41 lines
1.1 KiB
ArmAsm
41 lines
1.1 KiB
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 IEC_TMP2 ($A4 is a byte buffer according to http://sta.c64.org/cbm64mem.html)
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;; Carry flag set = signal EOI
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;;
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;; Preserves .X and .Y registers
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iec_tx_command:
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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 command (pulled ATN)
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jsr iec_pull_atn_clk_release_data
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;; Give devices time to respond (response is mandatory!) by pulling DATA
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jsr iec_wait1ms
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lda CIA2_PRA ; pulled DATA = highest bit 0 = 'plus'
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bpl +
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jmp iec_return_DEVICE_NOT_FOUND
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*
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;; At least one device responded, but they are still allowed to stall
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;; (can be busy processing something), we have to wait till they are all
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;; ready (or bored with DOS attack...)
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;; Release back CLK, keep DATA released - to ask devices for status.
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jsr iec_release_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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clc ; Carry flag set is used for EOI mark
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jmp iec_tx_common
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