15 Nov
2007
15 Nov
'07
4:45 a.m.
On 14 Nov 2007 at 23:23, Roy J. Tellason wrote:
What was oddball about DRAM (Is that what you meant?)
for those machines?
No, he means DAM - Data Address Mark. It's a combination of data and
clock bits that marks the start of the data portion of a sector.
Normally this would not be encountered elsewhere. For example, the
"normal" DAM for FM disks is FB with a C7 clock. But the 1771 could
write DAMs with values from F8 to FB. IIRC (I'd have to go back to
my notes to make sure), the TRS-80 marked directory sectors with a
DAM of FA. Now, a DAM of FA is no problem with the 179x FDC, except
that the 1771 returned a two-bit status giving record type, but the
1791 collapses this to one bit. So the 1791 can't tell the
difference between an FA and FB DAM, so the 1771 is needed for those
old TRS-DOS diskettes that use the FA=directory sector scheme.
A case where a programmer being "clever" came back to bite the
vendor.
Cheers,
Chuck