Monday, September 19, 2011


Much recent development on the ZX Prism has been to integrate the video modes from the orignal (48K) Prism development into the new 256K one. This has thus far been pain free but of course there's nothing new to show! The only component I'd implemented in the original Prism which hasn't been integrated yet is the palette memory, and that's next on my list.

Remembering the issue with "offset" screen attributes in the original paletted solution, and having identified some other timing issues relating to the video output and interrupts, I'm rewriting the video creation module including making the clock resolution higher. This allows me to fit the palette lookups into the standard video read/store cycle. It should also mean a resolution of 512x384 will be possible.

The new video module will be written with the additional screen resolutions and colour decoding modes in mind (rather than them being retrofitted to Mike Stirling's video module).

Whilst designing the new video module, ideas for additional video modes have arisen, including an "overlay" mode where the shadow screen is used as a background for the main screen - any areas of the main screen which are colour 0 will allow the shadow screen to show through. This could be used, for instance, for games - where the sprites are animated on the main screen, and coloured backgrounds are drawn on the shadow screen. There'd still be attribute clash between the sprites, but not between the sprites and the background.  It's only 3 colours rather than two in each 8x8 attribute square, but it would provide a relatively easy way of sprucing up existing software.


  1. Hi.

    Myself and another chap (Andy Key)are currently in the feasibility/Early stages of putting a Memotech MTX512 onto a FPGA board.

    Having looked at your website I was VERY impressed at what you’d done for the ZX Spectrum!

    However we are having some timing issues and incompatibility problems with the T80. As it seems not to work like a true Z80A!

    I wondered if it was possible to ‘pick’ your large Brain and get some hints on what may be the problem ??

    Any guidance and help would be much appreciated.

    My Website is :

    One Chip Memotech MTX512 Computer

    Andy Key's site

    Andys Memotech site - Links ReMEMOTECH

    Kind Regards


    1. Hi Lez,
      Sorry for the delay in replying! I always wanted a Memotech back in the day, but never saw one anywhere except Your Computer magazine!

      I'm also having big issues with T80 - to the point where I'm going back to using a real Z80. the issues I've had were related to timing, and the way t80 seems to handle interrupts differently.

      Sorry I probably can't be much help :/

  2. If you remember the T80 is clocked by CLK_n, the timings you see in SignalTap seem to look close to those in thr Zilog manual.

    I have written something approximating at Z80 CTC and as far as I have been able to test thus far, I have it working with the T80.
    Little bit of a handshake going on between them.

    See REMEMOTECH link above.

    {{{ Andy