2 Mar
2008
2 Mar
'08
8:22 a.m.
From: "Roy J. Tellason":
I'm speaking from memory, so I may have the details wrong, but what I
remember is that the way the front panel works, is that the "Examine"
key works (for example) is that it forces a "JMP xxxx" onto the CPU
bus, then returns the CPU to the halted state, ready to fetch the
new location. Of course, this only works if the CPU understands an
8080 JMP instruction, etc.
My understanding is that other front panel functions were implemented
by this sort of strategic lying to the CPU. That makes it a different
beast than the front panels of old :-).
Generally, I'm assuming working hardware, but want to see how the
software went wrong. But whatever software I'm running is interacting
with the hardware (device drivers or the sort of low level programs
we wrote back then).
Remember that those old machines didn't have a debugged operating
system under them, with a virtualization or protection layer to
catch them when they crashed and save their memory and registers.
Most of the programs ran stand-alone and directly controlled relevant
hardware resources.
Vince
On Sunday 02 March 2008 00:44, Vincent Slyngstad
wrote:
Thanks :-).
I think of blinkenlights as way ahead of a ROM
with a debugger in it,
and not for just the aesthetic reasons folks are talking about. It's
a little like Tony's fondness for "things he can understand". When I
use a front panel, there's no question what's going on in the hardware
or what's in the register. The front panel is such a simple device
there is no way for it to lie to me. No way is it going to miss my
breakpoint and wander into the weeds, etc. I'm not acting through
an intermediary -- I really am in control of the machine, and I'm
seeing the real machine state.
I think that's a pretty good point, there. Later, this
got stretched a bit. The actual workings of an IMSAI
front panel leave some potential for strange things to happen, as
what going on there is a little like a debugger activated by switches.
At least you can really stop the machine, etc.
Which aspects of the IMSAI are you referring to here? The one I have
does not
have the original CPU board in it, and the front panel functions are
seriously limited, as in most of it doesn't do much of anything. I do use
debuggers in core, of course, but I percieve that as a
trade of ease-of-use vs knowing what's really going on.
It all depends on what you're trying to do. Get hardware working
at all? Or something just a bit beyond that...