ACA1221LC + ACA500Plus memory

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Don't Panic. Please wash hands.
  • Hello,

    I have the ACA1221LC + ACA500Plus config on a Rev6a A500 with 0,5 MB chip + 0,5 MB slow RAM.

    In the booklet that came with the ACA1221LC, it says that there's 11,5 MB usable RAM on the card. But my system sees 8 MB + 1,5 MB fast RAM.

    1. Why doesn't it sum up to 11,5 MB (I assume that chip ram comes from the original machine, but even if it doesn't it still adds up to 10,5 MB)?

    2. What is this separate 1,5 MB chunk of memory?

    3. Is it possible to somehow utilise the memory on ACA500Plus?


    Cheers! Sorry for noobish questions.

  • The 68ec020 CPU has limited address space - the same size as the "small" 68000 CPU. It is not possible to use both the on-board fastram of the ACA500plus and the fastmem of the ACA1221lc at the same time.


    It wouldn't make sense either, as the memory of the ACA500plus is considerably slower: It is only 16 bits wide, so a "long" access (for 32 bits) requires the CPU to make two separate accesses, and these wouldn't even happen at full CPU speed, but at slowed-down 14MHz bus speed of the A1200 connector.


    The whole 11,5MB can only be used on an A1200, as the ACA500plus places some of the registers in an area where the ACA1221lc can place additional memory. You can read all the technical details in the register description of the card.


    Yes, it is possible to access the "underlying" memory, and even more memory than the 11,5MB max. fastmem (note that the ACA1221lc has a total of 16MByte physical memory) - this would require special software, though. We do not provide that software, but the register documentation shows you how it's done: Either by shuffling around address spaces to gain access to "the other 5.5MB", or by switching off the ACA1221lc fastmem, which will give way to the ACA500plus memory.


    The best idea I'd have for that use would be a RAM disk, because it's not possible to map single chings of memory in for code execution. It's "all or nothing", so you could think of a block-wise access. If you took the time to program that...