Commodore Amiga

Commodore Amiga

The Commodore Amiga system consists of the M68k system with custom hardware for graphics and sound.

Setting up

• Assembler: VASM
• Emulator(s): Linux: fs-uae, Windows: WinUAE

Compile source to Amiga Kickstart1.x executable:

vasm -kick1hunks -Fhunkexe -o example -nosym

Amiga executable format

Commodore Amiga executables consist of a 32 byte header, as well as a 4 byte footer. Here's an example where deadc0de is the only code data. 0000 03f2 marks the end of the block.

00000000: 0000 03f3 0000 0000 0000 0001 0000 0000  
00000010: 0000 0000 0000 0001 0000 03e9 0000 0001  
00000020: dead c0de 0000 03f2

Memory Map

Here is the amiga memory map

• 

Video display

The Amiga OCS uses an interleaved planar memory layout to represent its various paletted display modes.

Getting something on screen

One of the shortest ways to get you started with getting anything on screen at all, is to initialize and use the intuition graphics library routines. These are relatively easy to setup and allow you to plot pixels, circles, rects and even polygons. However, as can be expected these routines are extremely slow in execution.

         incdir  "include"
        include "graphics/graphics_lib.i"
        include "intuition/screens.i"

        move.l  $170(a2),a6     ; intuitionbase from globvec
        move.l  ib_ActiveScreen(a6),a5  ; a5 = activeScreen (640x240 on most systems)
        move.l  $4.w,a6         ; execbase
        move.l  156(a6),a6      ; graphics.library

        ; clear screen
        lea sc_RastPort(a5),a1
        jsr _LVOClearScreen(a6)

frameloop:
	move.w	#240-1,d7		; y
yLoop:	
	move.w	#640-1,d6		; x
xLoop:
	; Set Pixel/Pen Color
	move.b d6,d0		; d0=x
	eor d7,d0	        ; d0=x^y
	lsr #3,d0		; d0>>=3
        jsr _LVOSetAPen(a6)

	; Write Pixel
	move d6,d0
	move d7,d1
	jsr _LVOWritePixel(a6)
	dbra d6,xLoop	
    dbra d7,yLoop

   bra frameloop

Copperlist

Another way to start drawing things on screen is to setup a (minimal) copperlist with commands for one or more planes, which can be manipulated indiviually

Copper list format:

To be added

As it happens, you can also insert color definition into the copperlist.

Planes

Every plane contains one bit of a pixel's colour index value. The bits of the binary representation of a colour index like %1010 (% Bit3,Bit2,Bit1,Bit0) will end up in 6 different planes (bits most significant to least significant aka left to right): Plane4 Plane3 Plane2 Plane1.

So basicly Plane1 contains all of the Bit0s of all pixels, Plane2 all Bit1s, Plane3 all Bit2s and Plane4 all Bit3s.

The first pixel on a plane is described by the leftmost (aka most significant) bit in a word, the second one by the second-leftmost etc. - just like this %0123456789abcdef with 0-f=pixels 1-16. %1000000000000000=$8000=pixel 1 in a plane word set. The 16th pixel will use the leftmost bit of the next word in this plane. etc.

Drawing pixels

Here are some examples for drawing pixels in 1 or more bitplanes:

1-bpp pixel plot

To be added

2-bpp pixel plot

To be added

Setting a palette

The Amiga OCS uses index values into a palette of colours. Palette format is a word per index with 3 nibbles for 4-bit RGB (e.g. $00f for blue). Here is some code that will help you setup an entire palette at once using the intuition graphics routine.

lea sc_ViewPort(a5),a0
lea paldata(pc),a1
moveq #16,d0
jsr _LVOLoadRGB4(a6)

; Palette data
paldata:	
	dc.w	$000,$100,$200,$311,$422,$533,$644,$755
	dc.w	$575,$464,$353,$242,$131,$020,$010,$000

Alternatively you can set individual palette entries either by using a graphics function or using the copperlist for setting initial colors

Sound

To be added

Make some noise

So, here is a bit of sound code that outputs some sounds to the PCM output

To be added

Additional Resources

• AmyCoders
• Amiga Harware Reference Manual from Commodore (scans on Archive.org)
• Hardware Manual Guide
• Learning Amiga machine code
• Blogposts by Losso
• Coppershade articles
• Amiga Header information