Re: 6502 illegal opcodes questions

[Eric Smith <eric@brouhaha.com>]

"Michael J. Mahon" <mjmahon@aol.com> writes:
> Of course, the character generator ROM could have been an SRAM, and
> could have been made visible (byte sequentially, say) on an Apple.
> 
> It was a conscious choice to make the Apple character set fixed.
> (And it's nice to have a character set fully defined at power-up.)

The Apple III used an SRAM for the character generator, but it wasn't
directly memory mapped.

The boot ROM contained a very crude partial character set which was
decompressed and loaded into the character generator RAM at power-up.

Once the operating system was booted, the SOS driver for .CONSOLE was
responsible for the character generator.  It loads a default character
set, and provides a way for an application to change it.

The Apple II emulation disk loads the Apple II character set.

The way the hardware works is that you load the character set image
into main RAM, then set a bit on one of the VIA ports.  This makes
the video hardware copy the character set from main RAM into the
character generator RAM.  Pretty clever, and it avoids needing
faster main RAM that would be needed if the character generator was
mapped in main RAM.