First zeddy boots but....

[bwinkel67]

Normal ZX81 memory map:
0x0000 to 0x1FFF, 0 to 8191 - ROM
0x2000 to 0x3FFF, 8192 - 16383 - shadow/echo of the ROM
0x4000 to 0x7FFF, 16384 - 32767 - RAM
0x8000 to 0x9FFF, 32768 - 40959 - shadow/echo of the ROM
0xA000 to 0xBFFF, 40960 - 49151 - shadow/echo of the ROM
0xC000 to 0xFFFF, 49152 - 65535 - shadow/echo of the RAM, used by the hardware to produce the display

Mark

Question on this memory map. How does one use the shadow ROM area as RAM? I ask because the ZX81 Ram Disk Operating System seems to suggest it uses that area but I'm confused as to how it does so, unless there is extra hardware associated with it.

[1024MAK]

The reason for the full memory map despite there only being 8K of ROM and either 1K (ZX81), 2K (TS1000) or up to 16K of RAM (Sinclair 16K RAM pack) is due to partial address decoding.

To use the shadow/echo area where the ROM appears, logic on the expansion has to drive /ROMCS high. That disables the ROM in the computer at the appropriate addresses (as determined by the address decoding in the expansion). The expansion can then enable/select its RAM or ROM in that area.

And all expansion RAM such as Sinclair’s 16K RAM pack drive /RAMCS high to disable the internal RAM.

However, note that the shadow/echo of the RAM at the top has to be maintained when the Z80 executes an instruction cycle. As this is how the video display is generated. But extra RAM can be used in this area for data storage. Extra control circuitry is needed to control it though.

Mark