6502 Assembly programming for the Atari 800 & 5200

The early Atari games consoles weren't a true 6502, but by the time of the Atari 5200 things had gotten pretty good!

The Atari 800 and 5200 are basically the same machine, and we can write games that work on both pretty easilly... in fact the differences are INTENTIONAL.. they didn't want games to work on both the computer and console, as they were different operating companies, so the address of the sound and graphics chips were moved to stop the games working!

We'll be covering the Atari 5200 console, and Atari 800 home computer in these tutorials.

	Atari 5200	Atari 800
Cpu	1.79 mhz 6502	1mhz 65C02
Ram	16k	48k
Resolution	320x200 @ 2 color 160x200 @ 4 color	320x200 @ 2 color 160x200 @ 4 color
Sprites	8x128 pixel sprites... 5 onscreen	8x128 pixel sprites... 5 onscreen
Sound	4 channel POKEY	4 channel POKEY

ChibiAkumas Tutorials

	Lesson H4 - Hello World on the Atari 800 / 5200
	Lesson S4 - Bitmap Drawing on the Atari 800 / 5200
	Lesson S13 - Joystick Reading on the Atari 800 / 5200
	Lesson S23 -Sprite Clipping on the Atari 800 / 5200
	Lesson P2 - Bitmap Functions on the Atari 800 / 5200
	Lesson P11 - Joystick Reading on the Atari 800 / 5200
	Lesson P18 - Palette definitions on the Atari 800 / 5200
	Lesson P23 - Sound on the Atari 800 / 5200
	Lesson P31 - Hardware Sprites on the Atari 800 / 5200
	Lesson P51 - Sound on the Atari 800 / 5200 (ChibiSound Pro)
	Lesson P56 - Multiplatform Software tilemap on the Atari 800 or 5200 (Mintile)
	Lesson Photon4 - Atari 800 / 5200 - ASM PSET and POINT for Pixel Plotting

Useful Documents
Altirra Hardware Reference Manual - Extensive Atari tech manual
Atari System Reference Manual - Another great tech resource
AtariOSSrc - Disassembled OS Source code

Useful Disk tools
Mkatr - Disk Tool

Differences between the Atari 800 and 5200
The Atari 800 and 5200 have some differences, The GTIA (graphics) and POKEY (sound and io) are at different addreses, also the default cartridge base address and size are different.
The 800 cartridges are 8K, the 5200 has 32k ones.

	Atari 5200	Atari 800
Cart ROM	$4000	$A000
GTIA (Graphics)	$C000	$D000
POKEY (Sound)	$E800	$D200
PIA	Not present	$D300
ANTIC	$D400	$D400

Most documentation seems to use the Atari 800 addresses... so we'll do the same... if you're using an Atari 5200 just change $D0 to $C0 for GTIA code, and $D2 to $E8 for POKEY code... of course, it's best to just define a symbol for each system in your ASM code - which is what our example ASM code will do!

Screen Modes
In this tutorial we'll be looking at two screen modes...

ANTIC mode F is a 4 color 160x192 pixel screen mode with 'Rectangular' pixels like Mode 0 on the CPC... the 4 colors are defined by 4 registers
COLBK (&D01A) sets the background... Colors 1-3 are set by COLPF0 ($D016), COLPF1 ($D017) and COLPF2 ($D018)

ANTIC mode E is a 2 color 320x192 pixel mode screen with roughly square pixels.. technically it only has one color - with two shades... COLBK (&D01A) sets the background brightness and color, and foreground color... and COLPF0 ($D016) sets the foreground brightness

Screen Display lists

The screen is defined in a similar way to the Elan Enterprise... we provide a Display List... which provides commands to the screen hardware... we can do clever tricks like have different screen modes for different areas of the screen if we want - but we won't be using them in this tutorial - we'll just have a simple list that has the screen all the same mode!...
Our Screen will start at &2000... unfortunately we do have to do some tricks, to allow our screen to 'step' over the 4k boundary at $3000 (otherwise it would wrap) - we do this by putting a new 'start memory position' command ($40+screen mode) - to move the memory position to $3000 - getting over the wrap limitation!

We also have to put 3 blank strips at the top, and a 'loop' in at the end of the screen, to allow normal working of the screen!

