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A > ACORN COMPUTER  > Electron

Acorn Computer

More information about how the ULA works from Michael Harris (England):

The RAM read / write went through the custom ULA. The ULA also handled video and memory freshes among other things. This lead to the behaviour of the Electron running at slower MHz speeds depending on the CPU executing code in ROM or RAM, plus the video mode it was using at the time.

Using its best graphics modes (0 - 3) ment that the ULA would block the CPU more often for display RAM access. This would effect interrupt timing and sometimes lead to tape loading errors in these graphics modes. Disk access had a higher priority than the screen refresh which led to "snow" on the screen (later disk filing system code would blank the screen temporarily).

Due to the architecture of the ULA it prevented the Electron from being able to do smooth horizontal hardware scrolling. Due to it not as programmable as the BBC's 6845 CRTC chip, it could not create custom graphics modes.

The expansion port had all the CPU bus and interrupt lines so was as potentially as expandable as the BBC. Using BBC second processors it was technically possible to expand to a Z80 CP/M or even a hard disk UNIX setup. It is highly unlikely that this was ever done though.

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