6502

Introduction

Wanting to start sizecoding on a 6502 platform in this day and age can be tough.

So here is a bit of help to get you started:

The 6502 processor

The 6502 processor can be seen as the 8-bit micro ARM chip. It has only has 3 registers (Accumulator, X and Y registers) and a handful of instructions to work with.

Adressing modes

To be added.

Zero page

When using the 6502 for sizecoding, you'll mostly be working from zeropage

General 6502 Resources

6502.org http://www.6502.org/
6502 instruction reference http://www.6502.org/tutorials/6502opcodes.html
6502 books http://retro.hansotten.nl/6502-sbc/
6502 Assembler tutorial https://dwheeler.com/6502/oneelkruns/asm1step.html
Easy 6502 code tester https://skilldrick.github.io/easy6502/
Synthetic instructions https://wiki.nesdev.com/w/index.php/Synthetic_instructions#8-bit_rotate

Atari 8-bit family

The systems in this family are: Atari 400, 800, 5200, 1200XL, 600XL, 800XL, 130XE, 65XE, 800XE and XEGS.

The Atari 8-bit systems consists of the 6502 with custom hardware for graphics and sound.

Setting up

Setting up your development platform for the Atari 8bit systems is quite easy, first get the following tools:

Assembler: MADS Assembler - This assembler has nice macros for creating Binaries and SNA snapshot files out of the box. You can download it at https://mads.atari8.info/
Emulator(s): I Found Altirra to work best for my usecase. Make sure to use the original Rev2 rom for best compatibility.

Special Memory Adresses

FRAMECOUNTER_HIGH = 19
FRAMECOUNTER_LOW = 20

Video display

Video display on the Atari 8bit systems use the ANTIC and GTIA chips. Information can be found here:

Sync with frame

RTCLOK      equ $0012
      lda RTCLOK+2
waits
      cmp RTCLOK+2
      beq waits

Or if you don't mind trashing RTCLOK

RTCLOK      equ $0012
waits
      lsr RTCLOK+2
      bcc waits

Which is two bytes shorter.

Getting something on screen

;fill screen with charset,(88,89)=an address
 org $600; free 6th page:600-6ff
 ldy #0
fl: tya
 sta(88),y
 iny
 bne fl
 jmp *

To be added soon.

SDMCTL	= $022f
HPOSP0  = $d000
SIZEP0  = $d008
GRAFP0  = $d00d
COLPM0  = $d012

FRAMECOUNTER_HIGH = 19
FRAMECOUNTER = 20
WSYNC	= $d40a
VCOUNT	= $d40b

sinewave	= $0600		; to $06ff

		org $80

main	
	; disable all graphics/colors
	ldx #0
	stx SDMCTL	

	ldy #$7f
	sty SIZEP0	; size p0=127
		
	ldx #0
	ldy #$3f
make_sine:
value_lo
			lda #0
			clc
delta_lo
			adc #0
			sta value_lo+1
value_hi
			lda #0
delta_hi
			adc #0
			sta value_hi+1
 
			sta sinewave+$c0,x
			sta sinewave+$80,y
			eor #$7f
			sta sinewave+$40,x
			sta sinewave+$00,y
 
			lda delta_lo+1
			adc #8
			sta delta_lo+1
			bcc nothing
			inc delta_hi+1
nothing
			inx
			dey
			bpl make_sine

updateloop:
		; vblank
		lda VCOUNT
		bne updateloop

		; clear graphics
		sta HPOSP0
		sta GRAFP0

		ldy #0
		lda #47
		sta COLPM0
yloop:
		tya           ; graphics shape = y
		sta WSYNC
		sta GRAFP0

		; a = sin(frame+y)+48
		tya	
		adc FRAMECOUNTER
		tax
		lda sinewave,x
		adc #48
		sta HPOSP0
                
                iny
                bne yloop
		jmp updateloop

		run main

Sound

The Atari 8-bit use the POKEY chip to generate sound.