You'll need to set Smode to F or E for this example code to work!

Just creating a display list isn't enough, we need oto put it in the display list registers at $D402-$D403, and turn the DMA control on.

This will start the screen drawing with our chosen mode!

The following byte commands can be used in a display list:

Command	Function
$x0-$xF	Screen mode change
$70	8 Blank lines
$4x $BB $AA	Start Screen mode x at address $AABB
$41 $BB $AA	Wait for Vblank, and restart display list at $AABB

Screen Modes
The Atari 800 & 5200 have a variety of modes - we can change mode every line of the screen - but some modes are 'taller' than others... here are the options - note: we're only going to look at modes E and F in these tutorials

Antic Mode	Basic Mode	Colors	Lines	Width	Bytes per Line	Screen Ram (Bytes)
2	0	2	8	40	40	960
3	N/A	2	10	40	40	760
4	N/A	4	8	40	40	960
5	N/A	4	16	40	40	480
6	1	5	8	20	20	480
7	2	5	16	20	20	240
8	3	4	8	40	10	240
9	4	2	4	80	10	480
A	5	4	4	80	20	960
B	6	2	2	160	20	1920
C	N/A	2	1	160	20	3840
D	7	4	2	160	40	3840
E	N/A	4	1	160	40	7680
F	8	2	1	320	40	7680

Atari 800 / 5200 Palette

Atari 800 / 5200 Sprites
The Atari's hardware sprites are very weird!

Basically each sprite is 8 pixels wide and just 2 colors (1+transparent)... there are 4 'normal' ones that are 8 pixels wide.... and 4 missile sprites that are just 2(!) pixels wide... but we can position them together to give us 5 sprites.

Despite being 8 pixels wide... each sprite is up to 128 pixels tall (or 256 in hires mode) - the entire height of the screen!... if you can't guess this is because the systems is changing the data each rasterline.

The data used to draw the sprite is taken from a single pointer at $D407... if this pointer is set to $18 then all the sprites will use the $1800-$1FFF range - the exact address differs depending on whether the Resolution bit of $D400 is set to 0 or 1... in Res1 Sprites will be at $1800+$400 - $1C00 ... or in Res0 $1800+$200 = $1A00

On an Atari 800 where the GTIA is at $D000 this would give the following addresses for the sprite settings GTIA is at $C000 on the 5200)

The addresses controling the sprite are shown below... note we cannot set vertical position - we just write the sprite bitmap to a different address in the 'strip' of memory (eg between $1C00-$1CFF)

Player	Res0 Data	Res1Data	Width	Color	Xpos
0	$1A00+ypos	$1C00+ypos	$D008	$D012	$D000
1	$1A80+ypos	$1D00+ypos	$D009	$D013	$D001
2	$1B00+ypos	$1E00+ypos	$D00A	$D014	$D002
3	$1B80+ypos	$1F00+ypos	$D00B	$D015	$D003
4 (Missiles)	$1980	$1B00	$D00C	$D019* / $D012-$D015	$D004-$D007

*Missiles can be configured to use all 4 player colors for each 2 bit strip - or $D019 for all 4 2 bit strips ... this is set by PRIOR ($D01B)

The Sprites can be in front of, or behind the background... register $D01B (PRIOR) controls the order... and allows all 4 missiles to use color defined at $D019 as the sprite color - instead of the 4 player colors!
Note PRIOR is at $D01B... it seems to be incorrectly reported as $D10B or $D21B in some documentation!!!

To make use of sprites, we need to set the addresses shown above for the player sprite attributes, we also need to turn sprites on!

The example code to the right should do the job! note you needto set symbol GTIA to $D000 on the Atari 800, or $C000 on the Atari 5200

        lda #%00111110
        sta $D400            ;DMA control (SDMCTL)

        lda #$18 ;Sprites will be at $1800+$300 (or +$180 in low res mode)
        sta $D407    ;Store player sprite base

        lda #%00000011
        sta GTIA+$001D    ;Graphics Control (GRACTL)

        lda #%00010001   ;Priority: sprite 5 to use color 3
        sta GTIA+$1B        ;and put sprites in front of background

