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2 : Discs and Formats        


The chip used in the Master to control the disc interface is very versatile and can handle a number of different disc formats. For example, the Master allows single and double density disc formats via DFS and ADFS respectively. All DOS formats are double density. When you remember that the memory of a DOS system is about 512k, it makes sense not to limit the capacity of a disc to less than this, if at all possible.

DOS disc formats can apply equally to both 3.5 inch and 5.25 inch drives, so no distinction is made here. Both can be used on the Master and 512 but quite clearly a 3.5" drive cannot be used by a 5.25" disc and vice versa so clearly a physical incompatibility exists. However combined 3.5/5.25" disc drives are readily available nowadays and such problems can be avoided.

For the remainder of this discussion it is assumed that the physical size of the disc is irrelevant, though of course DOS disc formats were originally based on 5.25 inch discs, since DOS pre-dates widespread use of the 3.5 inch disc.

Standard PC discs are 40-track double-sided with a capacity of 360k. As 80-track discs are common with Master systems, Acorn provides for both 40 and 80 track formats. All DOS formats and all Master 512 formats use double density – commonly called MFM encoding.

The most important format for the 512 user is the 640k bootable DOS system disc. DOS, unlike the BBC micros Operating and Filing Systems, is contained on disc and not ROM. Hence, the Master 512 must be able to load the Operating System from disc (in this case DOS Plus) before full function is available.

All PC-compatibles operate in this way, and only have a relatively small program in ROM called the bootstrap. This bootstrap contains only sufficient code to load and run DOS from a cold start, ie when turned on for the first time. The 512 initially calls standard ADFS routines to load DOS Plus from disc, consequently the BOOT disc must be in a format which ADFS recognises, in this case ADFS 640k. Looking at the disc from ADFS, it appears just like a standard ADFS 640k disc, but with a single DOS file on it.

The disc size is set larger than normal to make the disc recognisable, and to prevent overwriting from ADFS. Hidden away behind this ADFS front, is a DOS partition, an area of disc which is reserved for DOS use only.

In addition to the 640k format, which itself is not seen on any other DOS system, there is an 800k format which maximises available disc space. Due to the way in which it saves data onto the disc, this format is very much faster than the others, and, as such, is possibly the best one to use whenever possible.

There are other formats to consider, a complete list is given in Table 2.1, and all these formats are fully explained in the Master 512 Technical Guide (see Appendix G).

      Format Type            Capacity       Equivalent
  Acorn DOS format   800k   800k ADFS
  Tandy DOS format   720k    
  Olivetti DOS format   720k    
  IBM DOS format   720k    
  Nimbus DOS format   720k    
  Altos CP/M format   720k    
  Acorn bootable DOS   640k   640k ADFS
  ADFS Acorn Z80 CP/M format   400k   400k DFS
  IBM DOS format   360k    
  IBM CP/M format   320k    
Table 2.1. The Master 512 disc formats.

No special commands have to be given to use a particular disc format. The DOS Plus software will automatically detect the format size of a PC disc. This means that it is possible to have different format discs in the disc drives. The main format used by the Master 512 is the 800k format so this is checked for first. DOS then works its way down the list until it has checked all the formats available. If the disc is not recognised an error message will be generated.

As a rule, if you wish to transfer data or software to or from other PCs but are unsure of the format to use, 360k is almost certain to be supported. Failing certain knowledge this is the best guess. However, if you seem to have trouble with this format it may be because two closely related forms were used as standard (!) and you have chosen the wrong one.

If you need to transport files by this means and are sure that you have the correct format but still encounter difficulty, you may find it solves your problem if you use discs formatted on the PC in question, rather than on the 512. This seemingly strange state of affairs can occur because there are minor differences between the speeds of the different disc drives, and while a fast disc drive has no trouble with a slow formatted disc, the reverse is not true. <But note here – YP>

Finally, in spite of its considerable facility in this area, there are two PC disc formats that the 512 cannot handle. These are generally seen on the more recent PCs and particularly on 386 machines. These are 1.2Mb and 1.44Mb, which are often referred to as 'high density' (HD) because they literally write twice as much data in each track of the disc. The Western Digital 1770 or 1772 controllers, used by Acorn, cannot support quad density, so there is absolutely no way these formats can be accommodated by the 512.

You can however format blank HD media to the lower formats on a 512, but it is a waste of money to do so, as HD blank discs cost much more than double density discs.

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