Hi guys.explanation as to why Apple bothered implementing this."
Obviously this can't be done in earlier Apple II computers.
But someone commented on my answers here: https://retrocomputing.stackexchange.com/questions/23854/can-you-read-the-character-definitions-font-in-an-apple-ii-using-peek-in-apple/
"Actually, on the Apple IIgs, there was a way to read the character ROM - it wasn't directly in the memory map, this was done in some backdoor way through other video-related registers. It's been far too long, I don't remember the details, or any
I've searched comp.sys.apple2 but came up empty handed.
Does anyone have the details of how to do this?
On 2/6/22 10:10 PM, Nick Westgate wrote:
Hi guys.
Obviously this can't be done in earlier Apple II computers.
But someone commented on my answers here:
https://retrocomputing.stackexchange.com/questions/23854/can-you-read-the-character-definitions-font-in-an-apple-ii-using-peek-in-apple/
"Actually, on the Apple IIgs, there was a way to read the character ROM - it wasn't
directly in the memory map, this was done in some backdoor way through other >> video-related registers. It's been far too long, I don't remember the details, or any
explanation as to why Apple bothered implementing this."
I've searched comp.sys.apple2 but came up empty handed.
Does anyone have the details of how to do this?
They _might_ have been thinking about the Apple ///, which you do need to go through some
register and interrupt chicanery to load chargen data.
Well, there are some vague clues:
1. Apple IIgs Firmware Reference, page 281, last line:
C02C:00 246 CHARROM DFB 0 ; Addr for tst mode read of character ROM
found the source code where I'd actually implemented character ROM reading
on the IIgs. I got the technique directly from an Apple engineer back in
the day - it's entirely possible I'm the only programmer outside of Apple
who ever used it.
(I'm the commenter from stackexchange mentioned in the first post.)back in the day - it's entirely possible I'm the only programmer outside of Apple who ever used it.
I did some digging through old disk images containing even older disk images (watch Inception for details), and found the source code where I'd actually implemented character ROM reading on the IIgs. I got the technique directly from an Apple engineer
I'm pretty sure that all the registers involved are part of the Mega II chip, so it's not actually specific to the IIgs.
- Jason Harper
This is very interesting. Is it possible for you to make the source code available?
Apart from the obvious questions, I'd also be interested to know why you actually wanted/needed to do this. Was it for fun or a project?
Ian, you're probably the one person in the world who might have this
source code already...
In reply to "Jason Harper" who wrote the following:
Ian, you're probably the one person in the world who might have thisI'll turn it into a standalone program to dump the character ROM and post the source + executable.
source code already...
I'm mainly interested in this from a preservation/emulation point of view. It's
another softswitch that is completely undocumented and none of the current emulators will even try to emulate. Let me fix that.
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