Hello all,
I am relatively new to vintage computing and this list,
although I've been working and playing with computers for a
long time. I am, however, a software guy and have only a
little skill with or knowledge of hardware.
I've recently acquired a pair of Altair computers (8800 and
8800a) both equipped with the "88-DCDD" single 8" disk
drives. Unfortunately, I am having various difficulties
with these machines. Hopefully the answers to some
questions will push me towards a better understanding of
these systems.
My main issue at the moment revolves around the EPROM cards
that hold the disk boot loader (DBL) PROMs for these
machines. The cards are Solid State Music ?2K/4K? cards
with 16 slots for 1702s. I asked the list about these cards
before and have since obtained an assembly/instruction
manual, but it is still unclear to me how these things are
configured and addressed.
There are four address switches which are supposed to
address the board as follows (this is taken from the
manual):
Address:
000 000 to 017 377 - All switches on
020 000 to 037 377 - A2, A3, A4 on
040 000 to 057 377 - A1, A3, A4 on
060 000 to 077 377 - A3, A4 on
100 000 to 117 377 - A1, A2, A4 on
120 000 to 137 377 - A2, A4 on
140 000 to 157 377 - A1, A4 on
160 000 to 177 377 - A4 on
200 000 to 217 377 - A1, A2, A3 on
220 000 to 237 377 - A2, A3 on
240 000 to 257 377 - A1, A3 on
260 000 to 277 377 - A3 on
300 000 to 317 377 - A1, A2 on
320 000 to 337 377 - A2 on
240 000 to 357 377 - A1 on
360 000 to 377 377 - None on
I see issues with this from the outset. First, assuming
this is octal, the addresses past 160 000 are beyond the
address space of the 8080 chip in the Altair (200 000 being
64K). As well, the gap of 16k at 160 000 to 200 000 is
either an error in the manual or something I am missing
since I can?t imagine the binary progression of switches
having that gap. How is this card addressed?
I?ve tried examining my way through the memory areas that I
think are supposed to be addressed into the EPROMs (based
upon various assumptions) but I get results that have me
confused. Answers to some of these questions might help:
Why would the address lights on the Altair not properly
correspond to the address switches being set? If I set the
switches at 1110111000000000, for instance, when I reset
then examine the light at A11 doesn?t come on (the lights
show 1110011000000000) even though I know that LED to be
functioning.
Why would the data lights ever show anything but on or off?
I sometimes get dim glows from the data LEDs.
What data can I expect to be in the RAM on boot-up? Is this
answer different for Static vs. Dynamic RAM?
What is the best way to execute the boot loader should I
locate it? Can I set the address switches to the proper
spot and run or do I need to code a simple program that will
?JMP? to the known address?
Much of my confusion might stem from the EPROMs themselves
since they may have been erased by time. I have a freshly
burned set on order which may help.
Please forgive my ignorance and this really long post.
Thanks in advance,
Erik
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Received on Fri May 17 2002 - 21:51:36 BST