Date   : Sun, 31 Jul 2005 13:27:36 +0000 (GMT)
From   : "Thomas Harte" <thomasharte@...>
Subject: Re: Hello/Recognize this?

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> A simple loop (time=0,for i=1 to 10000, next, print time) takes 7,98 sec on
> a normal Electron, with speedup it takes 5,27 secs. The BBC does the same
> loop in 5,22 secs.
> It only speeds up RAM-access in the lower 8K (a 6264 is used).

This has been emulated in ElectrEm (my Acorn Electron emulator) for a few 
years. To sum up for those who aren't very Electron hardware aware, the 
BBC's 4Mhz RAM chips were replaced with cheaper 2Mhz RAM, with the effect
that the 6502 may only access RAM when the video circuits in the ULA will
allow it. They are quite dense and limit you to 1Mhz RAM access in modes
4-6, much less in modes 0-3.

The change described moves the low 8kb so that it may only be accessed by
the CPU. It gets 2Mhz access there but obviously the screen may no longer
be placed in that area. The adjustment is supposed to have been originally
designed at Acorn.

The Slogger Master RAM Board duplicates this mode as its 'turbo' option (in
addition to a 'shadow' mode that provides an extra pageable 32kb of RAM accessable
at 2Mhz). Final stocks of Electrons were sold with this add-on prefitted.

Since ROM accesses are always at 2Mhz, small BASIC programs and any other
code located low down in the memory map on a machine with a low PAGE will
operate at very close to BBC speed.

-Thomas

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