80-Bus News


January–February 1983, Volume 2, Issue 1

Page 17 of 56


date. Each item, the amount, your autograph, etc, are assigned a specific place on the cheque. The bank gets cross if you put the wrong data in the wrong slots. So lets call the slots for your autograph, the payee, etc, the data fields because that is where the data is to go.

I hope that’s tidied up some definitions for you. A database is a chunk (large or small) of data. The database is composed of individual records each numbered in some way so they may be identified. Fach record has one or more fields, each assigned to a specific purpose in storing the data, quantity, date, item number, etc. The data base should be given a name, even if it’s something uninspired like ‘DATABASE’. The records would normally be numbered, and each field within a record would probably have a name, such as QTY for quantity, or ITEM for guess what. It helps if the field names are appropriate to the field use.

Now databases can come in many forms, and one of the most commonly sought after (but least commonly used) is the implimentation for the smaller machine, something like a 16K Nascom without disks for instance. Let’s take a fairly typical database, a name, address and telephone list. Now I’ve found from bitter experience that this requires at least 6 fields per record, four fields of 25 characters and two fields of 10 to 12 characters. Well one thing is certain, a 16K machine isn’t going to hold much data, as even at best using string arrays for the fields, the limitation is going to be the memory space available for those arrays. Let’s be generous, lets’s say the program to handle the database is no more than 4K long, then that leaves 12K available as string space. Now the way Microsoft Basic works is that it sets up a pointer to each string, which is, if I remember rightly, five bytes per string, so each field is going to have a five byte overhead for starters. So our name and address database is going to be about 150 characters per field. So we have room for about 85 names and addresses in the database before we run out of space. This is plenty big enough for the home user, but then the home user isn’t going to wait a couple of minutes loading up the data from tape just to look up Fred’s phone number. Far easier to look it up on the jotter by the phone. This is ignoring the difficulty of saving strings as data on a Nascom in any event. String saving has been hammered to death in past issues of INMC and INMC80 so I’m not going to waste time detailing them here.

Of course, there are ways of compressing the data, string compression where the data is shrunk arithmetically to reduce its size. Three characters into two is a typical compression ratio, or even 50% where lower case characters are converted to upper. Another way is ‘dynamic’ allocation of fields where empty fields are shuffled so that more space is available from the unallocated space. The Nascom version of the Microsoft Basic does this anyway when string space is getting tight. Have you ever noticed the Basic ‘hang’ for a few seconds, well that’s when it’s doing its garbage collection and re-allocating the string space. Whatever method is used for compressing data, this is done at the expense of program space, and what is gained on the swings is lost on the roundabouts, so the nett gain is usually trivial.

So what is the answer, well I’m afraid disks is (bad grammar, disks are) the answer, or of course oodles of RAM and some hairy software to get at it. That is, if anyone is intending to use a database seriously. With disk, access is slower than in RAM, but at least with sensible organisation at least the whole thing becomes feasible and the cost is not too astronomical when looked at in terms of price per bit saved. If on the other hand the intention is to simply provide a simple telephone numbers program, then a 16K machine is entirely adequate provided you can wait whilst the tape loads.

So I did get round to my topic after all, next issue I’ll be rabbitting on about how databases could be organised, starting from humble beginnings with a simple program for the Nascom. For those too impatient to wait for the next issue, go and get yourself some disks and get hold of Mike Hessey’s MANOR program. It’s well worth a look at and it’s simple to use.

Time and space preclude any more, so, that’s all folks, until next time, that is.

This is an OCR’d version of the scanned page and likely contains recognition errors.

Page 17 of 56