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DIGITAL-7-95-U

January 1966

7 pages

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0.2MB

Document:	ExecuteSubr
Order Number:	DIGITAL-7-95-U
Revision:
Pages:	7
Original Filename:	http://bitsavers.org/pdf/dec/pdp7/DIGITAL-7-95-U_ExecuteSubr_Jan66.pdf

OCR Text

IDENTIFICATION

1.1

Di gita 1-7-95-U

1.2

Execute Subroutine

1.3

January 25, 1966

PDP

LIBRARY

Digital-7-95-U
Page 1

ABSTRACT

2.1
The Execute Subroutine is designed to allow a user to execute any sequence of instructions
anywhere in the program, as if it were a subroutine. In essence, it acts as if the XCT instruction were
a I lowed to execute more than just a single instruction anywhere in memory.
The subroutine includes the instructions necessary for initialization, a pseudo command
(XXCT) to allow entrance and exit from the requested subroutines, and enough storage space for pushdown
stacks and nesting to five levels.
The subroutines can be used with either a PDP-4 or PDP-7.

REQUIREMENTS

3. 1

Storage

memories.

The subroutines occupy 105 8 locations. They are not designed to work with extended

3.2

Subprograms (None)

3.3

Equipment
Normal 4K or 8K PDP-4 or PDP-7

USAGE

4.1

The user is supplied with a symbolic tape which can be assembled with the main program.
The tape contains no undefined tags, no origin, and no final START address.
4.2

Ca II i ng Sequence

4.2.1
Before the Execute Subroutine is used for the first time, the user must initialize the system
by issuing the command XCTSET. XCTSET is defined as a JMS to a set of instructions which will see that
the stack pointers are reset and will replace any locations in memory which may have been changed by
any previous use of the subroutines. The latter could occur if the program were stopped and restarted
whi Ie under the control of the Execute Subroutine. The initialization, therefore, attempts to restore
memory to its correct original condition before continuing. Note that if the computer should be stopped
at certain locations within the subroutine itself, it might be necessary to reload the entire program to
restart.
4.2.2

The format for calling the subroutine is as follows:
XXCT
ADDRl
ADDR2

/PSEUDO COMMAND, defined as a JMS to subroutine.
/FIRST LOCATION to be executed
/FIRST LOCATION NOT to be executed.

ADDRl

should be the location of the first instruction of the loop to be executed. Fifteen
bi ts are used for the address.

ADDR2

is the location of the first instruction of the loop not to be executed, or, more simply,
it is the location which when reached causes a return to the main program at XXCT+3.

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Fifteen bits are used for the address. The user must be sure that ADDR2 is one which
will definitely be reached sometime during the execution of the subroutine. The contents of the accumulator and link are not destroyed when the subroutine is entered,
and will contain the correct results of the executed subroutine upon return.
4.2.3
There are no restrictions on actua I instructions being executed as a subroutine, except to
remember that an exit wi II be made the first time the instruction at ADDR2 is reached and before its execution. Execute loops may be nested to a depth of 5. If nesting is desired to any other level, the statement XCTLNG=5 should be changed to indicate the desired amount. Three locations are used for each
leve I of nesti ng.
4.3

Switch Settings (None)

4.4

Start Up

The subroutines can usually be reinitialized by issuing the XCTSET pseudo command. There
may be rare instances when the program should be reloaded in its entirety.

4.5

Errors in Usage
Only one halt can occur within the subroutines:
HALT Location

Meaning

Procedure

XCTTM

Execute loops have been
nested to a leve I greater
than 5.

If legitimate, reassemble the
program changing the XCTLNG=5
statement to the correct length
necessary.

RESTRICTIONS

5.1
The subroutines will not operate correctly if the subroutines themselves or the area to be
executed is in extended memories or if the computer is in extend mode when the subroutines are called.
The XXCT pseudo command should never be issued by a normal PDP-7 execute (XCT)
instruction.

DESCRIPTION

6.1

Discussion

The Execute Subroutine allows the use of any set of instructions anywhere in the program,
as a subroutine. It is most useful where running time and space are not an important consideration and
where, for one reason or another, a set of instructions is not coded as a subroutine. It has been found
very useful in the debugging stage to simply assemble the Execute Subroutine with the main program,
making corrections where necessary and where possible by executing a proper set of instructions anywhere
in the program. After debugging, the proper instructions can be coded as subroutines, if desired, and the
Execute Subroutine can be removed if necessary to save time and space. It has also been found useful
during testing to execute certain portions of a test program without necessari Iy executing those portions
before or after.

