💾 Archived View for uscoffings.net › retro-computing › systems › Tandy › oldskool › dmainf.txt captured on 2024-02-05 at 11:29:59.
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#: 39950 (P) S5/Tandy 1000 & 1200
24-Sep-85 23:55:58
Sb: #39907-HELP!!!
Fm: pbj 74736,1556
To: Craig K. 70003,2363 (X)
Craig,
You and others may be releived to know that we have finally determined the
cause of the problem, and have come up for a fix for it. Before, I give you
the sordid details, a history lesson is in order. It seems that the cause of
the problem is that the fact the DMA controller is improperly initialized to
provide extended write cycles rather than the normal write cycles that IBM and
the rest of the world use. For those of you that are interested, the offending
code is:
Address Wrong Code Right Code
F000:C7B7 24 20
F000:C7FC 20 00
This is what we found in BIOS 1.00.00. Extended write cycles may cause
problems, since the DMA Ready set-up time may be violated, causing needed wait
states to be omitted. There are NO problems provided the DMA is properly
programmed.
Further, we found that Tandy's Memory Plus expansion board suffers from
the same problem as does the MFB-1000, since the DMA circuitry is substantially
the same. However, only recently we have discovered that Tandy is now shipping
memory boards with 2 jumpers added to correct the problem.
There are three possible alternatives to correct your situation. The first
is to return the board to us for modification. This cannot be performed by the
user, since the mod requires re-programming of the U14 PAL.
The second alternative is to correct the errors in the BIOS by
reprogramming your BIOS EPROMS. This may not be satisfactory for some, since
Tandy is now shipping units with masked ROMs.
The last option is to contact your hard disk vendor and inform them of the
problem. It would be a simple matter for them to add the following line of
code to their firmware:
XOR AL,AL
OUT 08H,AL
Alright, so it's two lines! In any case, the various hard disk vendors should
be aware of this. We will attempt to contact as many vendors as possible, but
those of you who are having problems can help us out by informing your disk
vendors.
We will not be ready to perform these mods until Oct 1, since we must
re-program our PAL chips in vast quantities. And so that we are not swamped
with too many requests, please do not return