It is a storage medium like a tape ( not like a floppy!!! ), but faster and with a directory.

A tape is a magnetic stripe. On the QD there is also one stripe in a spiral. That's the reason why I say it is more like a tape than a floppy, because on a floppy there are many cylinders. So it has also the disadvantage of a tape: the stored data is one after an other, and it is difficult to delete data in the middle.

The directory is a trick and not really a directory like on floppy's. The whole QD is read and all found data is displayed. That's the directory. Well, it is much faster than a normal tape.

The Theoretical Capacity Limits of various Disks on the MZ-80K ( p. 5 )

On the MZ-80K, an original Sharp 5.25 ‘35-Track‘ disk contains 70 x 16= 1120 sectors. Using similar calculations a ‘40-Track‘ disk contains 1280 sectors and a 3.5" ‘80-track‘ disk contains 2560 sectors. But on all these disks the first 4 Tracks ( 64 sectors ) are used by the Boot Program, FAT, and Directory; so the theoretical user capacities of 35-track, 40-track and
80-track disks are 1056, 1216 and 2496 sectors respectively.

On the other hand, whatever the size of the disk, the FAT always contains 256 bytes, of which 4 are used for system info ( Vol. No. & Sectors Used ). So there are always 252 x 8 = 2016 bits available to ‘map‘ used sectors.

Therefore the theoretical capacity of a 35- or 40-track disk can be fully utilized, because all the sectors can all be mapped in the FAT with plenty to spare. But the 2496 sectors on an 80-track disk cannot all be utilize because the FAT is limited to 2016 sectors.

Theoretically it might be possible to increase the size of the FAT to allow it to map all the sectors on a 3.5" disk; but this would involve software changes that would make the new 3.5" disks incompatible with the old 5.25" disks, and this was considered to be an impracticable step at this stage.

The Hardware

If you look on the circuit diagram, you see a main part called LH 0084. This is a Z80 SIO. As a result the parallel incoming data is serialized with this part and afterwards it modulates a fixed frequency. This modulated frequency is written on the QD. Also some of the SIO handshake signals are used. Let's have a closer look to the connector T-4 which goes to the QD-drive:

If the QD is off, then the head is in the inner area of the disk and #HOME is 1. Now if you pull down #MTON for a short time the motor starts and the head is going fast ( 1.94s ) to the border of the disk. Than the head needs 5.52s to go back to it's end position. During this 5.52s the #HOME signal is 0 and the head is over an area which can contain data. If you pull down the #MTON signal continuously, this is repeating.

If the QD is only connected to power it needs 120mA. If the motor is on it needs 190mA at 5V.

The format of the QD ( by the hardware )

The SIO is used in a so called synchronous mode. This means you have a data line and a clock line and each data bit is valid after a clock edge. Now we have one problem: When are the bits synchronized?

Therefore the chip needs an defined sync-character. It inspects the incoming datastream until it detects the presence of the sync-character. First after this further data is valid.

In the QD case 2 sync-characters are used, both are 16h. That means, that in front of every data which is stored two bytes with the value 16h are stored. In front of this are normally a view bytes called break are stored. This "break bytes" has a value of 0.

At the end of each written data a checksum is stored. This is a 16bit CRC, but it will be never checked by the Sharp software. After the CRC there is one additional byte called FLAG.

So what do we know now:

If data is stored it looks like this:

The format of the QD ( by Sharp )

The above "frame" is given by the hardware, now Sharp has implemented their structure.

Each data in a frame starts with A5h.

The first frame on a QD tells the File Number of BLocKs ( FNBLK ).

With this information we can now build an empty formatted QD image:

But that is not all, because format also checks if the disk is ok. That means that every position on the disk is written and verified.

So a second frame is available on a formatted disk:

If you want to check this, use "MZQDTool format" to create an empty formatted QuickDisk-Image. This image can be viewed by any hex-editor.

If a file is stored it looks like this:

Like a tape file a normal file has a header- and data block. If we have one file stored on the QD it looks like this:

At first the filenumber frame

Now the fileheader frame

Now the filedata frame

That's all.