Screen Modes

Sam Coupé | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

MODE 1:32x24 character cells per screen, each cell with 2 colour capability.
MODE 2:32x192 cells, each cell 2 colour capability.
MODE 3:512x192 pixels (85 column mode), reduced colour capability per line.
MODE 4:256x192 graphics screen.

z80B processor speed

varies in each of these different modes
mode 1 slowest
mode 2 slightly better
mode 3 and mode 4 slightly faster
screen off in mode 3 or 4 fastest - as fast as rom or external ram code

Re: z80B processor speed

Nearly. The Z80B clock speed is always 6MHz, but ram delays mean it can’t use all the cycles it gets.

  • Instructions running in external RAM always run at full CPU speed.

  • Instructions in internal RAM get roughly the full number of cycles, but all instruction lengths are rounded up to boundaries of 4 t-states (because this is the next time at which RAM is available for fetching the next instruction). Typical code will go about 10%-20% more slowly for this reason.
  • With the screen on in modes 2,3 or 4, you lose about 20% of available memory cycles
  • With the screen on in modes 1 you lose about 35 % of available memory cycles

Some instructions which don’t use memory cycles are not affected by some of these conditions, so they can appear to go faster than would otherwise be expected. The full detail is rather complicated and was the subject of a very good article in Based On An Idea…

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