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DIGITAL-8-28-U

December 1966

10 pages

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Document:	digital-8-28-u-d
Order Number:	DIGITAL-8-28-U
Revision:
Pages:	10
Original Filename:	https://svn.so-much-stuff.com/svn/trunk/pdp8/src/dec/digital-8-28-u/digital-8-28-u-d.pdf

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IDENTIFICATION
Product Code:

DIGITAL-S-2S-U-SYM.

Product Name:

Single Precision Decimal-to-Binary Conversion
and Input ASR 33, Signed or Unsigned

Date Created:

January 14, 1966

Maintainer:

Software Service Group

PDP-B
LIBRARY

DIGIT AL-8-28-U-SYM

ABSTRACT
This routine accepts a string of up to four decimal digits (single precision for the PDP-8)

from the Teletype keyboard and converts it to the corresponding 2's complement binary number.
The string may contain as legal characters a sign (+, -, or space) and the digits from 0 - 9.

If the first legal character is not a sign, the conversion is unsigned. A back arrow (-) at any point in
the string erases the current string and allows the operator to reenter the correct value. Any character
after the first, other than another digit or back arrow causes the conversion to terminate and is found in
location SISAVE within the subroutine.
2.

PRELIMINARY REQUIREMENTS

2.1

Storage
This subroutine requires 74 core locations.

2.2

Equipment
Basic PDP-8 with ASR 33

LOADING OR CALLING PROCEDURE

3.1

Loading
The symbolic tape provided may be assembled with the user's main program by either PAL III

or MACRO-8. The symbolic tape has neither an origin setting nor a terminating "$", but does have a
PAUSE pseudo-instruction at the end.
3.2

Calling Sequence
The subroutine is called by an effective JMS to location SICONV. Return is to the location

immediately following the calling JMS with the binary number in the AC (accumulator).
4.

USING THE ROUTINE

4. 1

Errors in Usage

If a sign (+, -, or space) precedes the string of decimal digits, the maximum decimal number
correctly accepted is 2047 (2 11 _1 ). The sign, if any, must appear first. If a sign does not precede
the string of decimal digits, the maximum decimal number correctly accepted is 4095 (2 12 _1).
4.2

Recovery from Suc h Errors

If either of these maxima is exceeded, the results are unspecified.
5.

RESTRICTIONS

5.1

Status Active Registers
The status of the AC and I ink is not preserved.

5.2

Status Hardware
This subroutine should not be used when the interrupt is on.

5.3

Miscellaneous
The magnitude restrictions on numbers are described in section 4.1 .

DIGIT Al-8-28-U-SYM

DESCRIPTION

6.1

Discussion
This subroutine converts to the binary equivalent a signed or unsigned string of decimal

numbers read from the console keyboard of the PDP-8.
2's complement negative form.

If a minus sign is specified, the results are in

The first character is examined and, if it is a sign (+, -, or space),

a switch is set to provide the correct sign for the conversion. Regardless, a switch is set after the first
character to terminate conversion if a character other than a decimal digit or rub out appears.

If a

back arrow appears at any time, the conversion is reinitialized and the subroutine waits for the correct
entry.
The last four bits of the ASCII code for each of the decimal digits are identical to the
standard 8-4-2-1 BCD code. Thus, the BCD digit is extracted from the 8-bit code by the AND instruction with a "mask" of 178 . When the first BCD digit comes in, it is added to a cleared location
(SJHOlD) in memory and stored back in that location. When the next legal character comes in, location SJHOlD is multiplied by 10, then added to the BCD code of the character and returned to location STORE. This sequence holds true for a decimal number of any arbitrary length.
6.2

Example and/or Application
Since the PDP-8 can add and shift easily, the multiplication by 10 can be accomplished in

three instructions.

Since a shift left is equivalent to a multiplication by 2, a double shift left is

equivalent to a multiplication by 4. Assume that the number currently in STORE is 5, and the new
code just coming in is the number 1 stored in HOLD. The program sequence to perform the multiplication and storage is as follows:

Instruction
Sequence

Comment

Contents of AC

TAD STORE

/load C(STORE) into AC

000 000 000 101

Cll RTL

/Multiply C(STORE) by 4

000 000 a1a 100

TAD STORE

/ Add STORE giving C(STORE) by 5

000 000 all 001

Cll RAl

/Multiply by 2 giving C(STORE) by 10

000 000 110 010

TAD HOLD

/ Add in the next number

000 000 11 a all

DCA STORE

/Store back into STORE and return to wait for
next character

000 000 000 000

ClA

The number residing in location STORE is 00638 or 0051 10 .

DIGIT AL-8-28-U-SYM

If the next number to come in were "9," using the same sequence and conditions, the result
would be 001 000000 111, the binary equivalent of 519.
6.3

Scaling
This subroutine assumes an integral decimal number (signed or unsigned) and yields an integral

binary equivalent (signed or unsigned respectively).
7.

METHOD
The algorithm used is illustrated above (6.2) with details shown in the listing (I 0.1).

FORMAT

8. 1

Input
The input string mayor may not contain a sign (+, -, or space). Any character other than

a sign, 0 - 9, or back arro~ causes the subroutine to terminate, as does a sign in any but the first
position.
8.2

Core Data
The terminating character is found in location SISAVE.

8.3

Output
Spacing, tabulation, carriage return, etc., are not provided for in this subroutine. See

DIGITAL-8-19-U-Sym which contains short subroutines for the latter purposes.
9.

EXECUTION TIME

9.1

Average
This subroutine is input limited at a maximum of 10 hz.

10.

