Started by PulkoMandy, 14:48, 14 October 21
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Quote from: PulkoMandy on 14:48, 14 October 21A question for you all electronics electronics hacker out there. I'm looking at the schematics of the CPC and as far as I can see, the RAMDIS signal only disable reads from the main memory on the 464 and 664. On the 6128 on the Plus this was changed as the signal from the expansion port goes through the memory management PAL. However, I know there are several memory expansions that work just fine on these machines. Do they do anything special to prevent writes to the internal RAM when external RAM is mapped? How does this work?
QuoteThe 464/664 does not have the MMU PAL as seen in the 6128 as there is no memory expansion to deal with. The only controls on a 464/664 is with the 74LS153 address selection chips between the 6845 and Z80.The external memory expansion which implement the DK'Tronics standard use the RAMDIS to disable the internal memory so that accesses is correct on the 464/664.
QuoteThere is also the RAMROM which is a bit of an oddity, it is RAM in that it allows you to program it, but when in normal mode you lock it with a switch and all writes go to RAM even when ROM is enabled.
Quote from: rpalmer on 23:45, 14 October 21As for "C3" mode (correctly is defined as mode '3') ...
Quote from: rpalmer on 23:45, 14 October 21As for "C3" mode (correctly is defined as mode '3') as mentioned by others, is handled by completely disabling the internal memory so that all accesses go the external unit meaning the external memory looses 64K of its total expansion memory.
Quote from: zhulien on 20:49, 15 October 21Is mode 3 the reason CPM+ (unpatched) locks up on a 464 and 664 with additional RAM?
Quote from: TotO on 19:56, 15 October 21The 64K of the RAM expansion are only lost on the Revaldinho's memory expansion (shadow memory). It is a choice to be able to use this mode while the ROM is mapped.
Quote from: rpalmer on 00:00, 18 October 21This means mode &c3 (11000011) acts in the same as &D3 (11010011).
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