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revaldinho / cpc_ram_expansion

A series of Amstrad CPC PCBs including a backplane, ROM and 512K and 1MByte RAM expansions.
GNU General Public License v3.0
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464 RAM Card: KC-Compact unexpected behaviour & non functional shadow mode #2

Closed revaldinho closed 1 year ago

revaldinho commented 5 years ago

This is an enhancement request for the KC-Compact.

The documentation currently shows the 464 card to be compatible with the KC-Compact in overdrive mode only, providing a full 512KB DK'Tronics expansion. Shadow mode is not supported.

Rennert reports on compatibility testing with the Robotron KC-Compact

  1. Card is working in both overdrive (DIP=0100) and non-overdrive modes (DIP=0000).

    • Verified by Duke's RAM512KB.BIN test
    • Reports DK'Tronics C3 video mode operation
    • Notes that using FutureOS in C3 mode cursor trail differs when enabling overdriving

  2. Card doesn't work with shadow mode enabled - KC-Compact fails to start.

Item 1 isn't a problem since the card is working well with both settings, but it is unexpected.

The KC-Compact is a clone of the CPC464 and it is expected that the overdrive mode would have to be used to write protect the base RAM when writing to expansion banks. Rennert's report indicated that the base RAM is write protected simply by asserting RAMDIS as in a 6128.

Item 2 is related. Currently shadow mode is designated as unsupported in the documentation, but again if the KC-Compact is a true clone then this mode should work. Possibly a CPLD code change might be able to improve compatibility here.

Actions:

Need to check the KC-Compact schematics to understand:

  1. how the base RAM is write protected during external RAM accesses
  2. why shadow mode (A15 overdriving) doesn't work and whether a CPLD change might also allow the card to support this machine too.