Re: Char ROM length in IIe and IIc models

[Eric Smith <eric-no-spam-for-me@brouhaha.com>]

Stefano Priore <stefano.priore@inwind.it> writes:
> Looking at the file 342-0275 (German/USA charset) I've seen that it's
> layout is the following:
> 
> 256 German chars
> 256 garbage chars
> 256 USA chars
> 256 garbage chars

Those aren't "garbage chars".  They're the bits needed to make lores
graphics work.  The simulator probably doesn't need them, but the real
IIe and IIC hardware certainly does.  The hardware generates lores
graphics differently than on the original II/II+ hardware; the latter
used multiplexers and a recirculating shift register.

I don't remember whether the IIe and IIc funnel the hires graphics through
the character generator ROM.  I don't have the reference manuals handy, but
I'm guessing that they probably did because it would have saved a
multiplexer.

As far as determining the correct total size for the ROM, I'd recommend
looking at the schematics in the IIe or IIc technical reference manual.
If there are 12 address lines going into the ROM, it's a 4 Kbyte ROM.