61 lines
1.4 KiB
ArmAsm
61 lines
1.4 KiB
ArmAsm
// #LAYOUT# STD * #TAKE
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// #LAYOUT# * KERNAL_0 #TAKE
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// #LAYOUT# * * #IGNORE
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//
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// Tape (turbo) helper routine - byte reading
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//
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// Returns byte in .A
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//
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// Based on routine by enthusi/Onslaught, found here:
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// - https://codebase64.pokefinder.org/doku.php?id=base:turbotape_loader_source
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#if CONFIG_TAPE_TURBO
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tape_turbo_get_byte:
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phx_trash_a
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lda #$01
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sta INBIT // init the to-be-read byte with 1 (canary bit to mark loop end)
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!:
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jsr tape_turbo_get_bit
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rol INBIT
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bcc !- // is the initial 1 shifted into carry already?
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plx_trash_a
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// Compensate for tape speed variations
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lda __turbo_half_S // half of the last value for bit '0'
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clc
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adc __turbo_half_L // half of the last value for bit '1'
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sec
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sbc __pulse_threshold // now we have a diff between current threshold and calculated one
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beq tape_turbo_get_byte_done // branch if threshold correction not needed
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bpl !+
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lda __pulse_threshold
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cmp #($BF - 10)
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beq tape_turbo_get_byte_done // do not decrease threshold too far no matter what
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dec __pulse_threshold
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bne tape_turbo_get_byte_done // branch always
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!:
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lda __pulse_threshold
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cmp #($BF + 10)
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beq tape_turbo_get_byte_done // do not increase threshold too far no matter what
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inc __pulse_threshold
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tape_turbo_get_byte_done:
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lda INBIT
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rts
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#endif // CONFIG_TAPE_TURBO
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