DOCTOR DARK’S DIARY
“It isn’t in the book, so it doesn’t exist”
Now that we all know that IX and IY will behave as if they were
HL when asked nicely – but not always – (see
– Ed.) it
must be time for the missing (and mysterious) CB instructions to
see the light of day. Careful study of page 14 of the Hitch Hikers
Guide to the Z-80 (alias the Mostek Micro-reference Manual) and
meditation in a reverent posture has revealed all to me. The
missing codes are CB 30, CB 31..... to CB 37. All the instructions
on that page, with one exception, are in pairs; there should be a
mnemonic SLL, but you won’t find it. I think there were supposed
to be eight instructions which shifted a register, or a byte
pointed to by HL (and possibly IX and IY as well) one place left
into the carry flag, feeding a zero in on the right of the register.
Single step through this small program:–
0C50 06 8E LD B, 8EH
52 AF XOR A
53 CB 30 "SLL B"
You will find that bit 7 of register B has arrived in the carry
flag as expected, but B is not the 1C it should be. A 1 has been
fed in instead of the zero Zilog intended.
Having puzzled all that out for myself (honest!) the result gave
me a feeling of deja-vu. Get out your copy of Personal Computer
World (well, you should have bought it) Volume 1, Number 2, and
turn to an item called “Four Easy Pieces”. I knew it was there,
somewhere, the joystick circuit on the same page works, too.
“The bit isn’t displayed, so it doesn’t matter”
If you have such a thing as a Nascom handy, execute this complex
0C50 21 10 08 LD HL, 0810H
53 36 7F LD (HL), 7FH
55 76 HALT.
Now, replace the 7F with FF and run it again. Most of you got
the same result both times, the others have been connecting things
to the character generator socket. There are, you see, things
available to the brave soul who wants graphics on his Nascom,
called Bits and P.C.’s Add-on graphics boards. (We believe NM
are up to something too – Ed.) These useful items produce another
128 characters, and make your Nascom even further ahead of those
plastic boxes from across the Atlantic than it was. Trouble begins
when you try to run a program that uses 00 and 7F to display the
pieces for a game like Othello, for example. Mr Beal has written
a fine program, but it uses FF, and on my screen it looks like
Martian Hyper-Cricket (see my program, in INMC issue 476). Please
folks, if you mean 7F then use 7F. Ta.