Date   : Thu, 09 Dec 2004 14:18:59 +0000
From   : Richard_Talbot-Watkins@...
Subject: Video ULA shenanigans

All this 'revising' of the Video ULA details recently has got me
wondering...

I never tried this on a real Beeb: what are the effects of setting the two
unused combinations of 'characters per line' and clock rate in the Video
ULA?

We have:

MODE 0            80    high
MODE 1            40    high
MODE 2            20    high
MODE 4            40    low
MODE 5            20    low

plus, the Mode 8 hack:

MODE '8'    10    low

so what does a real Beeb do when given:

80 characters per line, low clock rate
10 characters per line, high clock rate

Extrapolating, we get a 0bpp mode and an 8bpp mode, but how does the ULA
actually implement these settings?  I would expect the first one to blank
screen, but do we actually get some kind of 8bpp 80 pixel wide mode from
the latter (obviously with the 16 repeating colours) or some other kind of
undefined effect?

Rich



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