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Quarts into pint pots (Computing with the Amstrad) | 200K DISC FORMATTER (COMPUTING WITH THE AMSTRAD) |
DAVID PATTEN provides a routine to format your discs for 200k AMSTRAD 3in discs are expensive, even after recent price reductions, so any way of getting more information on them is a good idea. Program I will enable any Amstrad CPC to format discs to store 200k, though the actual capacity is 198k because 2k is taken up by the directory. It is a disc formatter which writes 10 sectors per track rather than the standard nine. To achieve this the program plays on the flexibility of Amsdos by altering the disc parameter block. This is an area of memory which holds the variables the disc rom uses when accessing a disc. Amsdos is configured for 40 tracks a disc, each track containing nine sectors. Think of it as a round cake which has been cut into nine even wedges, each representing one sector. The sectors are numbered differently depending on their format. System format uses &41, &42, &43, &44, &45, &46. &47, &48, &49, whereas data format is numbered &C1, &C2, &C3, &C4, &C5, &C6, &C7, &C8, &C9. In practice the sector numbers are mixed on the surface of the disc to give an offset like this for system format: &41, &46, &42, &47, &43, &48, &44, &49, &45 This offset is referred to as the skew factor. It is used in most disc systems to allow the disc head to read from the disc, store the sector read in memory and still have time to prepare to read the next one. Amstrad discs have a skew factor of two, which means that when reading from, or writing to, the disc the system reads a sector, skips a sector, reads a sector and so on until all the sectors required have been read from the track. If a skew factor were not applied the disc access would be much slower. The system would have to read the first sector and then wait until the disc had made one complete revolution before it could read the second. If the sector numbers ran consecutively the disc would have to make nine revolutions to read all the sectors, whereas with the skew factor only two revolutions are necessary to read the entire track. As mentioned earlier, to achieve the extra capacity a sector is added to the end of each track, giving an extra 512 bytes on each. To do this we need to reduce the timing gap lengths when formatting or accessing the disc so that the head always lands in just the right place for each sector. The experienced programmers among you may like to acquire a copy of the data sheet for the floppy disc controller , which is a NEC uPD675A. This gives full details and helps in understanding the way in which the Amstrad disc system is implemented. I don't recommend messing blindly around in areas which directly affect the way in which the hardware performs, unless you understand what is expected and what the consequences may be. As the 200k format is alien to the Amstrad system, we must patch the disc parameter block to instruct the machine to access it. Consequently you need to use Program II, a short routine which will allow you to switch between the standard and 200k formats. When the machine code has been poked into memory it will be saved as a binary file, INIT.BIN. To use the new format either in your own programs or from a cold boot, run a small program to patch the disc parameter block in the form: 10 LOAD"INIT.BIN",&9C40:CALL &9C40Save it as INIT.BAS, then run it before using the 200k disc. If you want to use the 200k format within your own program this line should be included to load the code and initialise it. A CALL &9C76 or POKE &A8A8,0 instruction will reset the system to accept normal formats. Once the new format has been initialised the computer will operate as usual in all respects. In other words, all the normal commands associated with the disc system will work as expected under Amsdos. CWTA |
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