Most sideways ROM boards for the BBC microcomputer would let you plug in one RAM chip instead of a ROM. Then you could write your own ROM image, and it would work exactly as if that ROM was plugged in. Of course, you had to get the image somehow. While some people pirated ROMs, it was certainly possible to write your own image, or type it in from a magazine like Micro User or Acorn User. In fact, some images had to run from RAM, as they wrote back to the image!

Some boards would let you plug in a CMOS 62c64 (or 62c128) instead of a 6264 (or 62128), and would then keep it powered using a battery backup. This was not so good: if the ROM was corrupted in such a way as to cause the OS to recognise it on powerup, you had to take the battery out to get the computer to boot!

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