31 lines
714 B
ArmAsm
31 lines
714 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 303
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;; - [CM64] Compute's Mapping the Commodore 64 - page 224
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;; - https://www.pagetable.com/?p=1031, , https://github.com/mist64/cbmbus_doc
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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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unlsn:
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;; This routine is documented in 'Compute's Mapping the Commodore 64', page 224
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;; First check whether we have something to be sent in the buffer
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lda C3PO
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beq +
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sec ; send it with EOI
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jsr iec_tx_byte ; send the command regardless of the status
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*
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;; Buffer empty, send the command
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lda #$3F
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sta BSOUR
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jsr iec_tx_command
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bcs + ; branch if error
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jmp iec_tx_command_finalize
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*
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rts
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