80-Bus News


September-October 1984, Volume 3, Issue 5

Page 31 of 47


THE DH BITS By David Hunt

A few days ago a customer of ours popped into the shop and gave us a long tale about having exported some gear in 1982 and because of some customs problem could they have a copy receipt for the goodies? Well much as we try to please, trying to find a receipt for goods supplied on an unknown date in 1982 is alittle too much. So, as we remembered the customer, and were therefore pretty certain that he had bought something a long time ago, we suggested that we could give him a new receipt, but dated 1982, altogether a lot less hassle. This was fine, but could we make sure that the receipt was not dated on a Sunday or Bank Holiday or some other such suspicious date? I wonder how many of you have a 1982 diary or calendar to hand? We certainly didn’t. This whole thing was taking on the proportions of a farce, as trying to find a 1982 calendar looked like turning into as much fun as trying to find the original receipt. Then a thought occurred. A very long time ago, when Nascom first grew BASIC, I cobbled together a Calendar program. Did it still exist, had it been converted to run under CP/M, etc? A quick consult of the CAT program revealed CALENDAR.BAS, which when fired up, worked.

This turned out to be an interesting program, short and to the point, so for my sins, it’s offered here. It may even have been published before, although a quick look through the old back issues of INMC didn’t reveal it. Looking closely at the program, I’m pretty sure it’s not all original DH, so I must have pinched it, or bits of it, from somewhere. The basic algorithm for working out the start date seems to be attributable to David Ahl’s -101 Basic Programs’, but different, and I’m pretty sure, left to my own devices I wouldn’t have calculated the screen TAB positions the way it’s done here, but 1979 was a long time ago and premature senility is creeping over me, so lI don’t remember. One thing Ido remember was the trap for the 1752 start for the Gregorian calendar, and the nasty business of the (MOD 4000)}+2000 over the fact that the year 2000 won’t be a leap year when it should be. This doesn’t happen again until the year 6000, so it looks like I -short cut” the procedure and checked at the 1000 year boundary. This makes the effective operating range the 1250 odd year span from the year 1753 to the year 2999. The 1752 trap won’t allow earlier dates, and the 2999 trap stops the program thinking that the year 3000 is not a leap year when it should be. All pretty esoteric really as I doubt that anyone will be interested in dates outside a century span anyway. See Listing One.

Another thing which has been causing problems lately (and still on the subject of dates and times) is using machine code routines under dBASE II. Now versions 2.4 or later allow machine code calls, and the favourite for these is making the cursor display different on the SVC/IVC card, or reading either of the Gemini clocks (the clock on the GM816 1/0 card or the GM822 RTC) in as dBASE data.

The cursor first. The various permutations of Gemini CP/M do different things with the cursor when either waiting for a CP/M command, or actually executing a program (it’s documented in the manual, so I won’t repeat it here). Normally the cursor blinks whilst in the CP/M command mode, and becomes a non-blinking cursor when executing a program. I say normally, as the Winchester based Quantum machine I borrow when I visit Gemini, turns the cursor off completely when executing a program. I find this infuriating if the program I*m using doesn’t turn it back on again, and of course, dBASE is one program that doesn’t. I know this is very easily patchable with the program

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