Zx81 ram pack

Discussions about Sinclair ZX80 and ZX81 Hardware
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1024MAK
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Re: Zx81 ram pack

Post by 1024MAK »

Hard, no. But that does need extra circuitry unless you use a ULA replacement that has additional functionality built in as standard.

Remember, the Sinclair ULA ignores address line A15. Because of the partial address decoding the ROM chip appears in the Z80 memory range (map) four times, 0x0000 to 0x1FFF, 0x2000 to 0x3FFF, 0x8000 to 0x9FFF and 0xA000 to 0xBFFF. And the top area of RAM (0xC000 to 0xFFFF) has to be an ‘echo’ of the RAM between 0x4000 to 0x7FFF so that the Z80 can execute the display file.

To have RAM between 0x2000 to 0x3FFF, or between 0x8000 to 0xBFFF, the extra circuitry needs to disable the ROM and enable the RAM chip.

To have RAM (for data storage only) in the 0xC000 to 0xFFFF area is easier.

In practice, there is very little commercial software that needs or can make use of more than 16k bytes of RAM. Hence most users are happy with 16k bytes. And although it appears to be a waste by using a larger SRAM chip, it’s not really when you consider that 8k byte SRAM chips are a similar price, as are 128k SRAM chips…

Mark
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