Editing Psion Series 5mx Pro (section)

== How it boots ==

The Pro shipped with a '''16 MB CompactFlash card''' carrying the OS image. The file is called '''<code>Sys$rom.bin</code>''' and is about 10 MB.<ref name="wuffs">Ash Wolf (Ninji), [https://wuffs.org/blog/building-a-psion-emulator "\"16 Shades of Grey\" — Building a Psion/EPOC32 Emulator"], 27 December 2019. Source for the 5mx Pro's boot-from-CompactFlash arrangement, the <code>Sys$rom.bin</code> file name and size, its structure, and the difference from the standard 5mx.</ref><ref name="grp">Community discussion of the differences between the Psion 5mx and 5mx Pro, comp.sys.psion.misc. Source for the absence of ROM, the reserved RAM region, the 16 MB CompactFlash card supplied with the machine, and the boot-time prompt offering CompactFlash or a yModem serial load.</ref>

'''If the machine has no operating system available, it will not simply fail.''' On startup it asks whether to '''insert a CompactFlash card containing the SystemRom''', or to '''make a serial connection and load the system over a yModem transfer'''.<ref name="grp" />

'''That prompt is not a fault.''' It is the single most important thing to know about a 5mx Pro, because a Pro bought without its card looks like a dead or corrupted machine to anyone expecting a normal 5mx.