82 lines
1.8 KiB
ArmAsm
82 lines
1.8 KiB
ArmAsm
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//
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// Tape (normal) part of the LOAD routine
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//
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// We have the following possible CPU frequencies:
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// - C64 PAL: CPU frequency 0.985248 MHz
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// - C64 NTSC: CPU frequency 1.022727 MHz
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// Average is 1.0039875 MHz and we will take this value for calculation (but it really does not change much).
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//
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// We have to distinguish the following pulse lengths (http://sidpreservation.6581.org/tape-format/):
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// - S (short) 352 us
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// - M (medium) 512 us
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// - L (long) 672 us
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//
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// This gives us the following thresholds
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// - S vs M 432 us (108 or $6C 4-tick periods)
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// - M vs L 592 us (149 or $95 4-tick periods)
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//
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// 4-tick periods should be enough - TAP file format description (http://unusedino.de/ec64/technical/formats/tap.html)
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// tells it uses exactly 8 PAL ticks as a resolution
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// The following encoding is used:
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// (S,M) = 0 bit
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// (M,S) = 1 bit
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// (L,M) = new-data marker
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// (L,S) = end-of-data marker
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#if CONFIG_TAPE_NORMAL
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load_tape_normal:
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jsr tape_ditch_verify // only LOAD is supported, no VERIFY
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// Setup CIA #2 timers
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ldx #$03 // set timer A to 4 ticks
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stx CIA2_TIMALO // $DD04
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ldx #$00
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stx CIA2_TIMAHI // $DD05
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stx CIA2_TIMBHI // $DD07
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dex // puts $FF - for running timer B as long as possible
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stx CIA2_TIMBLO // $DD06
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// Let timer A run continuously
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ldx #%00010001 // start timer, force latch reload
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stx CIA2_CRA // $DD0E
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// Start playing
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jsr tape_clean_sid
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jsr tape_ask_play
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// FALLTROUGH
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load_tape_normal_header:
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// XXX replace this placeholder by a real routine
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jsr tape_load_get_pulse
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ldx #$00
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cmp #($100 - $6C - $05)
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bcs !+
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#if CONFIG_COLORS_BRAND && CONFIG_BRAND_ULTIMATE_64
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ldx #$0B
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#else
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ldx #$06
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#endif
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!:
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stx VIC_EXTCOL
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jmp load_tape_normal_header
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#endif
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