip on internal stop flag

SPSF
SKP
JMP SFR
SP MI

/Jump to stop Flag routine
/Skip on manual interrupt

SKP
JMP MIR
SPEF

/Jump to manual interrupt routine
/Sl<ip on edge flag

SKP

JMP EFR
SPES

/Jump to edge Flag routine
/Sl<ip on external stop Flag

SKP

JMP ESR
SPSP

/Jump to external stop routine
/Sl<ip on slave light pen routine

SKP

JMP SLR
RS l
AND (20

/Jump to slave light pen routine
/Read status I
/Test bit 7 For pushbutton flag

SZA CLA

JMP PBR

/Jump to pushbutton routine
/continue checking other I/O equipment

PROGRAMMED BUFFERED DISPLAY 338

/The following display File can be used
/to control the scale, intensity, and
/blink of a picture following the routine.
HEADRI ,

SKI 4O CLAT

/Sl<ip 2 locations if pushbutton 0 is a 0, clear after

SC UP

/test.
/Count the scale up if ploO was i.

0
SKI 20 CLAT

/Nop
/Sl<ip if pbl is 0, clear after test.

SCDN

/Count the scale down if pbl was I.

O
SKI l0 CLAT

/Nop
/Sl<ip if pr is 0, clear after test.

INT UP

/Count intensity up

SKI 4 CLAT

/Nop
/Sl<ip if plo3 is 0, clear after test.

INT DN

/Count intensity down

0
SK] 2 CLAT

/Nop
/Skip it plo4 is 0, clear after test.

BKON

/Turn the blink on.

/Nop
/Sl<ip if pb5 is 0, clear after test.

SKI l CLAT

BKOF

/Turn the blink off.

/Nop

/Place the rest of the display
/File here.

PROGRAMMED BUFFERED DISPLAY 338

/The Following routine is used to
/translate a picture if the picture is
/closed in x-y and contains no point
~/or graphplot mode instructions.
HEADRZ,

SK2 40

SVEC EDS

4l
SK2 20

/Skip if plo6 is off.
/Short vector enter data state.
/Ax=l; shift picture to the right.
/Sl<ip if pb7 is off.

SVEC EDS

/Ax=—l; shift picture to the left.

SK2 l0

/Skip if plo8 is off.

SVEC EDS
140
SK2 4

/Ay=l; shift picture up.
/Sl<ip if pb9 is off.

SVEC EDS

2140
SK2 2 CLAT

CCB CSB
0
SK2 l
SK2 74 CLAT
0
0

STOP

/Ay=-l; shift picture down
/Sl<ip if plolO is off; clear after test.
/Clear coordinate and sector bits.

/Nop
/Skip if ploll is off.
/Clear pb's 6,7,8,9 if
/pbll is on Causes single
/move each time one of the buttons is pushed.
/Stop the display and record the new starting
/ coordinates
.

PROGRAMMED BUFFERED DISPLAY 338

/This routine produces a raster
/with every eighth point on the
/screen intensified. The raster is
/put up if pb5 is on. The y overflow
/must be set at greater than 10 bits.
RAST ,

RAS ,

SK] 1 INV

JUMP

/Sl<ip if pb5 is on
/Jump over the raster

MAST
SC8 LPON INT 6 CCB CSB

/ Routine
/Set beam to lower left corner.

VEC EDS
4001
4177

/INTENSIFY Ay=l
/ESCAPE Ax=l77 (across the screen at scale 8).

POINT EDS
2000
4000

SNSZ
JUMP
RAS

LPOF

YVECI,
XVECI,
MAST ,

VEC EDS CCB CSB
0
4000

JUMP
PICT

/Do not change y coordinate
/ESCAPE Set x to 0.
/Have we run off the top of the screen yet?
/Nol Go back and generate
/Another line of the raster.
/Shut light pen oft.
/This vector
/is changed when picture is
/shifted to get back to virtual starting point.
.

/Continue on

PROGRAMMED BUFFERED DISPLAY 338

/Program to keep box under the
/light pen, assuming startup IOT's
/were given and standard INTERRUPT
/system is some place in core.
CLA

/PDP-8 got to this location through interrupt handler

/given above
LPR,

/Read in contents of l2 pushbuttons.

AND (76

/l_<eep contents of buttons 6-l0 set ll to 0.

TAD (300
LBF

/Add~ enable and bank bit.
/Give the IOT to clear pbll
/IOT's do not clear AC.
/Resume display sequence after light pen hit.
/Turn interrupt system on
/Wait For next pen hit.

CLA
REST
ION
JMP

DISSTT,

RPB

5C2 LPON INT6
VEC EDS CSB
4050

/Ay=40; intensify

4000

/Ax=0; escape

SK2 l INV CLAT COAT

SVEC EDS
61

VEC EDS
4000
4050

/Ay=0,Ax=-l; escape
/Ay=0; intensify
/Ax=40; escape

5K2 l INV CLAT COAT

SVEC EDS
T40

/Ay=l ,Ax=0; escape

VEC EDS

6050

4000

/Ay=—40; intensify
/Ax=0; escape

SK2 l INV CLAT COAT

SVEC EDS
4T

/Ay=0, Ax=l; escape

VEC EDS
4000
6050

/Ay=0; intensify
/Ax= -40; escape

SK2 l INV CLAT COAT

SVEC EDS

2140

/Ay=—l Ax=0; escape
,

JUMP

DISSTT

PROGRAMMED BUFFERED DISPLAY 338

/Progrom works as follows:
/Pushbutton H is sensed ot the end of
/each side. If the button is on the
/next side is drawn, if the button is
/ofF a short invisible vector is executed
/in the proper direction to keep the box
/Under the pen

”(créhgrl §e£i9E5

/
/

1'
light pen 1—9
.

KDirection
V

————————

[:I

old box position

ment

/The button is turned back on
/then the next side is drawn
.

new

box position

PROGRAMMED BUFFERED DISPLAY 338

APPENDIX3

REFERENCE

TABLES

(to be supplied)

DIGITAL EQUIPMENT CORPORATION
PRINTED IN

MAYNARD. MASSACHUSETTS

USA

10 / 66