From: Andre Fachat (afachat_at_gmx.de)
Date: 2004-06-02 21:07:32
Hi Ruud,
indeed the 8032 writes a "40" into the "number of columns" register.
But the hardware actually reads two bytes per CRTC "column", so
in fact 80 characters are displayed.
With a computer with 1MHz video memory access speed (Actually
2MHz, but half of the 1MHz the cycle is for video reads, the
other half is for the CPU, so only 1MHz video access) and 1 byte/cycle
bus width, you can only read video data for 40 columns.
For 80 columns you either have to use 2MHz video memory access speed
or instead read 2 byte/cycle.
In the normal 8032 (as opposed to 8296), IIRC there are two video RAM
chips and both are read during the normal video access cycle, so
2 byte/cycle are read and then shifted out to the screen.
Alas, you can display 80 chars within 40 cycles - and this is
what is written to the CRTC.
Hope that helps,
Andre
On Tue, Jun 01, 2004 at 04:00:34PM +0200, Baltissen, GJPAA (Ruud) wrote:
> Hallo allemaal,
>
>
> http://www.funet.fi/pub/cbm/firmware/computers/pet/d/petdis is the
> disassembly from a 8032 as the header says. I needed the values to
> initialize the 6845 and the route $FFFC, $FD16, $DF1B, $E000, $E04B, $E07A
> lead me to $E72A. The value on $E72B, $28, means there are only 40 columns
> displayed. I checked if there are other initializationroutines but didn't
> find any. So my conclusion is: this is the disassembly of a 4032. Any
> comment on this conclusion is welcome.
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