TS1000 Internal RAM: Part 4 (Full 32KB from BASIC)

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Published on November 3, 2023 11:43:44 AM ● Video Link: https://www.youtube.com/watch?v=Vv4TltCvYuM

Duration: 19:21

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Made a silly mistake last time (https://youtu.be/DMaPGB8HnQ8) - thanks to @filipstamate1564 for pointing it out. After increasing RAMTOP, I should've executed a NEW command.

After a few seconds, the machine returns and you can now access the full 32KB from BASIC (it seems) ... for example, reserving a 25KB array with DIM A(5000) works.

Summary:
[1] Increase RAMTOP with POKE 16389,255
[2] Execute NEW
[3] Check RAM with PRINT PEEK 16388+256*PEEK 16389
* Returned 49152 in my case, or could return 40960
[4] Test with DIM A(5000)
* Should return 0/0 rather than 4/0 (out of memory)

I was also very lucky to get some of Charlie's (vLA81 designer) 32KB RAM "shims". These are much simpler boards that slot in-between the 32KB RAM chip and the socket, meaning that they are also much lower profile.

I tested both Charlie's shim and my board and could verify that the full 32KB seems to be working ... when using Charlie's vLA81. When using an original ULA, some additional circuitry is required as the ULA doesn't fully decode the whole address range.

Some RAM size tests that can be done:
https://www.vintage-radio.net/forum/showthread.php?t=120165

My RAM board:
https://github.com/0ddjob/ZX81/tree/main/RAM_Upgrade

Another design with surface mount SRAM:
https://github.com/redhawk668/ZX81-Internal-16K-Expansion

vLA81 modern ULA replacement:
https://www.vretrodesign.com/products/vla81-zx81-ula-replacement