https://en.wikipedia.org/wiki/POKEY

BASIC

; from Analog, 1985-09,pp.25-32
*=$2000
AUDF1=$D200
AUDC1=$D201
AUDCTL=$D208
SKCTL=$D20F

.MACRO SOUND ; voice,pitch,dist,vol;,dur
	lda #%2
	sta AUDF1+2*%1
    lda #[[%3 * 16] | %4] ;lda #[[%3 shl 4] or %4]
    sta AUDC1+2*%1
.ENDM

 lda #0
 sta AUDCTL
 lda #3
 sta SKCTL

 SOUND 0,121,10,8

 jmp *

Make some noise

To be added soon.

Additional Resources

Sizecoding resource for the Atari 8bit are:

Mapping the Atari https://www.atariarchives.org/mapping/
Atari 8bit Memory map https://www.atariarchives.org/mapping/memorymap.php
Fready's undocumented 6502 opcodes https://github.com/FreddyOffenga/6502
Atari OS Rev2 disassembly for MADS assembler https://github.com/ilmenit/A800-OS-XL-Rev2
Fready's github https://github.com/FreddyOffenga/

Commodore 64

The Commodore system consists of a 6502-compatible MOS 6510 processor with custom hardware for graphics and sound.

Setting up

Setting up your development platform for the Commodore systems is quite easy, first get the following tools:

Assembler: To be added
Emulator(s): VICE is the way to go

Memory map

    0-1          $0-$1      PROCESSOR PORT REGISTERS, not usable as RAM
   0-255         $0-$FF     ZEROPAGE, mostly used by BASIC interpreter and Kernal
  256-511      $100-$1FF    SYSTEM STACK
  512-600      $200-$258    INPUT BUFFER
  601-630      $259-$276    FILE SYSTEM DATA
  631-640      $277-$280    KEYBOARD BUFFER, filled by Kernal ISR upon key presses 
  641-678      $281-$2A6    VARIOUS KERNAL VARIABLES
  679-767      $2A7-$2FF    Unused (89 byte)
  768-779      $300-$30B    KERNAL VECTORS
  780-783      $30C-$30F    REGISTER VALUES A,X,Y,SR for SYS
  784-819      $310-$333    KERNAL VECTORS
  820-827      $334-$33B    Unused (8 byte)
  828-1019     $33C-$3FB    DATASETTE BUFFER
 1020-1023     $3FC-$3FF    Unused (4 byte)
 1024-2023     $400-$7E7    SCREEN MEMORY (25 lines times 40 columns)
 2024-2039     $7E8-$7F7    Unused (16 byte)
 2040-2047     $7F8-$7FF    SPRITE COSTUME POINTERS
 2048          $800         must contain a value of 0 so that the BASIC program can be RUN
 2049-40959    $801-$9FFF   BASIC AREA (code, variables, arrays, strings)
40960-49151   $A000-$BFFF   BASIC ROM (unused RAM underneath)
49152-53247   $C000-$CFFF   UPPER RAM AREA (unused)
53248-53294   $D000-$D02E   VIC-II, memory-mapped graphics chip registers)
54272-54300   $D400-$D41C   SID, memory-mapped sound chip registers)
55296-56295   $D800-$DBE7   Color RAM, only lower 4 bits are stored, higher nibble undefined
56296-56319   $DBE8-$DBFF   Unused (24 half-bytes)
56320-56335   $DC00-$DC0F   CIA#1, memory-mapped I/O chip registers for keyboard, joystick, mouse, datasette, IRQ control
56576-56591   $DD00-$DD0F   CIA#1, memory-mapped I/O chip registers for serial bus, RS232, NMI control
57344-65535   $E000-$FFFF   KERNAL ROM
65534-65535   $FFFE-$FFFF   Execution address of interrupt service routine, can only be set if KERNAL is turned off

For a detailed explanation of the C64 memory map check out Commodore 64 memory map on Joe Forster/STA homepage.

