Psiren is a toolkit for sucking a computer's brain out through a straw (see Red Dwarf). Specifically, it's a client-server system for transferring a disk image over a serial port — because sometimes you don't have a better option. The protocol it uses is deliberately simple and robust, to ease writing new clients.
psiren is the server, running on a modern machine. The protocol it uses is described in comments at the top of the file.
ArcSendHD.bas is (so far) the only client, for the Acorn Archimedes series of machines; it sends an image of the machine's hard disk. It was developed on an Acorn Archimedes A310 with a Computerware hard disk podule, using RiscOS 2.00; it should work on any RiscOS system.