Thursday, December 15, 2016
Cromemco MFM Hard Disk Formatting
Subtitle: How long does it take Marcus to format a hard Disk?
Answer: About 1 day :-)
At first my answer to Agata about the formatting time of 1 day, or about 8 hours of Marcus' life would seem to be quite high, especially since the disk is only going to be 91MB in size, post formatting.
Presumably though this is exactly the point.
This is the disk:
Yes this disk however is now over 30 years old and it is somewhat of a miracle that it is still working electrically or mechanically, or that there is a computer that can use it, or a person that knows how to use it well.
And the capacity of just under 100MB is 500 times smaller than the storage card in my mobile phone. Oh mi, how things have moved on since the 80's
The following worked example of a disk format using Cromemco Cromix 167 my Operating System of choice in about 1981. Oh, and still my Cromemco Operating System of choice in 2016.
Unlike with today's hard disks there is no mechanism for the host to equire about geometry, power on hours that primarily the SMART standard has made so easily possible today
Using the Cromemco STDC and STDX hard disk controllers the data track is not split into conventional sectors. Instead a single read and write track command is used to write the 10K bytes contained on the surface. The STDC controller then buffers the output reads and writes back to the operating system in a more manageable fashion.
This does mean that if any part of a track surface is faulty then the whole track is marked as unreadable. In a more conventional schema of the time the track is divided into sectors and individual sectors are remapped if erroneous.
Example: Your disk contains 1000 tracks from 0-999. And there are 6 heads so meaning head 0 thru 5. You allocate the data part to be between 1-990 and leave 991-999 for bad track alternative mapping.
Whilst formatting you find track 146 surface 5 (head 5) has an issue. So you relocate this to track 990 head 0. So then when the Operating system tries to ready 146 surface 5 it gets remapped to track 990 head 0.
There is no 4 partition limit here as was imposed in later PC Operating System.
It is good practice to do this for these 35 year plus old disks (!!) so we can be sure that no 'extra' read errors crop up after the disk has been spinning for a few hours. Again, with media of this age, who design life was no doubt about 5 years, but is now over 35 years old, one has to be a little bit more careful.
Cromemco Cromix also provided a special rcopy command that would copy especially quickly from block devices, for example hard disks. So back in the day it was good practice to have pairs of hard drives. You could use the rcopy command to make a bit for bit copy from source to target hard disk. If the source disk should fail simply boot from target and repair source.
Today this practice lives on in the UNIX and Linux community using the dd command to copy disks, although metadata on hard disks can make this a bad idea.
I hope that I have shown why formatting a single disk on my 1980's Cromemco Cromix computer system can take the best part of the day. Well to do it professionally and in an error free manner, given that this ancient media can now have significant defects which must be detected and then remapped.