INMC 80 News


October–December 1981, Issue 5

Page 49 of 71

which is, of course the ASCII code for 5. Now it wasn’t really magic, it was knowing the effects of the DAA instruction which is the heart of this whole routine. I confess, that given the original specification, my approach would have been a lot more long winded, and I don’t suppose for one minute that I would have thought of using the DAA instruction in this way. The ADC A,40H can be thought of as a ‘fiddle factor’, but the result is what was required. The whole piece of code from line 4525 to 4540 must be though of as a single piece of bit manipulation, and can only be arrived be very careful though and thorough knowledge of the effects of the instructions.

Line 4550. Go and print the character on the screen.

Line 4555. Get the return address off the STACK and go back to where eve you came from.

And here endeth the lesson. What’s been learned? Well, firstly and most important, the fact that instructions may be intended for one purpose, but that does not impose a constraint on their imaginative use for other purposes. The two crucial instructions are the DAA and the ADC A,40H. Knowing exactly the effect of the DAA allows the programmer to deduce the required ‘fiddle factor’ to make the routine work correctly. Another thing has been the correct and orderly use of the STACK. See how neatly the whole thing comes together. The SP is always in the right place when data is wanted, and simple PUSH, POP, (not so simple) CALL and RET instructions always retrieved the correct data at the right time. The way the routine CALLed the second half the first time round, but simply dropped through the second time, thus saving one CALL instruction, and one additional RET instruction, so saving four bytes. (Not that space is usually at a premium, but there is an awful lot crammed into the 2K of NAS-SYS). I hope you all understand how the B2HEX routine works now, and hopefully, my commenting of the routine made for easier understanding.

Try commenting some of the other routines in the monitor. If you can’t see how they work from the listing, there’s nothing to stop you writing a simple piece of code which will pre-fill the registers with the right data and then CALLing the routine in question via a single step command. It’s quite revealing when you do it that way, you see all the registers change as the routine steps though, and you can see the sometimes hidden effects of the flags as it goes along. If you have NAS-DIS and D-BUG, so much the better, the display is so much clearer. Personally I find the register display of NAS-SYS 3 a retrograde step, as, although it gives much more information, it’s confusing to read.

Now for the ultimate confession. When I started to write this piece, I didn’t know how B2HEX worked either. Of course I’d looked at, but the significance of the DAA instructions hadn’t caught my eye. I had just taken it as a ‘black box’ module and not given it any attention. Imagine my surprise when I actually sat down and single stepped through it (I’ve got to do that, ’cos if I get it wrong, think of the delight you’d get writing me rude letters) and then tried to figure out exactly how it performed the magic conversion. I phoned Richard to confirm I’d got it right, and to ask how he’d invented that particular way of, doing it. It was then I discovered that Richard wasn’t the perpetrator of the particular piece of code. In writing NAS-SYS 1, Richard had run out of space by about 20 – 30 bytes, so he spoke to David Parkinson (of REVAS, NAS-DIS and GEMINI disk system fame) and asked if he could think of any ways of losing a few bytes. Dave came up with this one, and a couple in the KBD routine which saved the day. Incedently, I now know why I’ve never been able to fully understand how the keyboard routine works. David Parkinson has been at it!!!

Still, give David his due, he’s far cleverer at this programming lark than I will ever be. My excuse? I gave that in episode one, when I said that I started off knowing nothing, and I don’t suppose I learned much since. I’ll remember B2HEX though!!

Page 49 of 71