[TUHS] x/y/zmodem on Unix

John Cowan cowan at ccil.org
Tue Sep 19 13:58:42 AEST 2023

On Mon, Sep 18, 2023 at 7:08 PM Dan Cross <crossd at gmail.com> wrote:

but there was no total byte
> count, so it was assumed that the final block would be padded with a
> throwaway character.

CP/M, like RT-11 and OS/8 before it, did not track the sizes of files in
bytes or words, only in blocks.  Text files ended in ^Z and were then
padded with either NULs or more ^Zs if necessary; binary files were usually
padded with NULs.

ZMODEM was, as I understood it, designed for transfers across telenet,
> which was pretty reliable; instead of the highly synchronous
> send/wait-for-ack cycle of xmodem and ymodem, zmodem relies on error
> detection and correction and is basically a streaming protocol: a
> packet in a sliding window could be NAK'ed, thus rewinding the
> transfer, but otherwise it basically just sends data until done.

It's an analogue of TCP/IP, in fact.
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