MCCABE@MTUS5.BITNET (03/28/88)
Date: 27 March 1988, 13:56:33 EST
From: Jim McCabe MCCABE at MTUS5
To: INFO-ATARI16 at SCORE.STANFORD.EDU
Subject: Screen memory format
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?
Thanks in advance!
+-----------------------+-----------------------------+
| Jim McCabe | BITNET: mccabe @ mtus5 |
| G31 ECH MTU | jemccabe @ mtus5 |
| Houghton, MI 49931 | UUCP: mccabe @ m-net |
+-----------------------+-----------------------------+) (03/28/88)
>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 less reasonable and intelligent method has been used (to keep the hardware simpler and/or faster, I believe). Natuerlich! -------------------------------------------------------------- Loveletters & Hatemail to : wallman@yalecs Files to : WALLMANN@CTSTATEU (Bitnet) --------------------------------------------------------------
wheels@mks.UUCP (Gerry Wheeler) (03/29/88)
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. -- Gerry Wheeler Phone: (519)884-2251 Mortice Kern Systems Inc. UUCP: uunet!watmath!mks!wheels 35 King St. North BIX: join mks Waterloo, Ontario N2J 2W9 CompuServe: 73260,1043
unpowell@csvax.liv.ac.uk (04/18/88)
In article <429@mks.UUCP>, wheels@mks.UUCP (Gerry Wheeler) writes: > 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. The bits of this pixel are, in fact, 1101. ($78000 is the least significant word, whereas $78006 is the most). So the colour in pallette 1101=13 (dec.) will be displayed for this pixel. ******************************************************************************** "...there's no success JANET unpowell@uk.ac.lis.csvax like failure and UUCP {backbone}!mcvax!ukc!mupsy!lis-cs!unpowell failure's no success ARPA unpowell%csvax.lis.ac.uk@nss.cs.ucl.ac.uk at all..." B.Dylan ********************************************************************************