39 lines
825 B
ArmAsm
39 lines
825 B
ArmAsm
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;; Implemented based on https://www.pagetable.com/?p=1135, https://github.com/mist64/cbmbus_doc
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;; Expects byte to send in BSOUR; Carry flag set = signal EOI
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;; Preserves .X and .Y registers
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iec_tx_byte:
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;; Store .X and .Y on the stack - preserve them
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txa
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pha
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tya
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pha
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;; Notify all devices that we are going to send a byte
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;; and it is going to be a data byte (released ATN)
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jsr iec_release_atn_clk_data
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;; Common part of iec_txbyte and iec_tx_common - waits for devices
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;; and transmits a byte
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jsr iec_tx_common
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;; Give device time to tell if they are busy by pulling DATA
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;; They should do it within 1ms
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ldx #$FF
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* lda CIA2_PRA
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;; BPL here is checking that bit 7 clears,
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;; i.e, that the DATA line is pulled by drive
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bpl +
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dex
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bne -
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bpl iec_return_DEVICE_NOT_FOUND
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*
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jmp iec_return_success
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