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          INTRODUCTION TO ASSEMBLY LANGUAGE PROGRAMMING

                        CHARLES R. SMITH

REPRINTED FROM: THE MAPLE ORCHARD
                BOX 696, STATION B
                WILLOWDALE, ONTARIO

            WHAT IS ASSEMBLER AND MACHINE LANGUAGE ? 

MOST  OF  US  HAVE  SOME FAMILIARITY WITH BASIC, WITH ITS ENGLISH LIKE
COMMANDS SUCH AS 'PRINT', 'INPUT' AND 'FOR ..  NEXT'. THERE ARE  OTHER
HIGH-LEVEL  LANGUAGES,  LIKE  FORTRAN  AND PASCAL, WHICH SIMILARLY ARE
FORMED OF ENGLISH LIKE COMMANDS. THE APPLE'S CENTRAL  PROCESSING  UNIT
(CPU)  CANNOT  UNDERSTAND  SUCH  ENGLISH COMMANDS, BUT INSTEAD THE CPU
MUST 'INTERPRET' THE COMMANDS OR 'TRANSLATE' THEM INTO A FORM WHICH IT
CAN UNDERSTAND.  WHEN RUNNING A PROGRAM IN BASIC, THE APPLE TAKES EACH
LINE OF THE PROGRAM AND INTERPRETS IT INTO  A  MACHINE  LANGUAGE  FORM
WHICH  FOR  THE CPU. ONCE INTERPRETED, THE CPU THEN EXECUTES THE LINE.
APPLESOFT  IS  THEREFORE  CALLED  A  'BASIC  INTERPRETER'  OR  IS   AN
'INTERPRETED BASIC'. 

THE OPPOSITE OF THE HIGH-LEVEL, STRUCTURED LANGUAGES ARE ASSEMBLER AND
MACHINE LANGUAGE. THE TERMS 'ASSEMBLY LANGUAGE' AND 'MACHINE LANGUAGE'
ARE  OFTEN  USED  INTERCHANGABLY,  ALTHOUGH  THERE  IS  A  DIFFERENCE.
PROGRAMMING IN MACHINE LANGUAGE IS ACCOMPLISHED  WHEN  HEXADECIMAL  OR
BINARY DATA IS LOADED INTO THE COMPUTER BYTE-BY-BYTE (EG. ENTERING THE
MONITOR  BY  CALL -151 AND INPUTTING '300: A9 0A ...'). WHEN COMPUTERS
WERE IN THEIR INFANCY, THIS WAS THE ONLY OPTION AVAILABLE.  THIS  FORM
OF  PROGRAMMING  CAN  ONLY  BE INTELLIGIBLE TO PEOPLE WHO THINK LIKE A
COMPUTER. TO WE LESSER MORTALS, WRITING A PROGRAM IN MACHINE  LANGUAGE
IS  THE  ULTIMATE  EXERCISE  IN  FRUSTRATION.  (AN  ASIDE:  WHEN STEVE
WOZNIAK AND STEVE JOBS FIRST BUILT AN APPLE I COMPUTER, THEY  TOOK  IT
TO PARTIES AND ENTERED HUNDREDS OF HEX CODES BY HAND JUST TO LOAD IN A
PROGRAM  TO DEMONSTRATE THIS FIRST APPLE'S ABILITY. ALLEGEDLY, DOS WAS
FIRST WRITTEN THIS WAY!)

ENTER ASSEMBLERS. AN ASSEMBLER IS SIMPLY A TOOL WHICH ALLOWS A  PERSON
TO WRITE A PROGRAM USING SOMEWHAT MORE INTELLIGIBLE COMMANDS, AND THEN
THE ASSEMBLER PROGRAM CONVERTS ("ASSEMBLES") THE ENGLISH-LIKE COMMANDS
INTO THE CORRESPONDING MACHINE-LANGUAGE CODE. 

WHILE   THERE   IS  A  DEFINITE  AND  SIGNIFICANT  DIFFERENCE  BETWEEN
'ASSEMBLY' LANGUAGE AND 'MACHINE' LANGUAGE, THE TWO  TERMS  ARE  OFTEN
USED  INTERCHANGABLY.  THROUGHOUT THIS ARTICLE, SUCH INTERCHANGABILITY
SHALL BE USED. 


WHY PROGRAM IN MACHINE LANGUAGE ? 