Program file overhead

A standard CBM file consists of a 2 byte loading address followed by the data that is placed into memory starting at this address. In addition, if your machine program should be able to be started with "RUN", you need a BASIC stub that is loaded into memory from $0801 to $080c, typically followed by your machine program:

*=$0801
        .word $080b   ;address of next BASIC line
        .word LINENR  ;line number, can be chosen between 0 and 63999
        .byte $9e     ;token for SYS command
        .byte $32,$30,$36,$31  ;address to jump to in ASCII code: "2061" 
        .byte $00     ;end of BASIC line
        .word $0000   ;address of next BASIC line, $0000 means end of BASIC program  
start
; your code here

Together with the 2 byte load address, this makes an overhead of 14 byte for your program. To reduce this, you can

omit the BASIC line and tell users to start your program by directly entering "SYS address"
save your program from $0802 on, the byte at $0801 is recovered automatically after loading by the relink function. This reduces your program file size by 1 byte
accept the loss and use the 2 byte establishing LINENR for something good, they are copied to zeropage addresses $39/$3a automatically, saving you the effort of initializing the addresses
use an autostart method (see below)

Some compos also specify that the BASIC loader does not count towards the overall filesize, check the rules before you worry.

Autostart

Unlike the Commodore 128, the Commodore 64 has no dedicated built-in auto-boot feature, but it is possible to autorun a program after loading by specifying the load address so that it overwrites a vector or address that is called by the operating system. There are multiple possibilities:

Overwriting CHROUT vector at $326-$327

The vector at address $326-$327 points at the CHROUT routine, normally at address $F1CA. If you put the entry address of your program into $326/27, your code will be called when the operating system wants to print "READY." after loading. Note that the vector at address $328-$329 points at the STOP routine that checks the status of Stop key indicator. Since this routine is called multiple times before your code is done loading, its default value $F6ED needs to be preserved.

Application example:

*=$0326
        .word start              
        .byte $ed,$f6
start
; rest of code

Together with the two byte load address, the overwritten CHROUT vector and the preserved STOP vector, your program file will have a deadweight of 6 byte: $26 $03 $0a $03 $ed $f6, less than half of the standard version with a BASIC stub with SYS

Caveat:

If your program after $32a is longer than 214 byte, part of your program will load into the screen at $400. Thus, issuing a clear screen will overwrite your program. It is possible to move the screen memory somewhere else, but that requires storing a new value into address $D018, which when done with an LDA # / STA pair will cost you 5 byte
Since you changed the CHROUT pointer, standard KERNAL functions for printing won't work. For output of single characters, you can still use the direct address $F1CA. Alternatively, you can restore the vectors by calling $FD15, which will overwrite the memory $314 to $333 with the default values.

Video display

Video display on the Commodore, it has the following video modes:

To be added soon.

Getting something on screen

Output a character:

        lda     #C       ;C must be the char code according to the PETSCII code table
        jsr     $FFD2    ;you can also use the direct address $FC1A, function preserves A,X,Y and enables interrupt

Since the PETSCII code table contains also control characters, this method can be also used to clear the screen, move the cursor or set the text color. However, some functions are shorter (and faster) by directly calling the respective KERNAL function:

Clear screen             JSR $E544
Cursor home position     JSR $E566
Output newline character JSR $AAD7
Set text color           LDA #COLOR, STA $286
Set border color         LDA #COLOR, STA $D020   ;not possible with a PETSCII control code
Set background color     LDA #COLOR, STA $D021   ;not possible with a PETSCII control code

Copy characters directly to screen memory:

.macpack cbm   ;scrcode macro
SCREEN_BASE=$400
LINE=5    ;for example
COLUMN=10 ;for example

        ldx     #endtext-text-1
@loop:  lda     text,x
        sta     SCREEN_BASE+LINE*40+COLUMN,x
        dex
        bpl     @loop
text:   scrcode "hello world!"
endtext:

Note that the text needs to be stored in screen code format, not in PETSCII (see Commodore 64 PETSCII code to screen code conversion for the differences). Furthermore, the example above only writes to the screen memory, not the color RAM, assuming that the color RAM already contains the intended text color. This is the case after issuing a clear screen on most C64 machines, except for some old ROM revisions. To be on the save side you would have to write to color RAM as well, which costs another 5 byte:

