34 lines
1014 B
ArmAsm
34 lines
1014 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 304
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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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// - http://www.zimmers.net/anonftp/pub/cbm/programming/serial-bus.pdf
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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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UNTLK:
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// According to serial-bus.pdf (page 15) this routine flushes the IEC out buffer
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jsr iec_tx_flush
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// Buffer empty, send the command
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lda #$5F
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common_untlk_tksa: // common part of UNTLK and TKSA
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sta TBTCNT
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jsr iec_tx_command
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bcs !+ // branch if error
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// XXX - for TKSA, is it really the right place to do a turnaround? I've got some doubts
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// (it forces us to send the TKSA command 'manually' sometimes), but
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// Luigi Di Fraia (https://luigidifraia.wordpress.com/2017/06/27/codebase64-on-the-kernal-behaviour/)
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// claims TKSA Kernal routine actually does the turnaround
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jmp iec_turnaround_to_listen
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!:
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rts
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