WHY FORMAT?

BY: BOB BARKER

FROM: SCOTTDBORO COMMODORE USER
      GROUP NEWSLETTER DEC-85.

     To half of the Computer population formatting a blank disk is almost as
automatic as breathing out after having taken a breath in. To the rest of us
it appears to be some sort an ancient pagan ritual required as a sacrifice to
the 1541 god of ROM. Audio cassettes, reel to reel tapes, VCR tapes and other
magnetic media record data without any formatting, so why should an
"intelligent" device like a 1541 be different ? 
The answer is a study of the accuracy required of computers. A slight
distortion on a TV picture is acceptable. Look at a test pattern or cross
hatch pattern, and you will see that the circle of the pattern is not
perfectly round nor are the cross hatch lines parallel but as long as Tom
Selleck doesn't look like "TATOO" people won't adjust the Vertical or
Horizontal controls. Computers work with only 1's and 0's so there is no
"almost" zero or one. Close is not acceptable. Think how garbled a program
would be if it was off by just 1 bit. Take the sentence "This is a test of
being off by one" moved to the left one character becomes - "hisi sa t esto fb
eingo ffb yo ne ".
To avoid any chance of errors during saving or loading, a disk is first
formatted and the following occurs:

1) The head is bumped against the stop, then moved out 1/2 of a track. This
places the head in the center of the first track.

2) The Disk Operating System (DOS) which is the program in the disk drive ROM
writes a full track of 255's-syncronization (sync) marks-11111111 which
effectively erases anything on the track.

3) The DOS writes 4000 55's-01010101 

4) The DOS computes how many bits will fit on this track by calculating the
ratio of the 55's to the 255's. If the drive runs a little slow more bits will
fit as the bits are written at a controlled rate. If the drive is fast, fewer
bits will fit on the track.

5) The DOS now computes where to place all of the pulses to mark the start of
a block and the number of bits required to fill the gap after the block is
finished before the next block starts. NO space is left on the track that is
not written to. The header information is calculated, dummy data is created to
fill the block and check sums are calculated. Believe it or not, all of this
is calculated but the actual formatting has yet to start !!!

6) The track is totally erased by writing 55's again. Over 10,000 are written.

7) Formatting now begins with the writing of sync marks, headers, dummy data,
and finally the exact number of bits to fill the gap before the next block
begins.

8) This process is repeated for each block until the track is full.

9) The DOS then verifies everything it has done by reading it all back and
checking it.

10) If all is well, the head steps to track 2 and goes back to step 2 and
starts again.

11) After 35 tracks, the DOS does some cleanup work, writes a BAM then returns
control to you.


The purpose of this discussion is not to confuse you with details but rather
to point out that when a disk is formatted the DOS builds a bunch of magnetic
file cabinets to store data in, draws itself a map of where they are located,
and puts folders in each drawer, so that when the time comes to write data to
the disk it can be done quickly with a lot of error checking. If you think
about all of this it is obvious why you can't switch program disks between
different computer systems. COMMODORE, APPLE, ATARI, and TRaSh-80 all use
unique systems of track, sector, header, sync location etc. information that
the operating system uses to keep up with data. It is amazing that we can all
use the same 5 1/4 inch disks.
 
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