(Excerpts from a message dated 11-17-99, Jonathan de Boyne Pollard to
Murray Lesser)

Hi Jonathan--

    Thank you for your thoughtful comments.

JP>As you will no doubt remember, Murray, back in the mists of time,
  >MS/PC-DOS didn't actually support extended partitions originally. 
  >There was no such thing as primary partitions.  They were just
  >"partitions", and there was a maximum of four of them. 
  > And they could only hold up to 32MeB each.  So if one was rich
  >enough to have a hard disc bigger than 32MeB, one had to have a
  >second, third, or even a fourth "visible" partition.  The idea of
  >having an "extended partition", type 05, came along when this scheme
  >proved to be inadequate.

    I don't really remember when extended partitions were added to
MS/PC-DOS, perhaps because I skipped the 80286 generation, moving from a
PC/XT to an 80386-powered PS/2 model 80.  But, IIRC, there weren't any
hard drives for desktop machines having more than 32 MB capacity before
the PS/2 came along.

JP>The whole notion of extended partitions is an ugly kludge, and also
  >somewhat wasteful of disc space since because of cylinder alignment
  >requirements the secondary MBRs that form the linked list of
  >partition "subtables" waste a whole track each...

    With today's large-capacity cheap hard drives, losing a track's
capacity for each extended partition is most certainly not an important
criterion.  With the old, expensive, low-capacity drives, losing a track
didn't lose much capacity!  That "wasted space" argument is the same
meaningless red herring as is the "cluster size" argument when
discussing the choice between FAT and HPFS formats!

JP>   ...  It's certainly *not* how one would design a hard disc 
  >partitioning scheme if one were designing it from scratch (rather
  >than trying to retrofit something decent on top of the old "four
  >partitions" scheme and retain backwards compatibility).

JP>Given this, I suspect that the concept of having FOUR 
  >*visible* (/i.e./ type 0X) primary partitions is a very old one in
  >the DOS world.  It would certainly explain why almost all PC
  >operating systems, apart from OS/2, support it.

    I really wouldn't know from direct experience, never having owned a
PC-type machine that had more than one primary partition on its hard
drive(s).  My PS/2 had one primary partition and three "logical drives"
in a secondary partition, for ease in system maintenance.  My current
ThinkPad 365XD has a total of seven "drives" on one physical drive, only
one of which is a primary partition, again for ease in system
maintenance.

    But, I spent several years in one of my prior conditions of
servitude as a system architect for a large computer company that was/is
a firm believer in backwards compatibility (having learned it the hard
way a couple of times when "the dogs wouldn't eat" new products that
were were much better designs than, but breaks with, their
predecessors).  So I am still a follower of the first law of systems
architecture, as written by Euripides in the 5th century BCE: "The gods
visit the sins of the fathers upon the children."  Fortunately, unlike
some of those other PC operating systems, OS/2 (Warp 4) still contains
enough backwards compatibility to allow me to be writing this post using
my favorite vintage-1988 DOS text editor :-).

    Regards,

        --Murray

___
 * MR/2 2.25 #120 * Have a frabjous day." he chortled in his joy

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