The purpose of these brief notes is to give the reader an understanding of exactly what is happening when discs are in use on the MZ-80K. lt should be realised that in general what is said is with reference to the discs as used within BASIC 6015. When Sharp FDOS, CP/M or any other DOS is in use there may be considerable differences in how files are held. Everything in this article has been deduced from a study of the hex. code used within BASIC 6015 and the Sharp supplied utilities, together with considerable experimentation. Much of the work was carried out using the Sharp “Machine language“ tape and latterly with the disassembler written by R. Tanswell.

A disc is divided up into 70 tracks of 16 sectors each to give a total capacity of 1120 sectors. One sector can contain 128 bytes of data to give an overall capacity of 140K, the figure quoted by Sharp. Tracks are numbered from 0 to 69 with track 0 nearest the outer edge of the disc. Sectors are numbered from 1 to 16. In the Sharp System all even numbered tracks are on the top face of the drive and all odd numbered tracks in the lower face ( when the disc is in the drive ).

Not all the tracks are available for file storage, in fact the system uses tracks 0, 1, 2 and 3 for its own purposes. Thus a freshly initialised disc gives 1056 sectors i.e. 132K of user file space. Tracks 1, 2 and 3 are reserved for the directory of what files are held on the disc and track 0 is used in the following way. Sectors 15 and 16 contain the bit map giving which sectors of the whole disc are free and which ones are in use for file storage. The volume number and a count of the number of the potential 1056 sectors that are in use is also held in sector 15.

A master disc has the secondary boot code in track 0 sectors 1 - 14. A non-master does not have this code but the sectors cannot be accessed for file storage and so represent wasted space. Further space is taken up on a master by 6015 BASIC in tracks 4 - 13 inclusive, although no directory entry is present for the BASIC.

When a file is stored on disc an entry is made in the directory of the file name and the whereabouts of the first sector of data. At the end of the first data sector of the file is the address of the next sector and so on. These links use 2 bytes giving the track number and sector number, both in hex. Thus only 126 bytes of each sector can be used for data. The next 2 tables below show this linking. The important of these links will become more apparent in later paragraphs for although in many instances a file is stored in consecutive sectors - this will not always be the case.

lt is important that the system can determine which sectors on the disc are in use so that when a file is stored it does not overwrite part of what is already on the disc. This is achieved by keeping a map of all the sectors that may be used for file storage and indicating in the map which sectors are in use and which are free. As stated before there are 1056 sectors for file storage. One bit is used to represent each sector hence the map is 132 bytes long with 2 bytes i.e. 16 bits to represent each track. Track 0 sectors 15 and 16 are used for this bit map with some of the spare bytes holding the disc volume number and a count of the number of sectors in use. In the map a free sector is represented by the relevant bit being 0 and a sector that is in use being represented by the bit being 1. Track 4, the first track available for file storage, is mapped into bytes 4 and 5 counting the first byte of the sector as byte 0. Byte 4, bit 0 is sector 1, byte 4 bit 1 is sector 2 etc. until bit 7 is sector 8. Byte 5 bit 0 is sector 9, byte 5 bit 1 is sector 10 etc. Writing bytes 4 and 5 in binary as below makes it clear.

Byte 4				Byte 5
0000		0000		0000		0000
Sector 8		Sector 1		Sector 16		Sector 9

Bytes 6 and 7 represent track 5, bytes 8 and 9 track 6 etc., through to track 70.

lt should be noticed that tracks 0, 1, 2, 3 do not appear in the bit map since they are used by the System and hence never available for file storage. Byte 0 of sector 15 is unused. Byte 1 holds the volume number in hex. eg. if the volume number was 127, byte 1 would contain 7F. The next 2 bytes hold the number of sectors used for file storage.

When the directory is displayed the number of free sectors is calculated and the free sector count is displayed. Thus the overall picture is:

The purpose of the directory entry is to enable the System to “know“ enough about a file to be able to load it into the RAM correctly. The directory entry is directly analogous to the header written to a cassette when using tape storage with the BASIC 5025. The next table below shows the function of each byte in the directory entry. Note that each entry is 54 bytes long and hence each sector of the directory can hold 2 entries giving a total possible of 3 x 16 x 2 = 96 file entries.

From the previous sections most of what has to be done when a file is erased should be apparent; it is

1. Find all sectors on the disc that were part of the file and clear to zero the appropriate bits within the bit map.
   
   
2. Remove the directory entry.
   
   
3. Update the ‘sectors in use‘ count.

In the MZ-80K when one entry in the directory is removed all subsequent entries are moved down one place to close the gap formed. Thus after a file is removed all later entries are rewritten one place down. This can result in the final entry in the directory being duplicated. To prevent ambiguity arising the now redundant entry has a zero as the file type code digit. When the system is reading the directory a zero at this place indicates that the end of the directory has been reached.

By following the link in the directory entry of the file to be erased and following the links in the file all the sectors used for it are found. The bit map is then updated together with the ‘sectors in use‘ count. ( lt is this following of links that makes erasure so slow ). When a new file is saved on disk free sectors are used up in the order they appear on the disc. After many file erasures and fresh files being saved, some files may occupy sectors scattered across the disc, hence the importance of the links at the end of each sector.

When FD is typed from monitor, the following sequence of events takes place. The monitor examines the contents of address $F000. If it is found to be other than 00, the monitor retains control. ( on the disc controller card ROM is fitted that starts at address $F000 and the contents of the first byte is 00.) If the controller card is fitted the code beginning at $F000 is executed. This code is the primary boot which reads track 0 sectors 1 to 14 inclusive and loads this secondary boot into RAM starting at address $9800. The first byte of this code is then examined and if it is $C3 control is passed to $9800. This code then reads tracks 4 - 13 inclusive into RAM starting at address $1200. BASIC 6015 has now been loaded and control is handed over to it.