CPC ROM question

[zhulien]

Hi,

In a CPC,  are the external upper ROMs treated as ROM by the CPC, or is it only treated as ROM because it is ROM?  i.e. if it were changed to RAM, would writes go to it or does the GA prevent writes going to the ROMs?

[eto]

Afaik the ROMEN signal will not be low during a write, so it will still go to RAM. 

[andycadley]

The design of the CPC also relies on write-through being a thing. I.e. when ROM is paged in, it is expected (and required) that writes will still go to the underlying RAM that is present. Committing those writes to ROM instead of or as well as RAM would probably have unintended consequences.

[GUNHED]

Hi,

In a CPC,  are the external upper ROMs treated as ROM by the CPC, or is it only treated as ROM because it is ROM?  i.e. if it were changed to RAM, would writes go to it or does the GA prevent writes going to the ROMs?

It purely depends on the kind of expansion you use!
- EPROM will always stay ROM
- Flash can be rewritten, but won't work like RAM (because complete block must be written before they can be read)
- And the static RAM of the ROM-RAM-Box (RRB) and the Symbiface II can indeed be used as RAM. However, when writing to the SRAM of the RRB then the SRAM and the screen RAM will be written with the same value. In case of the SF2 the simulated ROM can be used as expansion RAM too, but nobody does (officially ;-))

[zhulien]

The design of the CPC also relies on write-through being a thing. I.e. when ROM is paged in, it is expected (and required) that writes will still go to the underlying RAM that is present. Committing those writes to ROM instead of or as well as RAM would probably have unintended consequences.

I actually take advantage of that with the SUPER ROM, |martha shows an image from ROM to screen RAM behind the ROM.