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View Full Version : Mystery ROM from MITS ROM Board



glitch
January 27th, 2016, 04:05 PM
I've been going through a lot of my S-100 boards, doing cleaning and testing. Tonight I started on a MITS 88-PMC, which had a ROM in the last socket (locations 0xFF00 - 0xFFFF) with no label. It's not the MBL, DBL, or TURMON. Here's a hex dump from the Dajen SCI monitor:

http://pastebin.com/24Gjh2kw

It starts with LXI SP, 0xFF13 -- a ROM location. I've seen the trick used elsewhere, to reduce code size by using an in-ROM jump table. Anyone recognize the code?

per
January 27th, 2016, 06:02 PM
The start of the code in ROM copies the block of code from 0xFF13 over to 0x2C00 and runs it.

When the code at 0x2C00 is run, it messes about with IO port 0x22, 0x23 and 0x10, then it waits for bits to be reset on IO port 0x8, and replies with setting IO port 0x9 (this is repeated with different bits). After a while it starts polling data from IO port 0x0A and stores it as a block in memory. Then the code gets more complex and it's quite a bit of effort finding out what without knowing more about where this came from.

It's also doing things like generating some simple code at address 0000, while another part of the code contains a jump to this.

glitch
January 27th, 2016, 06:41 PM
Hm, maybe this is the DBL then...the image I have may be for a Tarbell single-density controller, not the MITS controller. Does anyone have a copy of the MITS DBL ROM?

deramp5113
January 28th, 2016, 10:44 AM
Here are disassembled versions of the original Altair PROMs: http://deramp.com/downloads/index.php?dir=altair%2Fsoftware%2Froms%2Forginal_r oms%2F

Mike

per
January 28th, 2016, 11:32 AM
Here are disassembled versions of the original Altair PROMs: http://deramp.com/downloads/index.php?dir=altair%2Fsoftware%2Froms%2Forginal_r oms%2F

Mike
I see the MITS DBL code/hex is exactly similar to the "Mystery ROM", so I think we have the answer right there!

deramp5113
February 7th, 2018, 07:16 PM
Here are disassembled versions of the original Altair PROMs: http://deramp.com/downloads/index.php?dir=altair%2Fsoftware%2Froms%2Forginal_r oms%2F

Mike

Here is an updated link to the disassembled PROMs: http://deramp.com/downloads/altair/software/roms/orginal_roms/

Mike