38 lines
668 B
ArmAsm
38 lines
668 B
ArmAsm
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;;
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;; Official Kernal routine, described in:
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;;
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;; - [RG64] C64 Programmer's Reference Guide - page 279/280
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;; - [CM64] Compute's Mapping the Commodore 64 - page 224
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;;
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;; CPU registers that has to be preserved (see [RG64]): .X, .Y
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;;
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ciout:
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pha
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lda C3PO
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bne ciout_send_byte
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;; Nothing to send - set buffer as valid, store byte there
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inc C3PO
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;; Clear carry flag to indicate success
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clc
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ciout_store_in_buffer:
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;; Store in the buffer data to be sent next, return
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pla
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sta BSOUR
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rts
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ciout_send_byte:
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clc ; don't send with EOI
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jsr iec_tx_byte
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inc C3PO ; iec_tx_byte sets it to 0, but we still have something to send
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jmp ciout_store_in_buffer
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