.macpack cbm     ;scrcode macro
SCREEN_BASE=$400
COLOR_RAM  =$D800
LINE       =5    ;for example
COLUMN     =10   ;for example
COLOR      =1    ;white

        ldx     #endtext-text-1
@loop:  lda     text,x
        sta     SCREEN_BASE+LINE*40+COLUMN,x
        lda     #COLOR
        sta     COLOR_RAM  +LINE*40+COLUMN,x
        dex
        bpl     @loop
text:   scrcode "hello world!"
endtext:

Sound

The Commodore 64 uses the famous SID chip to generate sound. To be added soon.

Make some noise

To be added soon.

Useful Kernal functions

Get a pseudo random number

        lda     #1   
        jsr     $E09A
        lda     $63

The sequence of random numbers is always the same after a cold start or reset, the first 20 numbers are:

 4,24,231,4,177,170,193,101,231,18,196,1,75,48,149,36,124,53,65,154

Increment a zero page counter, return in X register, clear Y register

This is part of the string handling function in the BASIC ROM, but the code could be useful for some loop stuff.

        jsr     $B5FF    ;does a inc $23, ldx $23, ldy #$00, rts

Additional Resources

Codebase 64 https://codebase64.org/doku.php?id=base:start

Apple II

The Apple II is an 8-bit home computer and one of the world's first highly successful mass-produced microcomputer products. It was designed primarily by Steve Wozniak.

Setting up

ACME 6502 cross-assembler(https://sourceforge.net/projects/acme-crossass/)
Apple Commander (http://applecommander.sourceforge.net) for batch compilation
AppleWin emulator (https://github.com/AppleWin/AppleWin/releases). Supports Mockingboard card(AY-8910+speech synthesier), HDD, Z80 card(for CP/M), mouse etc.
CiderPress(https://a2ciderpress.com)

Compilation can be done as follows (master.dsk can be found with applewin)

acme hl.asm
java -jar AppleCommander-1.3.5.jar -d master.dsk hl
java -jar AppleCommander-1.3.5.jar -p master.dsk hl B 24576 < hl.bin

Memory Map

   0-255	 $0-$FF     ZERO-PAGE SYSTEM STORAGE
  256-511      $100-$1FF    SYSTEM STACK
  512-767      $200-$2FF    KEYBOARD CHARACTER BUFFER
  768-975      $300-$3CF    OFTEN AVAILABLE AS FREE SPACE FOR USER PROGRAMS
  976-1023     $3D0-3FF     SYSTEM VECTORS
 1024-2047     $400-$7FF    TEXT AND LO-RES GRAPHICS PAGE 1 <--- !!!
 2048-LOMEM    $800-LOMEM   PROGRAM STORAGE
 2048-3071     $800-$BFF    TEXT AND LO-RES GRAPHICS PAGE 2 OR FREE SPACE
 3072-8191     $C00-$1FFF   FREE SPACE UNLESS RAM APPLESOFT IS IN USE
 8192-16383   $2000-$3FFF   HI-RES PAGE 1 OR FREE SPACE <--- !!!
16384-24575   $4000-$5FFF   HI-RES PAGE 2 OR FREE SPACE
24576-38999   $6000-$95FF   FREE SPACE AND STRING STORAGE
38400-49151   $9600-$BFFF   DOS
49152-53247   $C000-$CFFF   I/O HARDWARE (RESERVED)
53248-57343   $D000-$DFFF   APPLESOFT IN LANGUAGE CARD OR ROM
57344-63487   $E000-$F7FF   APPLESOFT OR INTEGER BASIC IN LANGUAGE CARD OR ROM
63488-65535   $F800-$FFFF   SYSTEM MONITOR

Display

Graphics Modes

Text Mode 40x24, for Apple IIe available 80x25 - use PR#3 for switch mode, or hardware switch

