🚀 go-pugleaf

In article <8803271924.AA18903@ucbvax.Berkeley.EDU>, MCCABE@MTUS5.BITNET writes:
> I need to know what the format of the ST display memory is.  The Abacus
> Internals book gives a completely different discription than the Compute!'s
> VDI manual.  I tend to trust the Compute! publication more because it
> sounds a lot more reasonable and intelligent. (Let's talk low-res here)
> 
> Compute!:  Each ADJACENT four BITS specify a number between 0 and 15
>            that represents the color register used to plot that pixel.
> Abacus:    Four contiguous memory WORDS specify the color of 16 pixels.
>            To set a pixel, one must set a bit in each of the four words.
>            "in order to set the point in the upper left-hand corner,
>            the topmost bits of words $78000 (assume screen memory starts
>            at 78000), $78002, $78004, and $78006 must be manipulated."
> 
> Like I said, the Abacus method doesn't sound very logical.
> Which method is actually used?

The illogical method is the one that is used.  In low res mode, you have
to visualise the memory as sets of four words.  The corresponding bits
in each of the words are the four bits that specify the entry in the
palette.  For example, if the words are:

	1xxxxxxxxxxxxxxx	$78000
	0xxxxxxxxxxxxxxx	$78002
	1xxxxxxxxxxxxxxx	$78004
	1xxxxxxxxxxxxxxx	$78006

then the bits of the first pixel are 1011.

To switch your brain into medium res, just remember that the memory is
used in sets of two words, again with corresponding bits making up one
pixel. 

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