Disks and the Floppy

We are all familiar with the cassette recorder attached to our Nascom, it provides us with low cost mass storage, and permanent storage for programs and data. The method of recording is governed by the type of signal the cassette can record. Although stereo recorders allow two channels to be recorded, the Nascom in fact is designed to record on only one channel, thats because its easier and better to do it that way. In order to store our information on to one channel, we have to convert it to a serial stream of bits that can be recorded. Now this is the same process used for years with teletypes, and why the UART was invented. This device converts the input byte into a stream of 10 or 11 bits which consists of a start bit to tell the device to get ready, then the 8 bits of the byte, followed by either 1 or 2 stop bits to say its finished. When the tape is replayed, the reverse occurs.

This system is known as asynchronous transmission, as each byte is sent independantly of the others. When NAS-SYS records a block using the “W” command and reads it back again with the “R” command, it knows there are 256 bytes in the block but the electronics handles each byte quite seperately. As far as the electronics is concerned, there is no difference between the typed input and the recorded input.

Disks are not quite the same. The recording media is the same type of material but arranged in a disk. The disk has only one surface for recording purposes. (I know they are double sided, but so is paper. You only write on one side at a time.) The recording head of the disk can be moved radially on the disk surface, but does not move while reading or writing. This allows a single head to be moved to sit over different “tracks” of the disk. As the disk turns, the same piece of disk keeps coming round and can be read or written to each time it passes. The floppy turns about 5 times per second and can record one change of flux (one bit) in 8 micro seconds. Thus a single turn would allow 1000000/8/8/5 bytes. Or 3,125 bytes per track.

Formatting. To get the most out of this space, a different system of coding is used, synchronous data transmission. This way we don’t send a start bit and a stop bit, and so only send eight bits/​byte. We have of course to pay a penalty for this, we need to know when the data started, so as to know which is the first bit. Because we can move the head about, we also need to know where it is, and limit the number of places where it can be. This gives a fixed number of tracks on the disk. On 5 inch floppies there are 35 or 40 or 77 tracks depending on who makes the drive. This gives a maximum drive capacity of 3,125*35 (*2 for systems with two heads capable of writing to both sides of the disk) bytes. That’s a lot of storage, nominally 218,750 bytes for a double sided 35 track system. In order to access this data and to check for errors, we need to format it. This involves writing data onto the disk which does not change, but helps us find where we are.

Keep tracking. We divide the track up into sectors, each of which holds a nice round (HEXadecimal) number of bytes of data, 128, 256, or perhaps 512 bytes per sector. A few systems even use as much as 1K per sector. However, to know where we are, we must also include a number of bytes that will allow error checking and location. The most reliable method of error checking is known as ‘CRC’ or cyclic redundancy checking. This is simple and very fast to do in hardware but takes time in software (so CRC checks and generation is usually performed as a function of the disk controller chip). We also have the address for the sector, that is the track and sector header at the start of each sector with its own CRC, plus some spare bytes to allow time to switch from reading to writing, so that we do not have to rewrite the address data each time.