Atari 800 / 5200 Sprite Registers

Group	Name	Description	Address A80	Address A52	Bits	Notes
GTIA	HPOSP0	horizontal position of player 0	$D000	$C000
GTIA	HPOSP1	horizontal position of player 1	$D001	$C001
GTIA	HPOSP2	horizontal position of player 2	$D002	$C002
GTIA	HPOSP3	horizontal position of player 3	$D003	$C003
GTIA	HPOSM0	horizontal position of missile 0 (Player 4)	$D004	$C004
GTIA	HPOSM1	horizontal position of missile 1 (Player 4)	$D005	$C005
GTIA	HPOSM2	horizontal position of missile 2 (Player 4)	$D006	$C006
GTIA	HOPSM3	horizontal position of missile 3 (Player 4)	$D007	$C007
GTIA	SIZEP0	player 0 size	$D008	$C008	------WW	Width of sprite (0-3)
GTIA	SIZEP1	player 1 size	$D009	$C009	------WW	Width of sprite (0-3)
GTIA	SIZEP2	player 2 size	$D00A	$C00A	------WW	Width of sprite (0-3)
GTIA	SIZEP3	player 3 size	$D00B	$C00B	------WW	Width of sprite (0-3)
GTIA	SIZEM	missile size	$D00C	$C00C	wwWWwwWW	Width of sprite (Need to set all 4 parts)
GTIA	GRAFP0	player 0 graphics	$D00D	$C00D		(Used by DMA)
GTIA	GRAFP1	player 1 graphics	$D00E	$C00E		(Used by DMA)
GTIA	GRAFP2	player 2 graphics	$D00F	$C00F		(Used by DMA)
GTIA	GRAFP3	player 3 graphics	$D010	$C010		(Used by DMA)
GTIA	GRAFM	missile graphics	$D011	$C011		(Used by DMA)
GTIA	COLPM0	color/brightness, player/missile 0	$D012	$C012
GTIA	COLPM1	color/brightness, player/missile 1	$D013	$C013
GTIA	COLPM2	color/brightness, player/missile 2	$D014	$C014
GTIA	COLPM3	color/brightness, player/missile 3	$D015	$C015
GTIA	COLPF3	color/brightness of setcolor 3 / Player 5 (missile)	$D019	$C019
GTIA	PRIOR	p/m priority and GTIA mode	$D01B	$C01B	GGmMpppp	G=gtia mode (0=normal) C=multiColor M=Missile (player 5) pppp=priority setting (1=sprites in front 4=behind)
GTIA	GRACTL	graphics control	$D01D	$C01D	------L45	Latch Trigger / Enable 4 player / enable 5 (missiles)
ANTIC	DMACTL	Direct Memory access control (DMA)	$D400	$C400
ANTIC	PMBASE	player/missile address / 256	$D407	$C407

Pokey Sound

The Pokey is at memory mapped port $D200 on A800 and $E800 on A5200

Group	Name	Description	Address A80	Address A52	Bits	Meaning
POKEY	AUDF1	Audio frequency 1 control	$D200	$E800	FFFFFFFF	F=Frequency (0=highest tone)
POKEY	AUDC1	Audio channel 1 control	$D201	$E801	NNNNVVVV	N=Noise (0=noise / 10=Square wave) V=Volume (15=loudest)
POKEY	AUDF2	Audio frequency 2 control	$D202	$E802	FFFFFFFF	F=Frequency (0=highest tone)
POKEY	AUDC2	Audio channel 2 control	$D203	$E803	NNNNVVVV	N=Noise (0=noise / 10=Square wave) V=Volume (15=loudest)
POKEY	AUDF3	Audio frequency 3 control	$D204	$E804	FFFFFFFF	F=Frequency (0=highest tone)
POKEY	AUDC3	Audio channel 3 control	$D205	$E805	NNNNVVVV	N=Noise (0=noise / 10=Square wave) V=Volume (15=loudest)
POKEY	AUDF4	Audio frequency 4 control	$D206	$E806	FFFFFFFF	F=Frequency (0=highest tone)
POKEY	AUDC4	Audio channel 4 control	$D207	$E807	NNNNVVVV	N=Noise (0=noise / 10=Square wave) V=Volume (15=loudest)
POKEY	AUDCTL	general audio control	$D208	$E808	N1234HHS	N=Noise bit depth 1234=Channel Clocks HH=highpass filters S=main clockspeed