PROGRAM

DIGITAl-8-28-U-SVM
10.1

Program listi ng

ISINGlE PRECISION DECIMAL INPUT FROM KEVBOARD
ICAlLING SEQUENCE: JMS SICONV
IACC IGNORED, RETURN WITH BINARV WORD IN ACC
0200
0201
0202
0203
0204
0205
0206
0207
0210
0211
0212
0213
0214
0215
0216
0217
0220
0221
0222
0223
0224
0225
0226
0227
0230
0231
0232
0233
0234
0235
0236
0237
0240
0241
0242
0243
0244
0245
0246
0247
0250
0251
0252
0253
0254
0255

0000
7300
1274
3232
1274
3224
3310
3311
5257
3307
1307
1301
7450
5201
1302
7510
5232
1303
7740
5232
7300
1231
3224
1245
3232
5246
7300
1307
1304
7450
5274
1305
7450
5274
1306
7650
5273
5264
1310
7106
1310
7004
3310
1307
0300
1310

SICONV, 0
ClA Cll
TAD SISETl + 1
DCA SICTRl
TAD SISETl + 1
DCA SIXSWI
DCA SIHOlD
DCA SINEGI
JMP SINPUT
SIPROC, DCA SISAVE
TAD SISAVE
TAD SIRBUT
SNA
JMP SICONV + 1
TAD SIM260
SPA
JMP SICTRl
TAD SIM271
SMA SZA ClA
JMP SICTRl
SIXSW1, ClA Cll
TAD .+4
DCA .-2
TAD SINMBR -1
DCA SICTRl
JMP SINMBR
SICTRl, ClA Cll
TAD SISAVE
TAD SIMSPC
SNA
JMP SISETl + 1
TAD SIMPlS
SNA
JMP SISETl + 1
TAD SIMMNS
SNA ClA
JMP SISETl
JMP SIEND
SINMBR, TAD SIHOlD
Cll RTL
TAD SIHOlD
RAl
DCA SIHOlD
TAD SISAVE
AND SIMASK
TAD SIHOlD

IINITIALIZE PROGRAM SWITCHES

IClEAR NEGATIVE SWITCH
ISTORE AND THE PROCESS CHARACTER

liS IT A "BACK-ARROW" (IE. ERASE) KEV
IVES, REINITIALIZE
lIS IT lESS THAN 260 (IE. "0")
IVES. TRANSFER TO SEE WHAT CHAR. IT IS
liS IT GREATER THAN 271 (IE. "9")?
IVES, TRANSFER TO SEE WHAT CHARACTER IT
INO, FIRST CHARACTER WAS A DECIMAL DIGIT
IClOSE SWITCH TO GO TO "SINMBR" NEXT
ISET SWITCH TO SENSE TERMINATING CHAR.
ICONTINUE CHECKING

liS IT A SPACE?
IVES, SET SWITCH TO SENSE TERM. CHAR.
liS IT A "PLUS"?
IVES, SET SW TO SENSE TERM. CHAR.
liS IT A MINUS?
IVES, SET NEGATIVE X SWITCH AND TERM SW.
INO, IT WAS A TERMINATING CHAR.
IMUlTIPlV CURRENT ASSEMBLED NUMBER BV 10

IPIC K UP CURRENT DIGIT
IMASK OFF THE HIGH ORDER BITD
I ADD TO ASSEMBLED NUMBER

DIGITAL-S-2S-U-SYM

0256
3310
DCA SIHOlD
0257
6031
SIN PUT , KSF
0260
5257
JMP .-1
0261
6036
KRB
0262
6046
TlS
5211
JMP SIPROC
0263
/TERMINATING ROUTINE
0264
7300
SIEND, CLA Cll
0265
1311
TADSINEGl
0266
7010
RAR
0267
131 0
TAD SIHOlD
SZl
0270
7430
0271
7041
CMA lAC
0272
5600
JMP I SOCONV
0273
2311
SISETl, ISZ SIN EG 1
0274
7300
ClA Cll
0275
1245
TAD SINMBR -1
0276
3232
DCA SICTRl
0277
5257
JMP SINPUT
/CONSTANTS AND VARIABLES
0300
0017
SIMASK, 17
0301
7441
SIRBUT, -337
0302
0057
SIM260, 57
0303
7767
SIM271, -11
0304
7540
SIMSPC, -240
0305
7765
SIMPlS, -13
0306
7776
SIMMNS, -2
0307
0000
SISAVE, 0
0310
0000
SIHOlD, 0
0311
0000
SINEG1, 0

/STORE BAC K IN SIHOlD
/INPUT ROUTINE

/PUT NEGATIVE SWITCH INTO LINK
/IS THE LINK "1"?
/YES, NUMBER NEGATIVE. COMPLEMENT
/RETURN.
/SET NEGATIVE SWITCH
/ClOSE SW TO TRANSFER TO TERM.

/CODE FOR ERASE
/NUMBER USED TO GENERATE CODE "260"
/NUMBER USED TO GENERATE CODE "271"
/CODE FOR SPACE
/NUMBER USED TO GENERATE CODE "253" (+)
/NUMBER USED TO GENERATE CODE "255" (-)
/STORAGE lOCATIONS

DIGITAL-S-2S-U-SYM
11 .

DIAGRAMS

11 . 1

Flow Chart

INITIALIZE a ZERO
ASSEMBLY
LOCATIONS

WAIT FOR
INPUT FROM
KEYBOARD

SET SIGN
INDICATOR
TO NEGATIVE

MULTIPLY PARTIALLY
ASSEMBLED
NUMBER BY 10

DIGITAL-S-2S-U-SYM

12.

REFERENCES

12. 1

Other Li brary Programs

DIGITAL-S-19-U-Sym