added some comments to tape support coode

c64/kernal/tape/load_tape_normal.s

@@ -4,6 +4,28 @@
 //
 
 
+// We have the following possible CPU frequencies:
+// - C64    PAL:  CPU frequency 0.985248 MHz
+// - C64    NTSC: CPU frequency 1.022727 MHz
+// - VIC-20 PAL:  CPU frequency 1.108405 MHz
+// - VIC-20 NTSC: CPU frequency 1.022727 MHz
+// Since VIC-20 tapes are not expected to be loaded often, routine will be optimized for C64 timings
+
+// 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
+// This gives us the following thresholds (where average CPU frequency is 1.0039875 MHz):
+// - S vs M       432 us (54 8-cycle periods)
+// - M vs L       592 us (74 8-cycle periods)
+
+// The following encoding is used:
+// (S,M) = 0 bit
+// (M,S) = 1 bit
+// (L,M) = new-data marker
+// (L,S) = end-of-data marker
+
+
 #if CONFIG_TAPE_NORMAL