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43 lines
976 B
ArmAsm
43 lines
976 B
ArmAsm
; Serial output at 57600 bits/sec on controller port 2
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;
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; Uses stack and register A only, and doesn't mind page crossing
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; (uses subroutines instead of loops).
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; Initializes serial. If this isn't done, first byte sent to
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; PC might be corrupt.
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; Preserved: X, Y
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serial_init:
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sec
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lda #$FF
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bne serial_write_ ; always branches
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; Writes byte A to serial
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; Preserved: X, Y
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serial_write:
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clc
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serial_write_:
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jsr @bit ; start
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nop ; TODO: why the extra delay?
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jsr @first ; bit 0
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jsr @bit ; bit 1
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jsr @bit ; bit 2
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jsr @bit ; bit 3
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jsr @bit ; bit 4
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jsr @bit ; bit 5
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jsr @bit ; bit 6
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jsr @bit ; bit 7
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sec ; 2 stop bit
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@first: nop ; 4
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nop
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@bit: ; 6 jsr
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pha ; 3
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rol a ; 2
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and #1 ; 2
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sta JOY1 ; 4
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pla ; 4
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ror a ; 2
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.if CLOCK_RATE = 1789773
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nop ; 2
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.endif
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rts ; 6
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