Subj : docs/zmodem_comparison.md
To   : Git commit to main/sbbs/master
From : Rob Swindell (on Debian Linux)
Date : Sun Aug 30 2026 17:56:37

https://gitlab.synchro.net/main/sbbs/-/commit/60b70e526213285b61ae11f8
Modified Files:
	docs/zmodem_comparison.md
Log Message:
docs/zmodem: zmtx/zmrx supports CRC-16, it just cannot be asked for

The feature table listed CRC-16 as "--" for zmtx/zmrx, which reads as
not supported.  That is wrong in a way that matters to anyone pairing it
with a CRC-16-only peer.

want_fcs_32 is hard-coded true in zmdm.c and neither program offers a
CRC flag, so it cannot be asked to prefer 16-bit -- but the machinery is
present in both the header and data paths and the selection is
can_fcs_32 && want_fcs_32, so it falls back whenever the peer omits
CANFC32.  Distinguish "supported but not selectable" from "absent",
since sexyz and lsz can both force CRC-16 and zmtx/zmrx can only fall
into it.

Measured rather than read: zmtx to sexyz -o puts 4,311,016 bytes on the
wire against 4,312,065 with CRC-32, and sexyz -o to zmrx 4,310,744
against 4,311,816.  A 4 MiB file is 512 subpackets at 8 KiB and two CRC
bytes saved each is about 1,024, which is what those deltas are.  Both
verified byte-identical, so the fallback is real in both directions.

Also corrects a stale count in the prose above the table.
--- SBBSecho 3.37-Linux
 * Origin: Vertrauen - [vert/cvs/bbs].synchro.net (1:103/705)

.