LowRes 40x48, 16 colors: https://en.wikipedia.org/wiki/Apple_II_graphics

Hires mode 280x192,6 colors: https://www.xtof.info/blog/?p=768

https://mrob.com/pub/xapple2/colors.html https://archive.org/details/HiRes_Color_Graphics_on_the_Apple_II_Computer_by_Wozniak

However for sizecoding, you almost never want to do direct-access to graphics for Apple II in size-coding because the Apple II graphics modes are horrible. The only fast way to do things is with large lookup tables. To do hires you need to divide by 7 which as you can imagine is a bit difficult to do compactly on 6502. Double-hires is even crazier on top of that. Deater did manage a color-bar style effect in double-hires in 128B but that was doing some crazy tricks with the firmware BASIC routines, definitely not direct-access.

Lores and Hires can be mixed modes and full-graphics The screen structure is called memory holes(https://retrocomputing.stackexchange.com/questions/2534/what-are-the-screen-holes-in-apple-ii-graphics). The GBASCALC($F847) procedure is used to calculate the address of the horizontal line : IN:reg.A=Y, out : GBASL/GBASH($26/$27)=address. See also https://www.callapple.org/uncategorized/use-of-apple-ii-color-graphics-in-assembly-language/

Getting something on screen

Here is an example of a XOR texture, created by g0blinish

 *=$6000
 !to "HL.bin", plain	; set output file and format
 !cpu 6502		; set processor type

GBASL	=	$26
GBASH	=	$27
SETGR    =     $FB40 ; setup LoRes
GBASCALC = $F847 ; calc Address

CLRTEXT =  $C050 ;display graphics 
SETTEXT =  $C051 ;display text 

CLRMIXED = $C052 ;clear mixed mode- enable full graphics 
SETMIXED = $C053 ;enable graphics/text mixed mode 

PAGE1 =    $C054 ;select text/graphics page1 
PAGE2 =    $C055 ;select text/graphics page2 

CLRHIRES = $C056 ;select Lo-res 
SETHIRES = $C057 ;select Hi-res 

TMP= $FA

	JSR   SETGR      ;GR
	BIT CLRMIXED ; full screen

	LDA #0 ; A=0
	STA TMP ; POKE $FA,A

YLP ;
	LDA TMP ; A=PEEK($FA)
;	LSR ; A=A/2
	JSR GBASCALC
	LDY #0;Y=0

XLP TYA ; A=Y
	EOR TMP ; A=A xor PEEK($FA)
	and #$0F ; A=A and 15
	TAX ; X=A
	LDA COLORS,X ;A=PEEK(COLORS+X)
	STA(GBASL),Y ; POKE PEEK($26)+256*PEEK($27)+Y,A
	INY ; Y=Y+1
	CPY #40 ; Y=40?
	BNE XLP
	INC TMP ; POKE $FA,PEEK($FA)+1
	LDA TMP ; A=PEEK($FA)
	CMP #24 ; A=24?
	BNE YLP
	
M1 JMP M1 ; replace to RTS

COLORS ;N*17, pixel format is AAAABBBB, AAAA - upper dot, BBBB - lower dot
!byte $00,$11,$22,$33,$44,$55,$66,$77
!byte $88,$99,$AA,$BB,$CC,$DD,$EE,$FF

Sound

Here is an example for using the speaker, based onthe following basic program:

; 50  POKE 768,V: POKE 769,P - 255 *  INT (P / 256): POKE 800,1 + P / 256
; 60  CALL 770: RETURN 
; 95  FOR K = 1 TO N: READ V(K),P(K): NEXT K
; 100  FOR K = 1 TO N:V = V(K):P = P(K)
; 110  GOSUB 50
;!byte 173,48,192,136,208,5,206,1,3,240,9,202,208,245,174,0,3,76,2,3,206,32,3,208,240,96

 *=$6000
 !to "HL.bin", plain	; set output file and format
 !cpu 6502		; set processor type