IF ASSEMBLY LANGUAGE IS TOUGHER THAN BASIC, WHY BOTHER BEING A GLUTTON
FOR PUNISHMENT? IS THERE SOMETHING TO  BE  SAID  FOR  SUCH  AN  ARCANE
LANGUAGE?  CONSIDER THESE THREE REASONS: SPEED, FLEXIBILITY AND MEMORY
MANAGEMENT. 

THE MOST PROMINENT BENEFIT OF A MACHINE LANGUAGE PROGRAM CAN  BE  SEEN
ANYTIME  YOU  PLAY  A  COMMERCIALLY-PRODUCED,  ARCADE STYLE GAME. SUCH
GAMES  ARE  INVARIABLY  WRITTEN  ON  AN  ASSEMBLER  (THE   PEOPLE   AT
SOUTHWESTERN  DATA  SYSTEMS  TELL  ME  THAT  THE GAMES FROM SIRIUS ARE
WRITTEN ON MERLIN). THE EXECUTION SPEED OF A MACHINE LANGUAGE  PROGRAM
IS MUCH FASTER THAN A CORRESPONDING BASIC PROGRAM ARCADE QUALITY GAMES
SIMPLY CANNOT BE WRITTEN IN BASIC.  EVEN BASIC WHICH HAS BEEN COMPILED



INTO  MACHINE  LANGUAGE  (BY  APPLESOFT  COMPILERS  SUCH  AS  TASC  OR
SPEEDSTAR) IS STILL SLOWER AND MORE MEMORY-HUNGRY THAN  AN  EQUIVALENT
PROGRAM  WRITTEN  IN  ASSEMBLER. THE ADVANTAGE OF SPEED CAN BE USED IN
OTHER  SITUATIONS  WHICH  WILL  SIGNIFICANTLY  ENHANCE   THE   APPLE'S
CAPABILITY.  SORTING,  MATRIX  OPERATIONS AND DISK UTILITIES ARE BUT A
FEW REASONS TO USE THE SPEED OF MACHINE LANGUAGE. 

THE SECOND REASON TO PROGRAM IN  MACHINE  LANGUAGE  IS  THE  INCREASED
FLEXIBILITY WHICH IT OFFERS. THE COMMANDS IN APPLESOFT CAN BE ENHANCED
BY  THE  USE OF SHORT MACHINE LANGUAGE ROUTINES HIDDEN AWAY IN MEMORY.
PRINT FORMATTING FUNCTIONS, SOUND GENERATION, SORTING, STRING HANDLING
AND SWAPPING, DECIMAL  TO  HEX  CONVERSIONS,  GARBAGE  COLLECTION  AND
MEMORY  CLEARANCE,  AND  ARRAY  OPERATIONS  ARE  BUT  SOME OF THE MORE
COMMONLY USED SUBROUTINES.  WHILE SUCH MACHINE  LANGUAGE  ENHANCEMENTS
ARE  AVAILABLE  COMMERCIALLY, THEY CAN ALSO BE HAD AT A VERY LOW COST,
AS THEY ARE LISTED IN SUCH PUBLICATIONS AS NIBBLE AND CALL-APPLE. 

THE THIRD ADVANTAGE RELATES TO  MEMORY  MANAGEMENT.  MACHINE  LANGUAGE
PROGRAMS  ARE  MORE  EFFICIENT IN THEIR COMSUMPTION OF MEMORY THAN ARE
THEIR BASIC COUNTERPARTS. BECAUSE OF THIS,  IT  IS  POSSIBLE  TO  HAVE
SEVERAL  MACHINE  LANGUAGE  ROUTINES  RESIDING IN MEMORY CUNCURRENTLY,
THEREBY  ENHANCING  APPLESOFT  WHILE  AT  THE  SAME  TIME  AVOIDING  A
SIGNIFICANT REDUCTION IN THE MEMORY AVAILABLE TO THE USER. 


WHAT IS NEEDED TO GET STARTED

TO  BEGIN  PROGRAMMING  IN  ASSEMBLER/  MACHINE  LANGUAGE,  IT  IS NOT
NECESSARY TO PURCHASE ANY SOFTWARE TOOLS. A GOOD BOOK, HOWEVER, CAN BE
INVALUABLE. ONCE YOUR INTEREST IS SPARKED, YOU MAY  DEVELOP  A  SUDDEN
URGE  TO  PURCHASE  AN ASSEMBLER. OTHER MACHINE LANGUAGE UTILITIES ARE
HELPFUL. 

