44 lines
1.1 KiB
ArmAsm
44 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 TBTCNT ($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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#if CONFIG_IEC
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iec_tx_command:
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// Store .X and .Y on the stack - preserve them
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phx_trash_a
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phy_trash_a
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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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#endif // CONFIG_IEC
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