Date   : Mon, 03 Jan 2011 14:26:32 +0000
From   : percy.p.person@... (Ed Spittles)
Subject: OT: ULA book / spectrum serial port

(Now 100% off topic, but surely bit serial decoding at assembly level
is interesting...)

> So you'd also need to look
> into the Spectrum ROM, which this book doesn't cover.

But see Geoff Wearmouth's disassembly:
cassette section:
http://www.wearmouth.demon.co.uk/zx82.htm#L04AA
save 8 bits:
http://www.wearmouth.demon.co.uk/zx82.htm#L0525
load 8 bits:
http://www.wearmouth.demon.co.uk/zx82.htm#L05CA
distinguish a single bit of input:

; -------------------------
; Check signal being loaded
; -------------------------
;   An edge is a transition from one mic state to another.
;   More specifically a change in bit 6 of value input from port $FE.
;   Graphically it is a change of border colour, say, blue to yellow.
;   The first entry point looks for two adjacent edges. The second entry point
;   is used to find a single edge.
;   The B register holds a count, up to 256, within which the edge (or edges)
;   must be found. The gap between two edges will be more for a '1' than a '0'
;   so the value of B denotes the state of the bit (two edges) read from tape.

http://www.wearmouth.demon.co.uk/zx82.htm#L05E3

Cheers
Ed