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translation table (known as logical sectors). In other words, using the table, when sector 0 is required we find that the data will be read from sector 0 but when sector 1 is required, we actually read from sector 3. If you compare the results of the skew table and the translation table you will see that they have identical effect with one exception A disk that has been written using Sector translation must be read using the same translation table, otherwise rubbich will result as the sectors will be read in the wrong order. If, however a skewed disk is read on a machine that would normally use a different skew factor (ie: perhaps the skew was set up for a 2MHz processor and we are reading on a 4MHz processor), the only penalty would be one of speed of access. It is worth noting perhaps that Gemini skew the system track of the disk differently from the date tracks. On the system track, the sectors are recorded in the order shown below while the sectors on the data tracks are skewed by 0, 1, 2 or 3 as chosen by the user. The example sector skew shown a little earlier had a skew factor of 2.

Example of sector skew on the system track:

0 2 4 6 8 1 3 5 7 9

Getting back to the physical disk format, let us now look at what is physically written to the disk during formatting. Most disk controllers format the disk by writing a complete track at a time. To do this, the host microcomputer will have assembled a memory image of the track which it will then transfer to the FDC (Floppy disk controller) chip. The data consists of gaps, index and address marks, track, side and sector numbers, CRC bytes and of course the areas for saving the data.

Let us break the track up into its component parts and examine them in detail starting with the track, side and sector information and the area for the data.

Each disk sector is preceded by an identification block containing information about that sector. This block is six bytes in length and is identified to the FDC chip by an ID address mark immediately before the block. The data in the block is as follows:

The track and sector numbers on the disk may lie in the range 0 to 255 although this will obviously be limited to the number of tracks that the disk drive is capable of accessing and the number of sectors that will fit on one track of the disk. The side number takes the value 0 or 1 The sector length byte (with the FD1797) may fall in the range 0 to 3 and the values correspond to sector sizes of 128, 256, 512 or 1024 bytes respectively. The two CRC bytes are automatically computed by the FDC chip and will be written to the disk when instructed to do so by the host computer.

When the FDC is instructed to read a sector, it first waits for an ID address mark and then reads the ID block. If the side, track and sector information matches the values given by the host computer, and if no CRC error has occurred, the FDC will transfer data from the data area following the ID block. The data area will be of the length indicated by the sector length byte and is preceded by a Data Address mark on the disk. Initially, the data contains the value 0E5H for every byte. This value is set during formatting but in fact any value could have been used. 0E5H is used, by convention, because IBM used this value in their original floppy disk formats (back in the dim and distant past when 8″ floppies were used with mainframe computers and micros had not yet been invented). Finally, the data area is terminated with 2 CRC bytes for error checking.

Writing a sector is similar to reading but after the correct ID block has been found, a data address mark is written to the disk followed by the number of data