forked from Apple-2-HW/gp2io
28 lines
1.3 KiB
ArmAsm
28 lines
1.3 KiB
ArmAsm
ORG $0300
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ANN0HI EQU $C059
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ANN0LO EQU $C058
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ANN1HI EQU $C05B
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ANN1LO EQU $C05A
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BYTETOSEND EQU $EF
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SENDBYTE LDY #$08 ; load 8 for a full byte, loop counter
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STA BYTETOSEND ; put byte in zero page for safe keeping
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RTS STA ANN0HI ; annunciator 0 high, RTS
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SENDBIT ROL BYTETOSEND ; rotate byte left, high/MSB out to carry (sendbit)
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BCC SHORTLOOP ; "branch on carry clear" - if carry/bit = 0, goto #311 (short loop)
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LONGLOOP LDX #$14 ; if carry/bit = 1, load X with 20 (long loop)
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JMP SETANN1 ; jump over SHORTLOOP
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SHORTLOOP LDX #$0A ; if carry/bit = 0, load X with 10 (short loop)
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SETANN1 STA ANN1HI ; set annunciator 1 HIGH
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COUNTDOWN DEX ; decrement, countdown to setting ANN1 low (COUNTDOWN)
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BNE COUNTDOWN ; if X > 0, keep counting (goto COUNTDOWN)
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STA ANN1LO ; if done, set ANN1 LOW, countdown to getting next bit
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LDX #$05 ; reset counter for short "reset" transition. This lets the GP2IO stage the bit before the next interrupt
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COUNTDOWN2 DEX ; count down again (COUNTDOWN2)
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BNE COUNTDOWN2 ; if X > 0, keep counting (goto COUNTDOWN2)
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DEY ; decrement y, countdown bits sent
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BNE SENDBIT ; if y > 0, next bit (SENDBIT)
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RTSOFF STA ANN0LO ; annunciator 0 low, sending RTS OFF
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RTS ; return
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