6502 Machine Language

You guys are so lucky. I used to own this book back in the day. Now it is online for free. I think the last publication was 1983.

http://www.atariarchives.org/mlb/

Here is part of the code from a game called microchess:

;***********************************************************************

;

;  MicroChess (c) 1996-2002 Peter Jennings, peterj@benlo.com 

;

;***********************************************************************

; I have been given permission to distribute this program by the

; author and copyright holder, Peter Jennings.  Please get his 

; permission if you wish to re-distribute a modified copy of 

; this file to others.  He specifically requested that his 

; copyright notice be included in the source and binary images.

; Thanks! 

;

; modified by Daryl Rictor to work over a serial terminal connection, August 2002.

;

; 6551 I/O Port Addresses

;

ACIADat	= 	$7F70

ACIASta	=	$7F71

ACIACmd	=	$7F72

ACIACtl	=	$7F73

;

; page zero variables

;

BOARD   =	$50 

BK      =	$60 

PIECE   =	$B0 

SQUARE  =	$B1 

SP2     =	$B2 

SP1     =	$B3 

INCHEK  =	$B4 

STATE   =	$B5 

MOVEN   =	$B6 

REV	=       $B7

OMOVE   =	$DC 

WCAP0   =	$DD 

COUNT   =	$DE 

BCAP2   =	$DE 

WCAP2   =	$DF 

BCAP1   =	$E0 

WCAP1   =	$E1 

BCAP0   =	$E2 

MOB     =	$E3 

MAXC    =	$E4 

CC      =	$E5 

PCAP    =	$E6 

BMOB    =	$E3 

BMAXC   =	$E4 

BMCC    =	$E5 		; was BCC (TASS doesn't like it as a label)

BMAXP   =	$E6 

XMAXC   =	$E8 

WMOB    =	$EB 

WMAXC   =	$EC 

WCC     =	$ED 

WMAXP   =	$EE 

PMOB    =	$EF 

PMAXC   =	$F0 

PCC     =	$F1 

PCP     =	$F2 

OLDKY   =	$F3 

BESTP   =	$FB 

BESTV   =	$FA 

BESTM   =	$F9 

DIS1    =	$FB 

DIS2    =	$FA 

DIS3    =	$F9 

temp    =       $FC

;

;

;

		*=$1000			; load into RAM @ $1000-$15FF


		LDA     #$00		; REVERSE TOGGLE

		STA     REV

                JSR     Init_6551

CHESS		CLD			; INITIALIZE

		LDX	#$FF		; TWO STACKS

		TXS	

		LDX	#$C8

		STX	SP2

;		

;       ROUTINES TO LIGHT LED

;       DISPLAY AND GET KEY

;       FROM KEYBOARD

;		

OUT		JSR	pout		; DISPLAY AND

		JSR	KIN		; GET INPUT   *** my routine waits for a keypress

;		CMP	OLDKY		; KEY IN ACC  *** no need to debounce

;		BEQ	OUT		; (DEBOUNCE)

;		STA	OLDKY

;		

		CMP	#$43		; [C]

		BNE	NOSET		; SET UP

		LDX	#$1F		; BOARD

WHSET		LDA	SETW,X		; FROM

		STA	BOARD,X		; SETW

		DEX	

		BPL	WHSET

		LDX	#$1B		; *ADDED

		STX	OMOVE		; INITS TO $FF

		LDA	#$CC		; Display CCC

		BNE	CLDSP

;		

NOSET		CMP	#$45		; [E]

		BNE	NOREV		; REVERSE

		JSR	REVERSE        	; BOARD IS

		SEC

		LDA	#$01

		SBC	REV

		STA	REV		; TOGGLE REV FLAG

		LDA	#$EE            ; IS

		BNE	CLDSP

;		

NOREV		CMP	#$40			; [P]

		BNE	NOGO           	; PLAY CHESS

		JSR	GO

CLDSP		STA	DIS1          	; DISPLAY

		STA	DIS2         	; ACROSS

		STA	DIS3          	; DISPLAY

		BNE	CHESS

;		

NOGO		CMP	#$0D            ; [Enter]

		BNE	NOMV          	; MOVE MAN

		JSR	MOVE          	; AS ENTERED

		JMP	DISP

NOMV		CMP     #$41		; [Q] ***Added to allow game exit***

		BEQ     DONE		; quit the game, exit back to system.  