THERE ARE THREE DIFFERENT LEVELS OF SOPHISTICATION AT  WHICH  YOU  MAY
ENTER MACHINE LANGUAGE PROGRAMMING: 

  1) MACHINE LANGUAGE USING THE MONITOR
  2) ASSEMBLY LANGUAGE USING THE MINI-ASSEMBLER
  3) ASSEMBLY LANGUAGE USING AN ASSEMBLER. 

LET'S LOOK AT EACH OF THESE IN ORDER. 


MACHINE LANGUAGE USING THE MONITOR

ALTHOUGH  NOT  THE MOST FLEXIBLE APPROACH TO MACHINE LANGUAGE, THIS IS
THE CHEAPEST ROUTE TO GO, AND IT FORMS A GOOD STARTING PLACE. THE  WAY
TO BEGIN IS TO EXIT APPLESOFT AND GET INTO THE MONITOR BY MEANS OF THE
COMMAND: 

   ]CALL -151

WHICH YIELDS THE '*' PROMPT. 


FROM  HERE  WE  CAN ENTER A PROGRAM BY GIVING THE STARTING ADDRESS AND
THE HEXADECIMAL DATA. TRY THE FOLLOWING: 

*300:AD 30 C0 88 D0 FD 4C 00 30 <RET>

WHERE YOU ENTERED THE NUMBER 300, A COLON, THEN 9 SETS OF  HEXADECIMAL
DATA.  BE SURE THAT YOU DO NOT LEAVE A SPACE BETWEEN THE COLON AND THE
FIRST NUMBER. AT THE END OF THE LINE, ENTER A <CARRIAGE RETURN>. UP TO



85 HEXADECIMAL NUMBERS CAN BE ENTERED AT ONCE THIS WAY. 

WHAT THIS PROGRAM DOES IS TOGGLE THE SPEAKER BY ADDRESSING ITS  MEMORY
LOCATIO  (AD  30  C0), COUNTING DOWNWARDS FROM 255 TO 0 (88 D0 FD) AND
THEN JUMPING BACK TO THE START (4C 00 30). IN OTHER WORDS, THIS  SHORT
ROUTINE  KEEPS  A  CONSTANT  TONE  GOING  ON THE SPEAKER UNTIL YOU HIT
<RESET> TO GET OUT OF IT. TO START THIS CPROGRAM ONCE YOU  HAVE  TYPED
IN  THE  ABOVE LINE, SIMPLY GO TO LOCATION $300 AND DO WHAT'S THERE BY
THE COMMAND: 

   *300G <RET>

YOU HAVE JUST  ENTERED  YOUR  FIRST  MACHINE  LANGUAGE  PROGRAM  (THIS
PROGRAM EXAMPLE IS COURTESY OF SC MACRO ASSEMBLER). 

NOW GO ON TO GREATER THINGS. GRAB AN ASSEMBLY/MACHINE LANGUAGE LISTING
AND  ENTER  IT  INTO  YOUR  COMPUTER.  IF YOU ARE ENTERING AN ASSEMBLY
LISTING, YOU WILL SEE THE HEX CODE  IN  THE  LEFT  HAND  COLUMNS.  FOR
EXAMPLE, THE ABOVE PROGRAM LOOKS LIKE: 

0300  -  AD 30 C0 1 TONE LDA $C030 0303 - 88 2 LOOP DEY 0304 - D0 FD 3
BNE LOOP 0306 - 4C 00 03 4 JMP TONE

REMEMBER TO SAVE YOUR WORK ON  DISK.  THIS  IS  DONE  WITH  THE  BSAVE
COMMAND.  IT  IS  NECESSARY TO SPECIFY THE STARTING ADDRESS AND LENGTH
PARAMETERS USING THE 'A$___' AND 'L$___' COMMANDS. IF A  PROGRAM  WENT
FROM  0300  TO  0320, IT WOULD BE (HEX) 20 OR (DECIMAL) 32 BYTES LONG.
THEREFORE IT WOULD BE SAVED BY THE COMMAND: 

*BSAVE MYPROGRAM,A$300,L$20

WHERE A$ IS THE STARTING ADDRESS IN HEXADECIMAL AND L$ IS  THE  LENGTH
IN HEX. IT COULD ALSO BE SAVED BY THE COMMAND: 

*BSAVE MYPROGRAM,A768,L32

WHERE  A IS THE STARTING ADDRESS IN DECIMAL (768=300 HEX) AND L IS THE
LENGTH IN BYTES. 

