Helios II disk drive - Sol-20

From: Allison J Parent <allisonp_at_world.std.com>
Date: Wed Mar 4 14:52:27 1998

<garbage confined to the last two lines of the screen? (I'll have to go
<dig up my close-up screen shots photos). If so, this could mean a dead
<video ROM.

That would also be least likely as the device used is a mask programmed
component.

Sounds more like the cpu is starting up and then crashing either due to
soft bits in rom or more likely some dead ram(or the bus logic that)
connects the cpu to the ram and rom.

FYI: if the VDM1 (display board) is not accessed by the cpu or the cpu
runs amuck it's contents will be trash. If the cpu is getting nothing
or starts executing from a location where there is no memory the cpu
stacks itself to death and fills the screen (VDM1 is memory mapped).
The reason is an open bus (no memory addressed at all) = RST7 instruction
0ffh and it does a jump to 38h and starts executing from there and if
nothing answeres at that address the same thing repeats itself, each cycle
the return address is put on the stack and it fills memory with 00h, 38h.

Allison
Received on Wed Mar 04 1998 - 14:52:27 GMT