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The subroutines operate rough Iy as follows:

1) Save the accumulator.
2) Check to find out whether the nesting limit has been exceeded. If it has, halt;
otherwise continue.
3)

Save subroutine starting location (ADDR 1) .

4) Save the address of the last location (ADDR2) on the last location pushdown
stack (XCTLOC).
5)

Save the contents of the last location on the contents pushdown stack (XCTCON).

6) Place a JMP instruction in the last location whi ch returns to the Execute Subroutine when the subroutine is terminated.
7) Save the address of the return to the main program on the return address pushdown
stack (XCTREn.
8) Index a II stack pointers, restore the accumu lator, and jump to the subrouti ne at
the first location specified.
When the subroutine has been completed, it wi II return to the Execute Subroutine
where the following procedure is followed:
9)

Save the accumu lator.

10) Decrement the last location and contents of last location pushdown stack
pointers (XCTLOC and XCTCON).
11)

Restore the ori gina I contents of the last location.

12)

Decrement the return address pushdown stack pointer.

13) Restore the accumulator, and return to the main program at the location
saved on the return address stack (XCTRET) .
The initialization procedure (XCTSET) operates basically as follows:
14) Check to see if the contents pushdown stack pointer (XCTCON) is at its
base position.
15)

If the stack pointer is at its base position, exit and return to the main program.

16)

If the stack pointer is higher than its base position, repeat steps 10 through 12.

17)

Repeat steps 14 through 16 unti I a II items have been reset.

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6.2

Example

The following is an example of how the Execute Subroutine might be used. The example is
not to be taken as one which is either particularly applicable or efficient I but simply a use of the subroutine.
In the PDP-7 Bidirectional DECtape Subroutines (Digital-7-22A-1/0) the following 35 millisecond delay loop is used before changing unit selections:
MMWAIT,

MMSCH9,

MMRS
/READ DECTAPE STATUS
AND (400)
/SAVE CONTROL TYPE
JMP MMSCH9
/NO DELAY FOR NEW CONTROLS
LAC MMCHK1+1
/PICK UP UNIT NUMBER
SAD MMSEL
/COMPARE TO PREVIOUS SELECTION
JMP MMSCH9
/NO DELAY IF UNIT IS THE SAME
DAC MMSEL
/SAVE NEW UNIT NUMBER
CLA
/CLEAR THE ACCUMU LA TOR
MMSE
/SELECT UNIT 0, i.e., DESELECT
LAM DECIMAL-5000+10CTAL /COUNTER
DAC MMBLF
/TEMPORARY STORAGE AREA
ISZ I. -1
/DE LAY LOOP EQUALS 7 tJSEC
JMP .-1
/DELAY 35 MSEC
LAC MMCHK 1+1
/PICK UP SELECT
ETC.

If the user desired to deselect all DECtapes and delay 35 msec somewhere else in his program the following sequence of instructions could be used:
XXCT
MMSCH9-6
MMSCH9

/FIRST INSTRUCTION TO EXECUTE
/INSTRUCTION TO CAUSE RETURN

Note also that the following set of instructions could be used if the user simply required
a 50 msec de lay:
LAM DECIMAL-7143+1 OCTAL /50000 DIVIDED BY 7 !-,SEC
XXCT
MMSCH9-3
/FIRST INSTRUCTION TO EXECUTE
MMSCH9
/INSTRUCTlON TO CAUSE RETURN
The user should be aware that in these particular examples use of the subroutines themselves
causes an additional delay (see Section 9); however, for purposes of illustration it is being ignored.

METHODS (See Section 6)

FORMAT (Not Applicable)

EXECUTION TIME

The subroutines require 77 cycles or approximately 135 tJsec on a PDP-7 or 616 tJsec on a
PDP-4 to enter and exit any requested subroutine.

Di gital-7-95-U
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10.