;start
; 95  FOR K = 1 TO N: READ V(K),P(K): NEXT K
; 100  FOR K = 1 TO N:V = V(K):P = P(K)
ini:
	lda #70
	sta cnt+1
	lda #music&255
	sta gotbyte+1
	lda #music/256
	sta gotbyte+2

lop:
;V
	jsr gotbyte
	sta L300
	jsr gotbyte
;P
	jsr gotbyte
	sta L301
	jsr gotbyte
	clc
	adc #1
	sta L320
	jsr beep
	
	dec cnt+1
cnt lda #70
	bne lop
; 110  GOSUB 50
; 50  POKE 768,V: POKE 769,P - 255 *  INT (P / 256): POKE 800,1 + P / 256
; 60  CALL 770: RETURN 
	jmp ini
gotbyte
	lda music
	inc gotbyte+1
	bne noinch
	inc gotbyte+2
noinch
	rts
;!byte 173,48,192,136,208,5,206,1,3,240,9,202,208,245,174,0,3,76,2,3,206,32,3,208,240,96
beep:
	ldy #1
	ldx #1
loc_302:
		LDA	$C030

loc_305:
		DEY
		BNE	loc_30D
		DEC	L301
loc_30B:
		BEQ	loc_316

loc_30D:
		DEX
		BNE	loc_305
		LDX	L300
		JMP	loc_302
loc_316:
		DEC	L320
		BNE	loc_30B
		RTS
L301 !byte 0
L300 !byte 0
L320 !byte 0
music
 !word 76,192,85,64,96,64,102,64,114,128,114,64,96,64,102,64,114,64,128,64
 !word 114,64,152,64,171,64,152,512,76,192,85,64,96,64,102,64,114,128,114,64
 !word 96,64,102,64,114,64,128,64,114,64,152,64,171,64,152,512,85,64,85,64
 !word 85,64,96,64,144,128,144,64,128,64,76,128,85,64,96,64,144,128,114,64
 !word 96,64,102,128,114,64,128,64,128,128,114,64,128,64,114,512,85,64,85,64
 !word 85,64,96,64,144,128,144,64,128,64,76,128,85,64,96,64,144,128,114,64
 !word 96,64,102,128,114,64,128,64,128,64,128,128,96,64,85,64,96,64,102,64,114,64,114,64

Additional Resources

Deater's page on Apple II sizecoding http://www.deater.net/weave/vmwprod/demos/sizecoding.html
Article on double hi-res http://www.battlestations.zone/2017/04/apple-ii-double-hi-res-from-ground-up.html
Applesoft Hi-Res Subroutines : http://hackzapple.org/scripts_php/index.php?menu=5&mod=ASM&sub=AAL&sub2=8112&PHPSESSID=f65fabfd0cdbf56b6bdc0ddac25117c6#a2

Atari Lynx

The Atari Lynx consists of the 6502 with custom hardware for graphics and sound.

Setting up

Setting up your development platform for the Atari Lynx:

Assembler: -
Emulator(s): -

Video display

To be added soon.

Getting something on screen

To be added soon.

Sound

To be added soon.

Make some noise

To be added soon.

Additional Resources

Sizecoding resource for the Atari Lynx are sparse

42Bastian's website (link to be added)

6502

Contents

Introduction

The 6502 processor

Adressing modes

Zero page

General 6502 Resources

Atari 8-bit family

Setting up

Special Memory Adresses

Video display

Sync with frame

Getting something on screen

Sound

BASIC

Make some noise

Additional Resources

Commodore 64

Setting up

Memory map

Program file overhead

Autostart

Overwriting CHROUT vector at $326-$327

Video display

Getting something on screen

Sound

Make some noise

Useful Kernal functions

Get a pseudo random number

Increment a zero page counter, return in X register, clear Y register

Additional Resources

Apple II

Setting up

Memory Map

Display

Graphics Modes

Getting something on screen

Sound

Additional Resources

Atari Lynx

Setting up

Video display

Getting something on screen

Sound

Make some noise

Additional Resources

Navigation menu

Search