6 MHz CPC

The 6 MHz CPC is a DIY hardware modification which allows to run the CPC6128 with a CPU speed of 6 MHz instead of the usual 4 MHz.

Technic

The 16 MHz crystal of the CPC is replaced by a 24 MHz crystal. Both crystals should kept on the main board, but a switch (2 lines at least, better 3) selects between them. You have to select the CPU and bus speed before you switch on the CPC. Switching the speed while the system is switched on is not advised. None of the authors of this article is responsible for danger on your CPC, in case you try to use this DIY.

Advantages

• Z80 runs with 6 MHz
• Bus runs with 6 MHz
• FDC is 50% faster
• Disc formats with 50% more sectors can be used
• Sound is better, since PSG runs more quick
• Games run more fluid

Disadvantages

• Not every hardware expansion is able to work with the 6 MHz bus speed
• FDC can't read old disc formats any longer
• Sound must be reprogrammed

Software

• A lot of games run well with 6 MHz. For example: Nebulus, StarFox, Starstrike and others.
• There is a 6 MHz version of FutureOS

Advanced testing

• Only horizontal timing of the CRTC is affected. In order to get a stable image, you need to program register 0 at 95 (96 characters line) and Register 2 at 61 (centering for 40 characters default)
• Pixels pitch is 66.7% of their original pitch.Theses pictures show this fact (320×200 mode 1 pixels with black border)

• 

  Original CPC @16Mhz

• 

  Overclocked CPC @24Mhz

• This speed improvement is 16.66667%.This has been tested with this simple program :

10 i = 0
20 after 500 goto 100
25 cls
30 i = i +1
40 ? i
50 goto 30
100 end

Score is 258 on normal CPC versus 301 on 24 MHz CPC.

(Theses results can be subject to discussion as we are not sure interrupts timing is not affected)