68000 Assembly programming for the Atari ST

The Atari ST completely passed me by in the 80's... While it has the same CPU as the Amiga, it has inferior graphics and sound , with no hardware graphics layers, and only AY sound...

These days the ST has one major advantage... it's much easier to emulate! Often getting Amiga games working requires messing with different configurations, whereas the Atari ST version will run instantly and while usually has inferior sound, will otherwise typically be identical to the Amiga version!

Specs:

Cpu	8mhz
Ram	512k
Resolution	320x200 @ 16 colorr
Sprites	Software Only
Palette	16 colors onscreen from 512 colors
Sound chip	AY

ChibiAkumas Tutorials

	Lesson P2 - Bitmap Functions on the Atari ST
	Lesson P8 - Joystick Reading on the Atari ST
	Lesson P18 (Z80 series) - Making Sound with the AY-3-8910 on the Atari ST!!
	Lesson P14 - Palette definitions on the Atari ST

Memory Map

Address	Purpose
000000	512k ram
07FFFF
FA0000	128K Rom
FC0000	System Rom
FEFFF
FF8000	Io Area
FF8200	Io Area
FF8400	Io Area
FF8600	Io Area
FF8800	Io Area SOUND AY-3-8910
FFFA00	Io Area MFP 68901
FFFC00	Io Area 2 ACIA:s 6580

Registers
Because the 68000 uses a 24 bit address bus, These can also be addressed at $FFFFxxxx - for example $FF8004 is the same as $FFFF8004

Address	Mode	Bits	Purpose
FF8004	RW	----xxxx	Memory Config
FF8201	RW	HHHHHHHH	Video Base H
FF8202	EW	LLLLLLLL	Video Base L
FF8205	R	HHHHHHHH	Video Counter H (current drawing line)
FF8207	R	MMMMMMMM	Video Counter M
FF8209	R	LLLLLLLL	Video Counter L
FF820A	RW	------SR	Sync Mode
FF8240	RW	-----RRR -GGG-BBB	Palette Color 0
FF8242	RW	-----RRR -GGG-BBB	Palette Color 1
FF8244	RW	-----RRR -GGG-BBB	Palette Color 2
FF8246	RW	-----RRR -GGG-BBB	Palette Color 3
FF8248	RW	-----RRR -GGG-BBB	Palette Color 4
FF824A	RW	-----RRR -GGG-BBB	Palette Color 5
FF824C	RW	-----RRR -GGG-BBB	Palette Color 6
FF824E	RW	-----RRR -GGG-BBB	Palette Color 7
FF8250	RW	-----RRR -GGG-BBB	Palette Color 8
FF8252	RW	-----RRR -GGG-BBB	Palette Color 9
FF8254	RW	-----RRR -GGG-BBB	Palette Color 10
FF8256	RW	-----RRR -GGG-BBB	Palette Color 11
FF8258	RW	-----RRR -GGG-BBB	Palette Color 12
FF825A	RW	-----RRR -GGG-BBB	Palette Color 13
FF825C	RW	-----RRR -GGG-BBB	Palette Color 14
FF825E	RW	-----RRR -GGG-BBB	Palette Color 15
FF8260	RW	------SS	Screen Mode (00=320x240x4bpp)
FF8400			Reserved
FF8600			Reserved
FF8602			Reserved
FF8604		-------- XXXXXXXX	Disk controler data access
FF8606	R	-------- -----DSE	DMA Status Mode control
FF8606	W	-------W FD0RGAB	DMA Mode Control
FF8609	RW	HHHHHHHH	Dna Base & Counter H
FF860B	RW	MMMMMMMM	Dna Base & Counter M
FF860D	RW	LLLLLLLL	Dna Base & Counter L
FF8800	R	DDDDDDDD	IO Port B Data
FF8800	W	RRRRRRRR	PSG Register Select
FF8802	W	RGCDRFGH	PSG Write Data – IO Port A
FF8802	RW	DDDDDDDD	IO Port B Data
FFFA01		M-IW---P	MFP GP IO
FFFA03		DDDDDDDD	MFP Active Edge
FFFA05		DDDDDDDD	MFP Data Direction
FFFA07		DDDDDDDD	MFP Interrupt Enable A
FFFA09		DDDDDDDD	MFP Interrupt Enable B
FFFA0B		DDDDDDDD	MFP Interrupt Pending A
FFFA0D		DDDDDDDD	MFP Interrupt Pending B
FFFA0F		DDDDDDDD	MFP Interrupt Interrupt In Service A
FFFA11		DDDDDDDD	MFP Interrupt Interrupt In Service B
FFFA13		DDDDDDDD	MFP Interrupt Mask A
FFFA15		DDDDDDDD	MFP Interrupt Mask B
FFFA17		DDDDDDDD	MFP Vector Base
FFFA19		DDDDDDDD	MFP Timer A Control
FFFA1B		DDDDDDDD	MFP Timer B Control
FFFA1D		DDDDDDDD	MFP Timers C & D Control
FFFA1F		DDDDDDDD	MFP Timer A data
FFFA21		DDDDDDDD	MFP Timer B data
FFFA23		DDDDDDDD	MFP Timer C data
FFFA25		DDDDDDDD	MFP Timer D data
FFFA27		DDDDDDDD	MFP Sync Character
FFFA29		DDDDDDDD	MFP USART control reg
FFFA2B		DDDDDDDD	MFP receiver status
FFFA2D		DDDDDDDD	MFP Transmitter status
FFFA2F		DDDDDDDD	MFP USART Data
FFFC00		DDDDDDDD	Keyboard ACIA Control
FFFC02		DDDDDDDD	Keyboard Data
FFFC04		DDDDDDDD	Midi ACIA Control
FFFC06		DDDDDDDD	Midi Data

