Re: [9fans] A shot in the dark

[Digby Tarvin <digbyt@acm.org>]

Way back in the dim distant pre-IBMPC days of Z80 PCs, my favourite
debugging technique was to use the screen memory.

My original Exidy Sorcerer had 1920 bytes of character oriented
screen memory (this was when 8K was a respectable amount of main
memory). Each byte displayed a 8x8 character defined by 8
bytes of the character generator memory, to top half of which
(the non-ascii characters) was writeable memory.

Not only could diagnostics be written to the display with a single
assembly language instruction, but it was even possible to just
run the entire program on the screen, which was entertaining as
well as often quite informative. I had no assembler, so with
hand assembly 1920 bytes seemed like quite a large piece of code.

Running the firmware monitor memory test routines on the screen
ram was also an endless source of entertainment.

I have recently been tinkering with using the memory on PC text
mode VGA adapters to provide a realtime visual display of what is
going on inside the Linux kernel, but it seems to be one of the less
well documented areas of the software and hardware, so there has been a
lot of trial and error and reverse engineering involved..

If anyone knows of any good resources please let me know...

Regards,
DigbyT

On Wed, May 28, 2008 at 05:31:27PM +1000, Bruce Ellis wrote:
> When I did the port to the PS2 there wasn't even a light to blink. To
> get thru l.s I discovered a register I could write that resets the
> video.
>
> Only a hundred lines (most innocent) to binary chop.
>
> Ken has a better tale of a device that only had a speaker and
> debugging by tones.
>
> brucee
>
> On 5/28/08, Paul Lalonde <plalonde@telus.net> wrote:
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA1
> >
> > FWIW, we used a similar technique just last summer debugging some PS3

--
Digby R. S. Tarvin                                          digbyt(at)digbyt.com
http://www.digbyt.com