tape timing improvements

c64/kernal/tape/load_tape_normal.s

@@ -7,21 +7,23 @@
 // We have the following possible CPU frequencies:
 // - C64    PAL:  CPU frequency 0.985248 MHz
 // - C64    NTSC: CPU frequency 1.022727 MHz
-// Average is 1.0039875 MHz and we will take this value for calculation (but it really does not change much).
+// The clock difference is about 4% - it is unlikely to cause problems.
 //
 // We have to distinguish the following pulse lengths (http://sidpreservation.6581.org/tape-format/):
-// - S (short)    352 us
-// - M (medium)   512 us
-// - L (long)     672 us
+// - S (short)    tap value $30 (PAL: 352 us)
+// - M (medium)   tap value $42 (PAL: 512 us)
+// - L (long)     tap value $56 (PAL: 672 us)
 //
-// This gives us the following thresholds
-// - S vs M       432 us (108 or $6C 4-tick periods)
-// - M vs L       592 us (149 or $95 4-tick periods)
+// The TAP format (http://unusedino.de/ec64/technical/formats/tap.html) uses a resolution of 8 PAL ticks;
+// this implementation sets the timer for twice the precision (timer B is triggered every 4 CPU cycles),
+// probably an overkill, but in realityy it does not cost us anything.
+//
+// So -= we have the following thresholds:
+// - S vs M       $72 (in our units of 4 ticks)
+// - M vs L       $98 (in our units of 4 ticks)
+//
+// We need to recognize the following pulse sequences:
 //
-// 4-tick periods should be enough - TAP file format description (http://unusedino.de/ec64/technical/formats/tap.html)
-// tells it uses exactly 8 PAL ticks as a resolution
-
-// The following encoding is used:
 // (S,M) = 0 bit
 // (M,S) = 1 bit
 // (L,M) = new-data marker

c64/kernal/tape/tape_normal_get_byte.s

@@ -1,6 +1,6 @@
 
 //
-// Tape (turbo) helper routine - byte reading
+// Tape (normal) helper routine - byte reading
 //
 // Returns byte in .A, Carry flag set = error
 //
@@ -11,19 +11,13 @@
 
 tape_normal_get_byte:
 
-	// Init byte checksum and canary bit
+	// Init parity bit and canary bit
 	lda #$01
 	sta PRTY
 	lda #%01111111                     // canary bit is 0
 	sta INBIT
 
-	// First we need a byte marker - (L,M)
-!:
-	jsr tape_normal_get_pulse
-	cmp #($FF - $95 - $04)
-	bcs !-                             // too short for a long pulse
-	jsr tape_normal_get_pulse
-	bcs !-                             // too short for a medium pulse
+	jsr tape_normal_get_marker         // XXX this is probablyy not the proper place 
 
 	// Now fetch individual bits
 
@@ -44,7 +38,7 @@ tape_normal_get_byte_loop:
 	ror INBIT                          // put the bit in
 	bcs tape_normal_get_byte_loop      // loop if no canary bit reached
 
-	// Byte retrieved (on stack), now we need to validate the checksum
+	// Byte retrieved (on stack), now we need to validate the parity
 
 	jsr tape_normal_get_bit
 	bcs tape_normal_get_byte_error

c64/kernal/tape/tape_normal_get_marker.s

@@ -0,0 +1,16 @@
+
+//
+// Tape (normal) helper routine - marker reading
+//
+// Reeturns marker type in Carry flag, set = end of data
+//
+
+tape_normal_get_marker:
+
+	// (L,M) - end of byte, (L,S) - end of data
+
+	jsr tape_normal_get_pulse
+	cmp #($FF - $98 - $04)
+	bcs tape_normal_get_marker                             // too short for a long pulse
+
+	jmp tape_normal_get_pulse

c64/kernal/tape/tape_normal_get_pulse.s

@@ -1,6 +1,6 @@
 
 //
-// Tape (turbo) helper routine - pulse reading
+// Tape (normal) helper routine - pulse reading
 //
 // Returns pulse length (with a bias, see comments) in .A
 //
@@ -34,7 +34,7 @@ tape_normal_get_pulse:
 	// this is total 17 cycles, or $4 periods of 4 ticks (timer A) each - we need to take it into account
 	// every time we check the pulse length
 
-	cmp #($FF - $6C - $04)            // short vs medium is the default check
+	cmp #($FF - $72 - $04)            // short vs medium is the default check
 	rts