Aline commands
Aline commands are defined by DW $A0xx - these are handled by the vectors of the 68000 like an RST command on the Z80... see Anatomy of the Atari St for more details

Command	Command	Detail
$A000	Initialize	Determine address of required variable range
$A001	Put Pixel	Set point on the screen
$A002	Get Pixel	Determine color of a screen point
$A003	Line	Draw a line on the screen
$A004	Horizontal Line	Draw a horizontal line (fast)
$A005	Filled Rectangle	Fill a rectangele wit color
$A006	Filled Polygon	Fill a polygon line by line
$A007	BitBlt	Bit block transfer
$A008	TextBlt	Text block transfer
$A009	Show Mouse	enable mouse cursor
$A00A	Hide Cursor	Disable mouse cursor
$A00B	Transform Mouse	Change mouse cursor form
$A00C	Undraw Sprite	Clear sprite
$A00D	Draw Sprite	Enable sprite
$A00E	Copy Raster From	Copy raster form

Vblank!

It's possible to 'Wait for Vblank' by using the line counter at $FF8205/7/9.

First we calculate the address of the last line of our 32k screen, in this case by adding 320*200/2 to 'ScreenBase'

Next we wait for the line counter to point to this line - at this point the bottom of the screen is being drawn,

Here is an example of a possible solution - this assumes memory address 'ScreenBase' has a long pointer of the address of the first line of the screen.

waitVBlank:
    moveM.l d0-d1,-(sp)
        move.l ScreenBase,d1     ;Get screen pointer
        add.l #(320*200/2),d1    ;Add one full screen

WaitVblankAgain:
        jsr VblankGetLine
        cmp.l d1,d0
        blt WaitVblankAgain        ;Wait for last line to be drawn

WaitVblankAgain2:
        jsr VblankGetLine
        cmp.l d0,d1
        beq WaitVblankAgain2    ;Wait for line to end
    moveM.l (sp)+,d0-d1
    rts

VblankGetLine:
    clr.l d0
    or.b $FF8205,d0        ;Get current drawing line
    lsl.l #8,d0                ;from hardware regs
    or.b $FF8207,d0
    lsl.l #8,d0
    or.b $FF8209,d0
    rts

Joystick reading

I'm not a fan of using the firmware for doing things, but I can't find any other way to read the joysick on the Atari ST

The Atari ST joystick is connected as part of the keyboard... to read it we have to install a Joystick event interrupt handler...

Shown to the right is the simplest joystick handler I have working...
we store the two joystick bytes to a temporary variable called "Joydata"

    move.w    #$14,-(sp)    ;IKBD command $14 - set joystick event reporting
    move.w    #4,-(sp)    ;Device no 4 (keyboard - Joystick is part of keyboard)
    move.w    #3,-(sp)    ;Bconout (send cmd to keyboard)
    trap    #13            ;BIOS Trap
    addq.l     #6,sp        ;Fix the stack

    move.w #34,-(sp)    ;return IKBD vector table (KBDVBASE)
    trap     #14            ;XBIOS trap
    addq.l #2,sp         ;Fix the stack

    move.l d0,ikbd_vec     ; store IKBD vectors address for later
    move.l d0,a0             ; A0 points to IKBD vectors
    move.l 24(a0),old_joy     ; backup old joystick vector so we can restore it
    move.l #JoystickHandler,24(a0); Set our Joystick Handler
    rts

JoystickHandler:
    move.b (1,a0),joydata         ; store joy 0 data
    move.b (2,a0),joydata+1     ; store joy 1 data
    rts

ikbd_vec:    dc.l 0             ; old IKBD vector storage
old_joy:    dc.l 0            ; old joy vector storage
joydata:    ds.b 2            ;Joypad bytes F----RLDU

AY Sound Chip:

The AY uses a series of 8 bit registers to control the 3 sound channels.

To set a register, we must first select the register number by writing a byte to $FF8800, then send the byte of data to $FF8802

Register	Meaning	Bit Meaning	Details
0	Tone Pitch L - Channel A	LLLLLLLL	Lower value = Higher pitch
1	Tone Pitch H - Channel A	----HHHH	Lower value = Higher pitch
2	Tone Pitch L - Channel B	LLLLLLLL	Lower value = Higher pitch
3	Tone Pitch H - Channel B	----HHHH	Lower value = Higher pitch
4	Tone Pitch L - Channel C	LLLLLLLL	Lower value = Higher pitch
5	Tone Pitch H - Channel C	----HHHH	Lower value = Higher pitch
6	Noise Generator	---NNNNN	Higer = Faster noise
7	Mixer	--NNNTTT	N=Noise T=Tone (Channel --CBACBA 1=mute 0=normal)
8	Amplitude - Channel A	---EVVVV	E=Envelope (1=Enabled) VVVV=Volume
9	Amplitude - Channel B	---EVVVV	E=Envelope (1=Enabled) VVVV=Volume
10	Amplitude - Channel C	---EVVVV	E=Envelope (1=Enabled) VVVV=Volume
11	Envelope L (Volume over time)	LLLLLLLL	Lower=Faster Envelope
12	Envelope H (Volume over time)	HHHHHHHH	Lower=Faster Envelope
13	Envelope Selection	----EEEE	Envelope number (See PDF)

For more details, please see the AY sound chip PDF

Traps
Traps are used for firmware and OS functions... commands need to be pushed onto the stack before the call... see Anatomy of the Atari St for more details