TO CHECK A PROGRAM LISTING, ENTER THE START AND END ADDRESS, SEPERATED
BY A PERIOD. FOR THE SHORT TONE PROGRAM ABOVE, ENTER: 

     *300.308 <RET>

IF YOU WANT TO SEE THE DISASSEMBLED CODE, ENTER THE  STARTING  ADDRESS
AND "L" TO LIST 20 COMMANDS. TO SEE THIS PROGRAM, ENTER: 

     *300L <RET>

(TO MAKE THE ENTRY AND CORRECTION OF MACHINE LANGUAGE PROGRAMS, ONE OF
THE DISKS FROM THE INTERNATIONAL APPLE CORE (DISK 25, MACHINE LANGUAGE
UTILITIES) CONTAINS A UTILITY TO FACILITATE THIS PROCESS. IT IS CALLED
BEU  (FOR BINARY ENTRY UTILITY) AND WILL LOCATE ITSELF EITHER ON A 16K
RAM CARD OR IN THE APPLE'S 48K RAM.)

PROGRAMMING WITH THE MINI-ASSEMBLER

IN THE ORIGINAL APPLE II MACHINE WAS A MONITOR FUNCTION  ENTITLED  THE
"MINI-ASSEMBLER". ITS JOB WAS TO FACILITATE ENTRY OF MACHINE CODE INTO
THE COMPUTER. WHEN APPLESOFT WAS DEVELOPED FOR THE APPLE II PLUS, THIS
FUNCTION  (ALONG WITH STEP AND TRACE IN THE MONITOR ROM) WERE DELETED.
THAT IS THE BAD NEWS. NOW FOR THE GOOD NEWS. IF YOU  HAVE  A  16K  RAM
CARD,  LOADING INTEGER BASIC UPON BOOTING UP THE DOS 3.3 SYSTEM MASTER



DISK WILL LOAD THE MINI-ASSEMBLER  INTO  YOUR  MACHINE.  THIS  CAN  BE
ACCESSED BY ENTERING INTEGER WITH THE COMMAND: 

     ]INT

WHICH YIELDS THE '>' PROMPT. THEN ENTER: 

     >CALL -151

TO  GET THE MONITOR '*' PROMPT. THEN GO TO THE MINI-ASSEMBLER WITH THE
COMMAND: 

*F666G

WHICH YIELDS THE EXCLAMATION (!) PROMPT. 

THE MINI-ASSEMBLER IS STILL AVAILABLE TO THE OWNER OF A 48K  APPLE  ][
PLUS,  AS  IT  HAS  BEEN  MODIFIED  AND  INCLUDED ON THE IAC DISK #25.
INSTRUCTIONS FOR ITS USE ARE GIVEN IN AN ACCOMPANYING FILE, WHILE MORE
COMPLETE INSTRUCTIONS ARE GIVEN IN  THE  APPLE  ][  REFERENCE  MANUAL.
APPLE //E OWNERS CAN ACCESS THE MINI-ASSEMBLER THROUGH INTEGER BASIC. 

3) ASSEMBLER

ONCE  YOU'VE GOTTEN YOUR FEET WET BY GAINING ACCESS TO THE MONITOR AND
ENTERING A HEX LISTING, YOU WILL REALIZE THE FACT THAT THERE HAS TO BE
A BETTER WAY. OBTAINING AN ASSEMBLER IS THAT WAY. THERE ARE  AT  LEAST
TEN  COMMERCIALLY  AVAILABLE EDITOR/ASSEMBLERS, AND THE PERFECT ONE IS
PROBABLY STILL TO BE WRITTEN. BUYING AN ASSEMBLER  IS  LIKE  OBTAINING
ANY  OTHER  SOFTWARE,  AS  THE  AMOUNT  OF  MONEY  TO BE SPENT AND THE
CONFIGURATION OF YOUR MACHINE WILL BE  CONTROLLING  FACTORS.   SEVERAL
ASSEMBLERS  ARE  REVIEWED  IN  THIS  ISSUE.  A PUBLIC DOMAIN ASSEMBLER
CALLED MASM IS INCLUDED IN THE IAC'S SPECIAL MACHINE LANGUAGE  UTILITY
DISK. 

