area of the disc that contains no useful data, (it has only just been
allocated to the file), the BIOS has no need to pre-read the sectors from the
disc, and the system performance will increase as a result. However the BDOS
only tells the BIOS on the very first sector of the area, and not on every
sector. Thus the BIOS has to maintain a flag, (saying "I’m writing unallocated
data"), and maintain a record of the next drive/track/sector expected. If the
unallocated flag is set, it compares the next request against the stored
values to check that the write is following in sequence. As long as this
continues, it knows it can dispense with pre-reads.
As an example here are the results of doing a “SAVE 128 JUNK" on a
As normal (No unnecessary pre-read): 5.6 seconds
C always forced to O on Write: 18.5 seconds
C always forced to 1 on Write: 57.0 seconds
Finally a Double density system, but with a physical sector size of _
128 bytes. (i.e. no Blocking/deblocking) 11.5 seconds.
The latter is a bit artificial, as the timing figure can be varied widely
by altering the sector skew, but I hope I picked a figure in line with the
Gemini skew. Anyway it gives an indication of performance. :
I trust that equiped with the above in one hand, and the relavent section
of the BIOS in the other, you can make some sense of the blocking-deblocking
code within a few iterations.
Coming in the next issue: Wait states on the Nascom 2. Using 2716/2732
EPROMs in the byte-wide sockets.
This column is fueled by your letters, so write! [ ea. – fueled? Does this
mean that you burn them to keep warm?
1. O’FARRELL R., "IMPRINT – a review", INMC80-4, May/Sept 1981, pp58-59
2. PARKINSON D.W., "Parkinson’s Pep-up", PCW 3-10, Oct 1980, pp82-83,123
3. O’FARRELL R., "The Interrupt System of the Z80", 80-BUS News Vol 2 issue 1
Jan-Feb 1983, pp6-12
4. BEAL R., "Serial Interface problems made easy", 80-BUS News 2-3, May/Jun
5. HUNT D.R., "The Kiddies Guide to Z80 Assembler Programming", 80-BUS News 1-
3, Jul/Oct 1982
The East Kent Computer Users’ Club meets on the second Wednesday of each month
in room 111/112 of the Computer Science section of the University of Kent at
Canterbury. They are also affiliated to the Amateur Computer Club. The
meetings take the form of a talk on subjects of common, followed by a somewhat
less formal session in the university bar.
The membership contains only Nascom and BBC owners currently. For more
information on EKCUC contact either:
Kevin Wood, __ ______ _____, Sturry, Canterbury, Kent. ___ ___. ____-______ or
Laurence Fisher, __ _______ ___, St. Stephen’s, Canterbury, Kent. ___ ___.
Phone ____ _____.