Scor­pio News

  

October–December 1987 – Volume 1. Issue 4.

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When writing CP/M records to disk, the BIOS computes the physical sector number im the same manner as for reading. It then checks to see if that physical sector is already in the buffer and reads it if not. This pre-read of physical sectors explains why the disk performance is not quite as good when writing compared with reading. However, the performance achieved is still better than when writing byte sectors as the disk is still only accessed twice for every four CP/M records written.

The pre-read may be skipped under certain circumstances. CP/M tells the BIOS that the data written lies in an as yet unallocated block. If this is so, there is no data in the sector to be destroyed so the read it skipped. If the pre-read is required, before reading the sector into the buffer, there is another check that must be done by the BIOS as we shall see in a moment.

Having read the required physical sector, the buffer in use is set and the CP/M record may be put in it at the appropriate place. You will see that the other records in the sector have now been preserved. The BIOS doesn’t write the buffer to disk at this point as another write to the same physical sector may be requested by CP/M, so it simply flags this record as containing unwritten data and returns control to CP/M.

If another write is requested to the same sector, the pre-read of the disk will not be required as the wanted physical sector will already be is the buffer. However, if another write (or read) is requested that involves a different physical sector, the BIOS looks at the flag showing whether the buffer contains unwritten data and if it does, the BIOS now writes it to disk.

The only exception to the above explanation is when directory information is being written to disk. Since the BIOS maintains a separate buffer for physical directory sectors, the changed directory information MUST be written to disk immediately.

THE END !

A Review of two Modula 2s

by Doug Taylor

If you liken programming languages to cars, then Cobol is an Austin 7, old fashioned and requiring a lot of maintenance, BASIC is a Fiat 127, noisy, prone to rust and always running out of performance when you need it. Pascal is the Volvo of the programming world, careful, sale and always willing to cruse up the hard shoulder at 90 MPH. Whereas Fortran 77 is a Range Rover, bust go anywhere, do anything, break or remake all the rules language.

So where does Modula 2 fit? Well this is a kit car, built with strength of Volvo – it is after all a member of the Algol family, a child of Nicklaus Wirth, but bas all the versatility of Fortran. Modula 2 bas been used by Wirth to correct many of the mistakes of Pascal, the rather fussy and confusing syntax has bees tidied up, (I could never remember when I needed a semicolon and when I didn’t) and the most powerful feature of Fortran, the foundation in library based code, imported.

Modula 2, as it name suggest, encourages you to write modular code. These modules are combined into libraries, and programs created by importing code into the program from these libraries. The library will usually consist of Modula 2 code, it is a very recursive language, Modula 2 code is made from Modula 2 code which is made from Modula 2 code which is ...., but in most if not all applications, code from other repeatable code producing tools can be linked into the program, e.g. from an assembler or Fortran Compiler.

The two versions of Modula 2 I will review are Turbo Modula 2 from Borland International (only available for CP/M at the moment) and FTL Modula 2 from Workman and Associates (HiSoft in the U.K.) which is available for MSDOS and CP/M, the version reviewed being the MSDOS variant.

The Borland version of the language comes complete with a 544 page manual describing the language, the Wordstar like editor, the shell and the Standard library supplied. The standard library and reference directory to the modules is explained briefly with examples, covering 411 pages of the manual. If you have used Borland’s other minor work Turbo Pascal you will know the language high standard of their manuals – all examples in the manual will work and you can learn the language from these alone.


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