OTHER AIDS

OTHER  TYPES  OF AIDS ARE ALSO WORTHY OF CONSIDERATION. A DISASSEMBLER
IS A HANDY TOOL  WHICH  WILL  PERMIT  YOU  TO  TAKE  MACHINE  LANGUAGE
(HEXADECIMAL) CODE AND RE-CONVERT IT INTO AN ASSEMBLER SOURCE LISTING.
FROM  THERE,  YOU  CAN EDIT, ALTER AND RE-ASSEMBLE THE CODE TO MAKE IT
FUNCTION IN A  DIFFERENT  MANNER  OR  AT  A  NEW  MEMORY  LOCATION.  A
DISASSEMBLER  IS  AVAILABLE  IN  THE  APPLE'S  MONITOR  AND IS ABLE TO
DISASSEMBLE TO THE SCREEN, USING THE

*XXXX L <RETURN>

COMMAND,  WHERE  XXXX  IS  THE  MEMORY  LOCATION  AT  WHICH  TO  BEGIN
DISASSEMBLY.  A  PUBLIC DOMAIN DISASSEMBLER IS INCLUDED ON THE MACHINE
LANGUAGE DISK, AND SOME VERY GOOD ASSEMBLERS ARE  INCLUDED  WITH  SOME
ASSEMBLER  AS  PART  OF THE PACKAGE (EG. MERLIN AND LISA).  OTHERS ARE
AVAILABLE AS A STAND-ALONE PRODUCT (SUCH AS DISASM FROM RAK-WARE). 

DEBUGGERS ARE FOR THOSE WHO ARE VERY SERIOUS - THEY  PERMIT  STEP  AND
TRACE  FUNCTIONS  AND A HOST OF OTHER ADVANCED FEATURES.  TWO EXAMPLES
OF COMMERCIAL PRODUCTS ARE BUG  BYTER  AND  MUNCH-A-BUG.  BEU  ON  THE
MACHINE LANGUAGE DISK ALSO CONTAINS DEBUG ROUTINES. 

THERE  IS  EVEN  A  NEW  PRODUCT  WHICH  WILL TURN YOUR MACHINE INTO A
'VISIBLE' APPLE AS IT SHOWS YOU ON THE HI-RES SCREEN  HOW  YOUR  APPLE
WORKS,  DISPLAYING  THE  INSTRUCTIONS,  REGISTERS,  FLAGS  AND  MEMORY
LOCATIONS AS IT EXECUTES A PROGRAM ONE STEP AT A TIME. 




AND WHERE WOULD WE BE WITHOUT BOOKS? A VARIETY ARE AVAILABLE, EACH ONE
FOR A SLIGHTLY DIFFERENT READER. THERE ARE SOME GENERAL REFERENCES  ON
PROGRAMMING  THE 6502 MICROPROCESSORS, SEVERAL GOOD BOOKS SPECIFICALLY
WRITTEN FOR APPLE 6502 MACHINE LANGUAGE (REVIEWED IN THIS ISSUE),  AND
BOOKS  WHICH  SERVE  AS  A  ROAD-MAP OR ATLAS TO THE APPLE'S INTERNALS
(ALSO REVIEWED HERE).  MUCH IS WRITTEN IN THE MONTHLY  MAGAZINES,  AND
THERE  IS  EVEN  A MONTHLY PUBLICATION ON MACHINE LANGUAGE PROGAMMING.
BUT THE BEST STARTING PLACE IS THE APPLE ][  REFERENCE  MANUAL,  WHICH
LISTS  THE  OPCODES  AVAILABLE ON THE 6502 AND SOME IMPORTANT INTERNAL
ROUTINES. 

WHEN ALL IS SAID AND DONE, A PERSON WHO KNOWS A FEW BASIC COMMANDS CAN
EASILY FIND THEIR WAY AROUND MACHINE LANGUAGE.  THERE IS A CERTAIN JOY
IN ENTERING,  DEBUGGING  AND  RUNNING  A  PROGRAM,  BE  IT  IN  BASIC,
ASSEMBLER  OR  WHATEVER. AND ISN'T THAT JOY A BIG REASON FOR OWNING AN
APPLE? 


















































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