Johnny's home-built CPU corner (and other stuff, maybe)
XBUS SRAM
This card holds both SRAM and ROM.
The SRAM part are a 512K chip, wasting away lot of usefull RAM but that was what I had on hand when I designed the board.
The ROM part are a parallel FRAM 8K memory from Ramtron. It's an old chip but available from Ebay and AliExpress.
Using a FRAM memory simplifies the process of updatering the ROM section a lot - I can write new code to it by closing a jumper, instead of having to rip the ROM from the board and use a separate programmer.
Moreover, this FRAM memory can also be written to directly from the 8080 itself, if necessary.
This version of the memory board do not support bank switching.
The FRAM has a special timing requirement to accept writing information to it, but fortunately I, by pure luck, happened to built that into the 8080 memory handler from the beginning. FRAM where not on the table at that time.
There are a jumper to select how much FRAM/ROM memory to use at the bottom of the 64K memory space, the rest are filled in by the SRAM chip.
On the top right there are a manual jumper that has to be closed to allow write access to the FRAM, in conjuction with the write protection circuit.
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