80-Bus News

  

September–October 1983, Volume 2, Issue 5











Page 15 of 67











MBASIC RANDOM ACCESS FILES, AND SOME OTHER NOTES

By C. Bowden (G30CB)

The notes on MBASIC in 80-BUS NEWS Vol 2. Issue 3 by P.D. Coker gave a fair summary of many of the commands and functions found in MBASIC. As the author states, the manual is not a beginners book. Despite the examples in it the manual fails to show the power and scope of many of the commands. In this article I will explain a few of the features of MBASIC that are not obvious, particularly with regard to RANDOM FILES, which are an essential part of most serious programs.

First an explanation of what Random files are. There are two main types of DATA files in use on Computer systems – Sequential and Random. (Sometimes a mixture of both may be used.) A Data file will consist of something like a list of names and addresses, or a list of parts, prices and suppliers, etc. As the name implies, items in a Sequential file can only be read in strict sequence. Thus a file system on a cassette is almost certainly sequential. Disks are essentially Random access devices though, because the head can go to any part of the disk at any time. MBASIC supports Sequential and Random access files (the latter needs CP/M 2.x or better). I prefer to use Random files since they have a number of advantages over Sequential files. These include :–

1) A file can be ‘OPEN’ for ‘READ’ and ‘WRITE’ at the same time.
2) Any record (number) can be accessed directly.
3) There is no need to use ‘RENAMING’ routines to avoid losing Data files.
4) More than one file can be open at a time.

A particular advantage of Sequential files is that Data types can be mixed (i.e. String and Numeric). With Random files only String Data can be used. This does not mean that Numeric data cannot be used, as will be shown. Data files of both types are composed of ‘RECORDS’. The final output of the program may be the result of combining records from several different data files. The rest of this article applies only to Random files. You will see from the title of this article that I am a Radio Amateur. Let us assume that I wish to use Random files to keep a ‘card index’ of stations contacted, so that I can quickly find out if I have contacted a station previously. Assume that I decide to save the following data. Obviously I could have chosen to save much more Data such as full address, equipment details, radio conditions, signal strengths etc.

For each separate Record on Disk the following Data items :–

Name– allow 18 characters
Callsign– allow 8 characters
Town– allow 20 characters
County/​State (etc;)– allow 16 characters
Date of First contact– allow 8 characters

The first thing to note is that I have allowed a certain number of characters per item. The reason for this is that the length of the Record to be saved will be the total length of all of the items. Normally BASIC will not allow the length of a Random record to vary so it is necessary to guess at the MAXIMUM length of each item, and to add them all together. The total length of the above record is 70 characters. This space must therefore be left for each












Page 15 of 67