Thursday, July 18, 2013

ZX Prism 1024K mode Memory Map

This may go some way to explaining why ZX Prism's codename was "Project Overkill" for a while.

Some further explanation/clarification:

In 1024K mode, the memory page at 0xC000 is chosen using port 0x7FFD in the same way as the 1024K Pentagon (as documented in the ZX Profi documentation by Velesoft):
  • 7FFD bit 0 = Page # bit 0
  • 7FFD bit 1 = Page # bit 1
  • 7FFD bit 2 = Page # bit 2
  • 7FFD bit 6 = Page # bit 3
  • 7FFD bit 7 = Page # bit 4
  • 7FFD bit 5 = Page # bit 5
It should be noted that outside of 1024K mode, port 7FFD bit 5 when true works as expected (ie it locks the current memory configuration and prevents any further memory paging - known as "48K mode" in some circles).

Due to the way this locking works in ZX Prism, enabling 1024K mode from "48K mode" has the effect of re-enabling memory paging.


  1. you are not seriously aiming at 1MB of video ram?
    if you examine how the sam coupe asic gets 16 colours per scan line in hires mode 3
    you might be able to get 24bitru colour with only 1MB - 1080p
    still nothing from the border port with the screen switched off? bleed border colour into main pixel area?

  2. No, if you re-read the post carefully (and look at the diagram) you'll see that I said 64K video RAM (8K in 8 pages) which is held within the FPGA itself.

    I'm deliberately not looking at how the SAM Coupe does its video. This is not a SAM compatible project.

    As for the border port... if you read the next post (which you have because you've commented on it too), you'll see that there's a 256 colour mode which uses the border colour as "PAPER"and also a mode which ONLY displays the border colour across the whole display so yes, with the CPU set to a fast enough speed you could play around with that. I'm thinking of naming it Roger mode :*)