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39 lines
920 B
Plaintext
39 lines
920 B
Plaintext
.setcpu "6502"
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.export delay_ms
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.code
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;.align 256
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; Delay the number of miliseconds specified by X
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; This is hardcoded for a 1 MHz system clock
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; @in X The delay in ms
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delay_ms: pha ; 3
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txa ; 2
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pha ; 3
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tya ; 2
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pha ; 3
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ldy $00 ; 3 (dummy operation)
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ldy #190 ; 2
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@loop1: dey ; 190 * 2
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bne @loop1 ; 190 * 3 - 1
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@loop2: dex ; 2
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beq @return ; (x - 1) * 2 + 3
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nop ; 2
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ldy #198 ; 2
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@loop3: dey ; 198 * 2
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bne @loop3 ; 198 * 3 - 1
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jmp @loop2 ; 3
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@return: pla ; 4
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tay ; 2
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pla ; 4
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tax ; 2
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pla ; 4
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rts ; 6 (+ 6 for JSR)
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