Sharp Users Club - MZ-700 Section - Disks in detail

I would first like to give my greatest thanks to Ching Rashbrooke for supplying me with the Rom disassembly and boot disk, also to Danny Abondanza for his assistance with Disk Basic.

My objective was to get a disk system for my MZ700 without spending the vast amounts of money usually required for such nice things. I wanted my disk system to be compatible with existing disk systems but to also provide a more comprehensive operating environment.

The Hardware

From the Z80's point of view my hardware had to appear identical to the original hardware, this being the Kersten & Partner disk system from Germany. First I went through the disassembly of the Boot Rom and flowcharted it, this told me what IO addresses the system used, and how to write to each.

The Software

The firmware uses memory starting at 1000H, these bytes and their uses are :-

The interesting thing to note is that all disk accesses are done by reference to the logical sector numbers. Each disk is double sided, 35 tracks per side and 16 sectors per track. To convert a logical sector into physical side, track & sector the following calculation should be done :-

In practice however the software assumes it is a 70 track drive and automatically splits the track number ( 0 to 69 ) into track and side when accessing the disks.

The bootstrap ROM lives at F000H, when you type 'F' from the monitor, the processor looks at F000 and if it sees a ROM there then jumps to F000. This displays a prompt asking for the boot drive, press 1 to 4 or simply [CR] to default to 1. The system then reads in logical sector 0 to memory at CE00 ( 256 bytes ). If it is a boot disk this sector will contain a header with a file name beginning with “IPLPRO", a new memory load address, the size of the file and the start sector on disk. This is then loaded and run, the disk I had was the Disk Basic disk.

All the routines in the ROM get their parameters from memory pointed to by the IX register, this is usually set to 1008H. It is possible to use these ROM routines yourself by setting up a parameter block and calling them, however I am not sure that all versions of the boot ROMs contain the same code. To read sectors from disk into memory the following technique was used,

Using the above technique I was able to write a small MC program to read disks sector by sector and display their contents in Hex and ASCII form. The following are printer dumps of the results :-

First Sector of Boot Disk. File = MZ700D-BASIC, Size = 8800H, Load = 1200H, Exec = 9229H,
Sector 0030H. N.B. 02 = Boot Disk ( 03 if 1E05 is used ).

Analysis of the above for various disks enabled me to determine how the system uses various sectors.

Sector 0 = Boot information sector.
Sector 15 = Sector Allocation information.
Sectors 16 to 23 = Directory information.

Sector 15

Byte 0 indicates the volume number, 0 is a master, any other is a slave. A master contains the bootstrap program. Byte 1 is the first usable sector on the disk, for a non master this is 30H or for a master is B8H. Bytes 2 / 3 point to the next sector to be used for writing a file. Bytes 4 / 5 indicate the total number of sectors on the disk, this is 0460H which is 1120 decimal. Next follows a sector allocation map, this is a bit map of which sectors are used, a bit set indicates the sector is used, a bit reset means the sector is available. The last few bytes are I think a serial identification number.

Sectors 16 to 23

A maximum 0f 64 files can be held on disk, only 63 are usable as the first directory entry is reserved by the system. Each directory entry takes 32 bytes, i.e. 64 files * 32 bytes = 8 sectors * 256 bytes. Each of the 32 byte blocks contains the following Information :-

Sectors 0

This contains one directory entry in the above format. This is the program that gets run as a bootstrap, however this does not get included in the directory listing.

The Kersten & Partner System

The K & P system has the strange convention of Master and Sub-Master disks, a Master disk being the original supplied disk and a sub-master being one copied from a Master. The disk contains a utility allowing disks to be copied, however you cannot copy from a Sub-Master disk, presumably to prevent people like me using the system without buying the Hardware. However if your Master disk gets damaged you are stuck with however many sub-masters you managed to create. The way round this is simple, the disk copy routine contains a bit of code that checks whether you are copying a Master, if so it crashes out with a rude message.

1) Load the utility in the usual way and press the reset button.
2) Using the monitor 'M' command enter the following :-

13B9 20 18 [CR]   ( You enter the 18 and CR )
13BA 23           ( Press Shift / Break to exit M command )
J1200 [CR]

The disk copy option will now copy anything.

There is also what I consider to be a bug in the Disk Basic. There is a utility which allows machine code programs to be loaded from tape and saved to disk, these can then be executed from disk basic. However certain programs refused to run properly, on investigation I found it was programs using any of the monitor melody routines including the beep. In fact it was because Disk Basic changes the TEMPO byte at 119E, this causes all sound routines to take much longer to execute. A simple solution is to modify disk basic as soon as it loads by including the following two lines as part of your ‘AUTO RUN‘ program.

1 RESTORE : FOR I = 0 TO 4 : READ D : POKE $7AB3 + I, D : NEXT
2 DATA $3E, $04, $32, $9E, $11

This simply causes the original value of 04 to be restored before the interpreter jumps off to execute a loaded machine code program.

What Next

I have written assembler routines to perform the following :-

My intention is to use these to produce a BIOS / BDOS for a simple operating system which is to be tape compatible. The directory read / write and file load / save routines are compatible with the inbuilt tape system. Each time a header is loaded the software effectively reads the 'Next' directory entry into the tape header area of memory. The disk is therefore treated as a tape with many files, except access is direct as the directory points to the location of the files on disk. Any help or constructive comment concerning a simple DOS would be most welcome, also I am considering making a PCB of the controller interface, if anyone has a PCB design package or access to one and feels they could have a go then write to me and I will supply the circuit diagram.

Editor's comment:-

The Kersten and Partners system of Masters and Sub-masters sounds very similar to the system used by Sharp with their MZ-80K / A / B disc interpreters. In recent issues we have shown how to create Masters on the MZ-80B and the MZ-80A, and in this issue, thanks to John Edwards ( see p. 35 ) we reveal how it may be done on the MZ-80K. Now, thanks to Tim Cowell, we also see how to create a Master of MZ-700 Disc Basic.