Commodores (Was: Teaching kids about computers...)

Gordon JC Pearce gordonjcp at gjcp.net
Tue Nov 27 05:21:22 CST 2007


On Monday 26 November 2007 23:45:20 Chris M wrote:

>  Of course most other computers are more *normal*
> w/regards to the way their vid mem is layed out. This
> applies to all *known* peecee video cards also. Anyone
> know of exceptions?

The ZX Spectrum had a fairly bonkers layout, with three blocks of 2048 bytes, 
comprising eight rows of eight rows of 32 bytes.

So - the first 32 bytes were the top row of the display, the next 32 were 
eight pixels down, the next 32 were 8 pixels below that, until you got 2048 
bytes in.  Then you started again on the second row of the display, and so 
on.

Once the top third was done, you moved onto the middle third, then the bottom 
third.  After you'd got all the pixels done, you had 768 bytes of attribute 
memory, to set foreground and background colour, bright and flash.

Writing single-pixel vertical scrolling code was a pig.

Gordon



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