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49 lines
1.7 KiB
Plaintext
49 lines
1.7 KiB
Plaintext
; Assembler should use basic 6502 instructions
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processor 6502
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; Include files for Atari 2600 constants and handy macro routines
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include "vcs.h"
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include "macro.h"
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; Here we're going to introduce the 6502 (the CPU) and
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; the TIA (the chip that generates the video signal).
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; There's no frame buffer, so you have to program the TIA
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; before (or during) each scanline.
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; We're just going to initialize the system and put some
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; color on the TV.
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; 4K Atari 2600 ROMs usually start at address $F000
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org $f000
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; Typical initialization routine
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; ('start' is a label because it's on the left margin)
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Start sei ; disable interrupts
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cld ; disable BCD math mode
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ldx #$ff ; init stack pointer to $FF (grows upward)
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txs ; ... transfer X register to S register (stack pointer)
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; Another typical thing is to clear the zero page region ($00-$FF)
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; This includes a bunch of TIA registers as well as RAM ($80-$FF)
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lda #$00 ; set A register to zero ('#' denotes constant)
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; X register is already at $ff from previous instruction, so let's loop...
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Zero sta $00,X ; store A register at address ($0 + X)
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dex ; decrement X by one
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bne Zero ; branch until X is zero
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sta $00 ; the loop doesn't cover address 0
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; Set background color
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lda #$30 ;load value into A ($30 is deep red on NTSC)
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sta COLUBK ;put the value of A into the background color register
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; Nothing else to do, so let's start over.
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; There's no vertical sync logic, and the zero-page clearing routine
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; will run again, so you'll see alternating black and red lines.
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jmp Start
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; Here we skip to address $FFFC and define a word with the
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; address of where the CPU should start fetching instructions.
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; This also fills out the ROM size to $1000 (4k) bytes
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org $fffc
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.word Start
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.word Start
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