Autumn 1979, Issue 4

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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 issue 3 – 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 program:–

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.

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