Re: Interesting graphics hacks?

[dempson@actrix.gen.nz (David Empson)]

Pim Blokland <pDOTblokland@spcgroup.nl> wrote:

> Michael A. Balfour wrote:
> 
> > The ROM contains $1000 bytes. The last $800
> > contain the bit patterns used to draw the graphics.
> 
> Gee, I didn't know that. Honestly. I thought I roughly knew what the
> hardware was all about, but I didn't know that the bit patterns were
> programmable.
> 
> > My question about hacking the graphics ROM specifically refers to the
> > last $800 bytes.  $400 of these are used for lo-res, and $400 for
> > hi-res.  Changing these bytes could change the way things are drawn,
> 
> But why $400 for hires? Isn't $100 enough?

The address connections to the video ROM consist of eight bits from the
appropriate source (hi-res screen in this case), a graphics/text select
bit, and three further bits for a total of 12 bits, or 4K bytes.  (The
international version of the IIe has an additional input for character
set selection.)

In graphics mode, one of these bit indicates whether the system is
display hi-res or lo-res graphics.

In lo-res mode, the remaining two bits are need to distinguish between
the top and bottom half of a character cell, and between even and odd
columns (to correct the colour pattern).

In hi-res mode, the remaining two bits serve no practical purpose, but
they behave the same as in lo-res mode.

This means that hi-res mode requires four identical copies of the
unmodified graphics data.

> I can imagine different bit patterns for main mem than for aux mem

Nope - identical patterns are used for both banks.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand