This is an old post, preserved for reference.
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Here we have a ZX81 with an unusual RAM chip. It is a MK4801AP-5, a 1K static RAM made by Mostek (not to be confused with MOS technology owned by Sinclair's arch rivals Commodore).
this article for more info. The easier option is just to ignore half of the chip and get 16K RAM. That is fine because most of the software is designed for 16K.
K. So far so good. The usual test is to run
- PRINT PEEK 16388+256*PEEK 16389
UPDATE 2: Some more photos, this is the memory size check:
UPDATE 3: See also a version of this mod which does not require tracks to be cut.