PROGRAM

10.4

Program Listing

I~XE~UTE SU8~OUTtN~
I~M~
'~25~6~,
wILL
I~X~~UTES A qEnU~NnE

NOT WORK WITH E~TENnED ~EMnRy.
OF T~STRUCTIONS NOT IN SU8~OUTyNE FORM
1138US oER SU~ROUTINE.
,~5 (OCTAL. L"CATrON~)
IFO~MAT
XXCT
I
ADDR 1
IF 1 RST I NSTRIICT I nN TO BE EXECUTED
I
ADDR?
IFIRST INSTRIICTII")N NOT TO Sr: EXECUT~D

XXCT=.JMS •
t'J

OAC XCTAC
LAW 'l('CTR£T
SAO ~CTCON
~LT

LAC t XXCT-JMS
DAt:'! XCT"WA

r 82 'l('XCT ""Jr'1S
LAC I XXCT .. Ji"1S
DAC r XCTlnC
f'lAC XCrWA?
LAC T XCT .. I,~A2
TJAC

r XCTcnr\l

LAC C \.JMP XXCTRT)
nAr. r XCTWA2
r S 7 XXc T " J t.1 S
LAC XXCT-JMS
nAC T XCTR~T
rSl XCTLOC
ISl ycrCOIIJ

XeT"

1S'- )feTHEr

XCTAC
JMP r XCTwll

L.AC

IRETURN ,rROM SUBROUTINE
XXCTRT,
DAr. XCTAC
LAM
ADD
DAC
'_AM
ADn

-1

verloe
'<crlOC
-1
,<crCON

DAC )(eTCOl\1
l.AC T xCTC'iN

XCT xelLoe
LAM -1
ADn 'lCTRET

DAC
X~T2,

)feTHET

LAC XCTAC
JMP veTHET

IS AvE ACCUMULATOR
IFr,R COMPARISON
IC"MPARE TO CnNTENTs STACK POINTER
IT"O MANY RECURSIvE LEVELS
ISTARTING ADDRESS
ITrMPORARy STnRAG~ AREA
1 I ~'DEX ARGUMENT Po I t\ITER
IFTRST NON-EXECUTED LOCATION
ISAVE ON LAST LOCATION STACK
IT,MPORARy STnRAG~ AREA
ICnNTENTS OF lAST LOCAT I ON
ISAVE nN OONT~NTS STACK
IJMP INSTRUCTION
IRrPLACE OONT,NTS OF LAST LUCATION
IIMDEX ARGUMENT PnINTER
IPTCK uP RETURN AnDRESS
ISAvE ON RETURN AnDRESs STACK
IIMDEx LAST LnCATyON STACK poyNTER
II~DEX CONTENTS STACK POINTER
IINDEX RETURN ADDRESS STACK PoINTER
IREsToR~ AC, LINK UNCHANGEO.
ISTART EFPECTyVE SUBROUTINE

ISAVE ACCUMULAToR
IF"R StJBTRACTTON
ID~CREMENT LAsT AnDREss STACK POtNT~R
IR~STO~E POINTER
IFr!R SllBTRACTyQN
ID~CREMENT CONTENTS STACK POINTER

IRl='sTORE POIf\lTER

IPfCK

IJP ORIGrNAL

IR~STORE

CONT~NTS

CONTENTs UF LAST LCcATtnN
OF LAST LOCATION

IF"R SUSTRACTyON
ID~CREMENT RETURN ADDRESS SlACK porNTER
IR~STORE POINTER
IREsTORE AC, LINK UNCHANGEO
IJMP TO MAIN LOOP.

Digital-7-95-U

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IINtTTAlTZE VCT
TA,RLE
XCTS~T·JMS •

SURROUTI~ES.

WILL

R~SET

LOOPS TF AMY ITEMS REMAIN

lIN

iii

LAW

~CTCON+l

~An XCTCI')N
JMP r .-3
LAC (JMP XCT3)
DAC )(CT2
,J MP X XCT~ T+ 1
LAC XCT1
OAC )(CT2
JMP ~CTSET.l-JMS

)(CTAr.II)(CTS£T . . JMS
xCTLnc,
DAC ,
XCTU'lC+XCTLNr,+1/

.+,

XCTCON,
LAw .+1
XC TcnN+)(cTLNr,+1/
XCTR~T,
JMP I
'l(CT~r:.:T+)(CTLNr,+11

.+'

STA~T

11 .

DIA GRAMS (None)

12.

RE FERENCES (Not Applicable)

IW~RK AREA, XOTAC
IFnR CO~1PARISn~J
IIq POYNTER RESET
IEXTT
IR~TUR~ FROM CLEARING STACK
IRpPLACE LAC TNSTRUCTION
ICIEAR ONE ENTRY IN THE STACK
ILAC INSTRUCTTON
IR~STO~E LAC TNSTRUCTlnN
IC~ECK FOR COMPLETION.