Minimal Amiga Boot Code
From CiaransWiki
Pretty much the minimum required for an Amiga boot block to get rid of the operating system and do something visible. See this post for more details.
#Everything in the boot block must be PC relative anyway .org 0 CUSTOM = 0xDFF000 |Base address for all custom chip registers BPLCON0 = 0x100 BPLCON1 = 0x102 BPLCON2 = 0x104 INTENA = 0x9A INTREQ = 0x9C COPJMP1 = 0x88 COP1LCH = 0x80 DMACON = 0x96 SETCLR = 0x8000 COPEN = 0x80 BPL1PTH = 0xE0 BPL1PTL = 0xE2 BPL2PTH = 0xE4 BPL2PTL = 0xE6 BPL3PTH = 0xE8 BPL3PTL = 0xEA BPL4PTH = 0xEC BPL4PTL = 0xEE BPL5PTH = 0xF0 BPL5PTL = 0xF2 BPL6PTH = 0xF4 BPL6PTL = 0xF6 BPL1MOD = 0x108 BPL2MOD = 0x10A COLOR00 = 0x180 COLOR01 = 0x182 COLOR02 = 0x184 COLOR03 = 0x186 DDFSTRT = 0x92 DDFSTOP = 0x94 DIWSTRT = 0x8E DIWSTOP = 0x90 CopList = 0x20000 |We'll put our copper list here Screen = 0x21000 |We'll put our screen here ScreenWidth = 320 ScreenHeight = 200 Start: dc.b 'D','O','S',0 dc.l 0 |Checksum will go here dc.l 880 #We'll keep a6 pointing to the custom chip registers throughout... lea CUSTOM,a6 #Disable all interrupts and requests move.w #0x7FFF,INTENA(a6) move.w #0x7FFF,INTREQ(a6) #Disable all DMA move.w #0x7FFF,DMACON(a6) #Set up the screen geometry... move.w #0x1200,BPLCON0(a6) move.w #0,BPLCON1(a6) move.w #0,BPL1MOD(a6) move.w #0x38,DDFSTRT(a6) move.w #0xD0,DDFSTOP(a6) move.w #0x2C81,DIWSTRT(a6) move.w #0xF4C1,DIWSTOP(a6) #Set up some colours... move.w #0x000,COLOR00(a6) move.w #0x0F0,COLOR01(a6) #Fill the screen with some data... lea Screen,a0 move.l #0xF0F0F0F0,d0 move.w #ScreenWidth/32*ScreenHeight,d1 Loop: move.l d0,(a0)+ subq.w #1,d1 bne Loop #Generate our copper list... lea CopList,a0 move.l a0,COP1LCH(a6) #Set address of bitplane 1... move.w #BPL1PTH,(a0)+ move.w #(Screen>>16),(a0)+ move.w #BPL1PTL,(a0)+ move.w (Screen&0xFFFF),(a0)+ #Infinite wait stops copper until next vblank... move.l #0xFFFFFFFE,(a0)+ #Start the copper (the value written is irrelevant)... move.w d0,COPJMP1(a6) #Enable copper and bitplane DMA... move.w #0x8380,DMACON(a6) #Sit in a loop forever... ILoop: bra.s ILoop
