2. Plea for help
In the last newsletter we said that it was your newsletter.
Please will you send in helpful tips to this software section
of the newsletter. I promise to consider printing anything
that isn’t actually wrong!
The last newsletter said that to end a program you could jump
to PARSE. I did not write that – it was edited by our super
software? expert (Ta for the compliment Richard. Just wait until
you want some hardware sorted!! D.H.) and INMC chairman
David Hunt, who got it wrong! This is a complicated situation,
and the ONLY 100 percent safe way that ALWAYS works is to jump
to address 0. If you don’t want to clear the screen, put in a
76 (HALT), and press RESET to carry on when you are ready.
Nobody sent in a correct explanation of the problem, but I give
half marks to those who made the program work by putting in
code B7, which is OR A, before the INC A. The reason for the
problem is that although INC A and ADD A, 1 might seem the same,
INC A does not set the Carry flag, or change it at all. Once
the carry flag gets set, the DAA instruction gets completely
confused! To understand all this, use the S command to step
through, keeping an eye on the Carry flag. Also, have a look
at the Z80 programming manual, under INC, ADD, and DAA. If you
don’t understand the table describing DAA, you are not alone!
By the way, perhaps the Z80 should set Carry when INCing – but
if it did it wouldn’t be 8080
compatible, so you really have to
blame Intel for the 8080 design. Anyway, these little quirks
make programming interesting.
5. NASCOM Monitor Programs – a personal view
Here is a brief history of the monitors – also known as operating
systems – which have controlled my NASCOM 1 computer. By now
I suspect that there maybe quite a few confused NASCOM owners,
because so much change has occurred in the last year, so this may
also help to clear up misunderstandings.
In the beginning I had NASBUG 1. This lasted about two minutes,
then I blew it up – it got very hot, I burnt my fingers, and
pulled its little legs off getting it off the board! I then
had a replacement NASBUG 1, which worked, except for the tape
loading problem. This was cured when NASBUG 1T2 was produced,
and most NASCOMS probably use this monitor, which is the only
standard 1K monitor.
Eventually the magic of NASBUG wore off, and frustration with
slow tape loading set in, so I wrote a monitor called B-BUG,
which was NASBUG compatible but with extra features, the most
important of which was the read and write commands, for tape
loading at four times the speed. B-BUG used about 2/3 of the
second 1K EPROM, so there was still room for improvement.