6502 based CPUs

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Introduction

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

6502.jpg

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

The 6502 processor

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

Zero page

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

Atari 8bit family

The Atari XE/XL 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 TIA chip, it has the following video modes:

To be added soon.

Getting something on screen

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 8bit systems use the Pokey chip to generate sound. To be added soon.

Make some noise

To be added soon.


Additional Resources

Sizecoding resource for the Atari 8bit are sparse

  • Fready's github (link to be added)

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)

Commodore 64

The Commodore systems consists of the 6502 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

Autoboot

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

Will give you autoboot and more space directly. (though writing through to $0400 will load it onto the screen unless you move the pointers)

Video display

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

To be added soon.

Getting something on screen

To be added soon.


Sound

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

Make some noise

To be added soon.

Additional Resources

  • links to be added


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, 80x25 - use PR#3 for switch mode LoRes(https://en.wikipedia.org/wiki/Apple_II_graphics) 40x48, 16 colors Hires modes: 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/

Sound

To be added soon.

Additional Resources