Date   : Sat, 27 Aug 2011 05:55:14 +0200
From   : rick@... (Rick Murray)
Subject: Tube ROM

On 27/08/2011 01:12, J.G.Harston wrote:

> It uses the same 6845 and the same pixel bitmap.

What, even down to the quirky screen addressing?


Actually, back in the early '80s, pretty much every video system used 
the 6845 (like IBM's CGA card), and the stuff that didn't use the 6845 
probably used an ASIC with a souped-up 6845 (like EGA cards).

It is interesting to note that the 6845 really only deals with the 
timing and interfacing to generate a stable display. All the pixel 
reading and conversion is handled by other circuitry, which explains the 
large disparity in capabilities of 6845 based video systems in terms of 
available resolutions and colours, if any. Or in the case of the MDA 
adaptor, no actual graphics capability whatsoever.


Best wishes,

Rick.

-- 
Rick Murray, eeePC901 & ADSL WiFI'd into it, all ETLAs!
BBC B: DNFS, 2 x 5.25" floppies, EPROM prog, Acorn TTX
E01S FileStore, A3000/A5000/RiscPC/various PCs/blahblah...