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explaining the memory map
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@@ -160,3 +160,16 @@ Also I might, for a fee, program (and test) a 27C256 EPROM for you, if you do no
I wrote a small document about the use of jumpers on the zx81+38, what they do, where they are on the board etc.
But note that normally you don't have to change a single jumper, they are all pre-set with a small wirelink, you must cut to change the setting.
This document is just for the more curious people, or for those who do not want to use a 27C256, perhaps because they want to use the original zx-81 ROM
# The memory map of a ZX-80 or ZX-81
I'm writing this as this information is hard to find elsewhere.
The ZX81 memory map is divided into four 16K parts.
The first (lower) 16K part, contains the ROM (as it’s only 8K, you would normally get a echo of the first 8K in the second 8K, but the 27C256 contains 32K, so you can put something else there). this ROM-space is normally set to 16K default, and you can use the second 8K for ROM expansions, such as the SD-Card reader firmware on the (PSG) sound + SD-card expansion board that I'm planning to design.
The second 16K part, is the normal RAM area.
The third 16K of a real ZX-81 will have copies of the ROM, but the ZX-81+38 will have copies of the RAM, as ROM space is limited to the first 16K.
The fourth and last 16K, is more echoes of the RAM. However, it’s important to note that the hardware detects this, and uses it for the video generation logic.