		JMP	INPUT		; process move

DONE		JMP     $FF00		; *** MUST set this to YOUR OS starting address

;		

;       THE ROUTINE JANUS DIRECTS THE

;       ANALYSIS BY DETERMINING WHAT

;       SHOULD OCCUR AFTER EACH MOVE

;       GENERATED BY GNM

;		

;		

;

JANUS		LDX	STATE

		BMI	NOCOUNT

;		

;       THIS ROUTINE COUNTS OCCURRENCES

;       IT DEPENDS UPON STATE TO INDEX

;       THE CORRECT COUNTERS

;		

COUNTS		LDA	PIECE

		BEQ	OVER           	; IF STATE=8

		CPX 	#$08           	; DO NOT COUNT

		BNE	OVER          	; BLK MAX CAP

		CMP	BMAXP        	; MOVES FOR

		BEQ	XRT           	; WHITE

; 		

OVER		INC	MOB,X          	; MOBILITY

		CMP 	#$01           	;  + QUEEN

		BNE	NOQ           	; FOR TWO

		INC	MOB,X

;		

NOQ		BVC	NOCAP

		LDY	#$0F           	; CALCULATE

		LDA	SQUARE       	; POINTS

ELOOP		CMP	BK,Y           	; CAPTURED

		BEQ	FOUN          	; BY THIS

		DEY			; MOVE

		BPL	ELOOP

FOUN		LDA	POINTS,Y

		CMP	MAXC,X

		BCC	LESS          	; SAVE IF

		STY	PCAP,X         	; BEST THIS

		STA	MAXC,X         	; STATE

;		

LESS		CLC	

		PHP			; ADD TO

		ADC	CC,X           	; CAPTURE

		STA	CC,X           	; COUNTS

		PLP	

;		

NOCAP		CPX	#$04

		BEQ	ON4

		BMI	TREE          	;(=00 ONLY)

XRT		RTS	

;		

;      GENERATE FURTHER MOVES FOR COUNT

;      AND ANALYSIS	

;		

ON4		LDA	XMAXC        	; SAVE ACTUAL 

		STA	WCAP0         	; CAPTURE

		LDA	#$00           	; STATE=0

		STA	STATE

		JSR	MOVE          	; GENERATE

		JSR	REVERSE       	; IMMEDIATE

		JSR	GNMZ         	; REPLY MOVES  

		JSR	REVERSE

;		

		LDA	#$08       	; STATE=8

		STA	STATE        	; GENERATE

;		JSR	OHM          	; CONTINUATION

		JSR	UMOVE         	; MOVES

;		

		JMP	STRATGY       	; FINAL EVALUATION

NOCOUNT	CPX	#$F9

		BNE	TREE

;		

;      DETERMINE IF THE KING CAN BE

;      TAKEN, USED BY CHKCHK

;		

		LDA	BK           	; IS KING

		CMP	SQUARE       	; IN CHECK?

		BNE	RETJ          	; SET INCHEK=0

		LDA	#$00           	; IF IT IS

		STA	INCHEK

RETJ		RTS	

;		

;      IF A PIECE HAS BEEN CAPTURED BY

;      A TRIAL MOVE, GENERATE REPLIES &

;      EVALUATE THE EXCHANGE GAIN/LOSS

;		

TREE		BVC	RETJ          	; NO CAP

		LDY	#$07           	; (PIECES)

		LDA	SQUARE

LOOPX		CMP	BK,Y

		BEQ	FOUNX

		DEY	

		BEQ	RETJ          	; (KING)

		BPL	LOOPX         	; SAVE

FOUNX		LDA	POINTS,Y       	; BEST CAP

		CMP	BCAP0,X        	; AT THIS

		BCC	NOMAX         	; LEVEL

		STA	BCAP0,X

NOMAX		DEC	STATE

		LDA	#$FB           	; IF STATE=FB

		CMP	STATE        	; TIME TO TURN

		BEQ	UPTREE        	; AROUND

		JSR	GENRM         	; GENERATE FURTHER

UPTREE		INC	STATE        	; CAPTURES

		RTS	

;		

;      THE PLAYER'S MOVE IS INPUT

;		

INPUT		CMP	#$08           	; NOT A LEGAL

		BCS	ERROR      	; SQUARE #

		JSR	DISMV

DISP		LDX	#$1F

SEARCH		LDA	BOARD,X

		CMP	DIS2

		BEQ	HERE          	; DISPLAY

		DEX			; PIECE AT    

		BPL	SEARCH        	; FROM

HERE		STX	DIS1         	; SQUARE

		STX	PIECE

ERROR		JMP	CHESS

;		

;      GENERATE ALL MOVES FOR ONE

;      SIDE, CALL JANUS AFTER EACH

;      ONE FOR NEXT STE?

;		

;

<snip>

;

; end of file

It is scary that I can understand most of that without having touched it for 20 plus years.