Memory expansion hack for the BBC microcomputer. Not to be confused with sideways RAM (and certainly not with sideways ROM), this would essentially page the top 20K of RAM, which was used for the display. Of course, the 6502 (and 65c02) used in the beeb had nothing remotely like page mapping. Instead, the following clever hardware scheme was used:
- Well-behaved programs only accessed the screen via the OS, by calling the video driver.
- The OS was always in the address range &C000-&FFFF.
- The hardware switched the shadow RAM in and out. When PC (the program counter) was in the OS range, shadow RAM was out; when it was in the application range, shadow RAM was in.
Of course, game
s and the like weren't well-behaved applications, so you'd need to switch off (in software
, of course) the shadow RAM before running. But a surprising number of applications (and even sideways ROM
s!) worked with shadow RAM.