the full description of B protocol is available on page VID-17, i believe. it
is a 100K file, describing all aspects of the protocol.
there are also several files on A protocol in the Programmers' Forum. their DL
2 even contains the source for COMSH, a COMunications SHell for MS-DOS, which
implements both XMODEM and A protocols. can't remember the page number for that
forum, however.