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THIS SERIES IS FOR BEGINNERS. YOU WILL NEED AN ASSEMBLER AND A THOROUGH KNOWLEDGE OF APPLESOFT BASIC.

PREFACE:

ASSEMBLY LANGUAGE IS THE FASTEST AND MOST FLEXIBLE OF ALL PROGRAMMING LANGUAGES ON ANY COMPUTER, BECAUSE YOU ARE ACTUALLY TELLING THE COMPUTER WHAT TO DO DIRECTLY, WITHOUT A LOT OF "MIDDLEMEN" LIKE THE APPLESOFT BASIC INTERPRETER. ALSO FOR THIS REASON, ASSEMBLY IS A VERY HARD LANGUAGE TO LEARN. I HOPE THIS TUTORIAL WILL MAKE THE JOB A BIT EASIER.



THE ASSEMBLER:

THIS TUTORIAL WILL CENTER MAINLY ON THE S-C MACRO ASSEMBLER, BUT IF YOU HAVE A DIFFERENT BRAND (MERLIN/BIG MAC, ORCA/M, LISA, ETC.) YOU SHOULD BE ABLE TO FOLLOW ALONG. IT HELPS IF YOU HAVE A MANUAL SO YOU CAN SEE IF THERE ARE ANY DIFFERENCES IN YOUR ASSEMBLER AND THE ONE I USE.

MOST ASSEMBLERS HAVE TWO PARTS I WILL DISCUSS EACH AND THE MAKEUP IN THIS ARTICLE.

PART 1: THE EDITOR

THE EDITOR IS USALLY A TEXT EDITOR FOR CREATING YOUR PROGRAMS IT WILL HAVE FEATURES FOR EDIT, DELETE, INSERT AND IF IT'S GOOD MUCH MORE.

PART 2: THE ASSEMBLER

THIS PART ASSEMBLES THE TEXT INTO MACHINE LANGUAGE CODE.


THE VARIABLES:

JUST AS IN APPLESOFT, ASSEMBLY LANGUAGE HAS VARIABLES. BUT, WHERE IN BASIC YOU HAD THOUSANDS TO PLAY AROUND WITH, HERE YOU ONLY HAVE A FEW AND YOU ARE RESTRICTED IN THEIR USES. WHAT FOLLOWS IS A DESCRIPTION OF EACH VARIABLE AND IT'S FUNCTION.

THE 6502 HAS 5 REGISTERS, WHICH ARE USED SIMILARLY TO VARIABLES IN HIGHER LEVEL PROGRAMMING LANGUAGES.

1. THE ACCUMULATOR ("A-REGISTER")- IS THE MAIN VARIABLE FOR GENERAL FUNCTIONS.

2. THE X REGISTER- USED FOR COUNTING

3. THE Y REGISTER- USED FOR COUNTING

4. THE STACK ("S-REGISTER")- CONTAINS THE STACK POINTERS FOR APPLESOFT AND ASSEMBLY LANGUAGE PROGRAMS

5. THE P REGISTER- KEEPS TRACK OF THE SYSTEM STATUS. IT IS DIVIDED INTO BITS, LIKE THIS:


      7  6  5  4  3  2  1  0    >
      ^  ^  ^  ^  ^  ^  ^  ^    >   ALL OF THIS
                                >   IS CONTAINED IN
                                >   THE "P-REGISTER"
      N  V  *  B  D  I  Z  C    >

N. THE NEGITIVE BIT-0 IF THE LAST COMPARE WAS + 1 IF IT WAS -

V. THE OVERFLOW BIT-0 IF THE LAST RESULT DID NOT OVERFLOW



B. BREAK STATUS-1 AFTER A BRK COMMAND

D. DECIMAL MODE-0 AFTER CLD COMMAND 1 AFTER SED

I. INTERRUPT-0 AFTER CLI 1 AFTER SEI

Z. ZERO-0 IF LAST RESULT WAS NON-ZERO 1 IF IT WAS 0

C. CARRY-0 IF THE LAST RESULT DID NOT CARRY OR A CLC AND 0 AFTER A CARRY OR SEC

IF THIS ALL SOUNDS CONFUSING, YOU'RE ABSOLUTELY RIGHT. THIS WILL BE EXPLAINED MORE THOROUGHLY IN A LATER CHAPTER OF TUTORIAL



THE LINE:

THE LINE IN ASSEMBLY HAS 5 PARTS:

1. THE LINE NUMBER, THIS IS FOR EDITING PURPOSES ONLY AND YOU CAN'T GOTO A LINE BY LINE NUMBER.

2. THE LABEL, THIS IS FOR BRANCHING YOU CAN IN SENSE GOTO A LINE BY IT'S LABEL.

3. INSTRUCTIONS THESE ARE THE COMMANDS AND THERE ARE 56 OF THEM DEVIDED INTO 10 GROUPS:

    1. TRANSFER:LDA, STA, LDX, STX, LDY, STY, TXA, TAX, TYA, TAY, TXS, TSX
                PLA, PHA, PLP, PHP

    2. ARITHMETIC:ADC, SBC, INC, INX, INY, DEC, DEX, DEY

    3. LOGICAL:AND, ORA, EOR, BIT

    4. SHIFT:ASL, LSR, ROL, ROR

    5. COMPARE:CMP, CPX, CPY

    6. STATUS:SEC, SED, SEI, CLC, CLV,, CLD, CLI

    7. BRANCH:BCC, BVC, BNE, BPL, BCS, BVS, BEQ, BMI

    8. JUMP:JMP, JSR

    9. RETURN:RTS, RTI

   10. OTHER:NOP, BRK

YES, YOU MAY HAVE NOTICED THEY ARE ALL 3 LETTERS. THERE ARE ALSO ASSEMBLER COMMANDS THAT ARE DIFFERENT FOR EACH ASSEMBLER. NEXT WEEK I'LL LIST AND EXPLAIN THESE AND THE ASSEMBLER COMMANDS FOR S-C MACRO ASSEMBLER.

4. THE OPCODE: IS THE NUMBER USALLY FOUND AFTER THE COMMAND THERE ARE 4 TYPES:

      1. $00-MEANS WHATS IN THE LOCATION $00
      2. #00-MEANS THE DECIMAL NUMBER 0
      3. #$00-MEANS THE HEX NUMBER $00
      4. $300,Y-MEANS THE VALUE OF THE ADDRESS $300+Y SO $300,Y WHEN Y=2 IS $302'S VALUE

5. THE REM FIELD FOR REMARKS


GET THE IDEA? IT'S EASY!

NEXT TIME: THE COMMANDS



CREATED AND WRITTEN BY:

THE BBB
    B  B
    BBB
    B  B
    BBB  EST APPLE

(OF SADISTIC SOFTWARE AND THE BLACK HOLE BANDITS)

EDITED BY:
 
//   /
/ /  /
/  / /
/   // OWHERE MAN
 
(OF SADISTIC SOFTWARE AND THE BLACK HOLE BANDITS)



ORIGINALLY WRITTEN IN 80 COLUMNS, UPPER CASE ONLY, ON TUESDAY NOVEMBER 20 1984
  THIS MAY BE DISTRIBUTED AND POSTED ON BBS'S *AS LONG AS IT IS NOT CHANGED*