INMC 80 News


October–December 1981, Issue 5

Page 40 of 71

Each directory entry is 32 bytes long. Sixteen bytes for the block allocation map (telling CP/M where it put the blocks), and sixteen bytes used to store the file name, the extent record and the actual number of sectors used for that entry. Most files will have only one extent, but large programmes like MBASIC take more than 1. (MBASIC is 24k three times the Nascom BASIC).

BIT mapping

CP/M knows for each file which sectors (or strictly each block) that has been used, but needs to know how many and which blocks or sectors are free. This it does with a bit map. It uses a sector of 128 bytes to store the sectors used as 1 bit in each byte. It can then know exactly which and how many blocks are used up. A second advantage is that if it wants to set aside a portion of disk not to be used it only needs to set the bit map and the system will not use them. Clever, eh.

CP/M Commands

There are a number of commands within CP/M, such as DIR (directory), ERA (erase), etc. Note that these are single word commands. If CP/M was given a single word command, but could not find the name, it assumes it is the name of a file ending with the ‘file type’ .COM and loads that in and executes it, so you can add your own commands. Better still!

For example if you rename BASIC as RUN.COM and you type RUN STARTREK, CP/M will load BASIC and then load the basic program STARTREK. It is almost friendly.!

Well so much for this week/​month/​quarter/​year.


Review of the Arfon Speech Synthesis Board

by Steven Hanselman

The board comes well packed in a cardboard box. The board itself is contained in an anti-static treated bag. The board is Nasbus compatible although at 8″ x 4″ it is half the size of most Nasbus boards. Input to the board is through port F6 (hex) that’s 246 (decimal). Output from the board is audio through a small but adequate speaker attached to the pcb, or a 3.5mm jack socket to an external speaker or amplifier. A status bit is returned via port F6 to tell you when the board is ready to accept another word. The speech synthesis part contains 144 pre-programmed words contained in two 64k bit ROMs. These ROMs are the same type of ROMs as the Nascom Basic ROM, so if you plug them into the Basic socket of the Nascom, you could tabulate them and probably work out how the speech is stored. Having done that you could purchase some more 64k roms (Maplins sell them) and find some way of programming your own speech set. (Doubtful but interesting premise. Ed.)

The board is exceptionally easy to program in Basic, all that is required is that you use the WAIT statement to monitor the busy line until free and when free output the value of the next word that you want the board to say. Although the ROMs only have a 144 word capacity, it is possible to make up almost any word that you want by truncating parts of words and using truncated parts to make the new word. This is not ideal, as unfortunately, the way the Digitalker produces such good quality speech is by adding emphasis to certain characteristics within speech, such as raising the tone near the end of the word, and because of this when you put pieces of words together you sometimes find the words sounding slightly wrong. Apart from this the Digitalker board seems to be good value for money and great fun to use. The documentation fs splendid as it reveals everything that you require to know about the board, how to use it, and how it works.

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