Hi all,
Long time reader but first time poster. I'm troubleshooting my Rev.3 (7850). After isolating Ram problems and swapping 1 or 2 LS, it now does boot and start disk II.
The main default comes from HGR command, which doesn't clear the screen (Hires1), so I suspected the $2000-$3FFF RAM area, but corresponding chips are tested good. After issuing HGR command, the screen doesn't blank and shows partial white blocks. Otherwise, HGR2 works: Hires2 is cleared. Text (1&2) and GR work well.
Any hint appreciated ! Thanks.
White "blocks?"
Are you sure it's in hires graphics mode? Might be one of the display softswitches not functioning. Much less likely - a ROM issue.
I think jeffmazur may be on to something that the right mode might not be displaying.
One question though, how are you testing the memory? A RAM test program with the chips in the computer or in an external tester? It's possible for the chips to be good but the sockets bad. Also it could be some other address decode logic for the RAM or something else too.
Here's a few more tests you can do:
Excercise the graphics softswitches from the Monitor:
C050 (set text)
C051 (set graphics)
C054 (set page1)
C055 (set page2)
C056 (set lo-res)
C057 (set hi-res)
Do all of these work?
If so, try filling the hi-res page with single value. Select hi-res page 1.
Type 2000:00 (also try FF)
2001<2000.3FFFM
If 00 clears graphics portion and FF fills with white, then all of the hardware is working.
Note, this is backwards. :)
Of course! I usually remember this because all of the softswitch latches default to low (even address) on power up. Which explains why the Apple II starts in the Graphics mode when first turned on.