Fun fact: the ARM 1 was designed using some custom software on a BBC Micro and tested in simulation (sub 1KLOC) before they submitted the design for tape-out and production. First silicon actually worked!
Probably more like 64 KB, assuming 6502 second processor...
(Actual usable space is less! The second processor's onboard OS uses 2 KB. The BASIC interpreter uses 16 KB for its code plus another approx 1.1 KB for workspace.)
https://en.wikichip.org/wiki/acorn/microarchitectures/arm1