98 lines
2.1 KiB
ArmAsm
98 lines
2.1 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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// Tries to detect
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//
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// Carry clear - normal, Carry set - turbo
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//
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#if CONFIG_TAPE_AUTODETECT
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tape_common_autodetect:
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// Set screen color to something neutral, silence the audio
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lda #$0B
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sta VIC_EXTCOL
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lda #$00
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sta SID_SIGVOL
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// FALLTROUGH
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tape_common_autodetect_retry:
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// It is not trivial to distinguish between normal and turbo,
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// as short pulses of normal (and sync uses only shorts)
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// have similar lengths as long pulses of turbo
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// Thus, the routine here reads certain amount of pulses
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// looking for evidence for either normal or turbo system;
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// in case of no evidence found, assume normal
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lda #$00 // plus = probably normal, minus = turbo
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sta XSAV
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ldy #$60 // $60 pulses cover 12 bytes of turbo recording
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jsr tape_common_get_pulse // first pulse might be a garbage, ignore it
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// FALLTROUGH
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tape_common_autodetect_loop:
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jsr tape_common_get_pulse // read pulse for detection attempt
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// If garbage found, retry detection
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cmp #$28
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bcc tape_common_autodetect_retry // this is a VERY long pulse
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cmp #$E8
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bcs tape_common_autodetect_retry // this is a VERY short pulse
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// First try to distinguish turbo short signal (reference: $CD)
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// from normal signals (reference: $A5 or lower), these values
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// give us threshold of ($CD + $A5) / 2 = $B9
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cmp #$B9
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bcc !+
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dec XSAV // above $B9 (so signal is shorter), looks like turbo
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!:
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// Now try to distinguish turbo signals (reference: $B1 or higher)
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// from normal medium/long signals (reference: $7D or below), these values
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// give us threshold of ($B1 + $7D) / 2 = $97
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cmp #$97
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bcs !+
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inc XSAV
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!:
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// The sync for turbo is not too long - if we have enough evidence,
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// assume turbo recording already
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lda XSAV
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cmp #$FD
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beq tape_common_autodetect_turbo
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// Next iteration
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dey
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bpl tape_common_autodetect_loop
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// Last chance for turbo
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lda XSAV
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bmi tape_common_autodetect_turbo
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// FALLTROUGH
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tape_common_autodetect_normal:
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clc
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rts
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tape_common_autodetect_turbo:
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sec
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rts
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#endif
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