Sector interleave (was: H89 CP/M hard-secored boot diskette

From: Fred Cisin <>
Date: Thu Apr 3 16:54:00 2003

> > I always wondered, if it was so easy to change the sector interleave on
> > disks to make them load faster, why didn't the original
> > programmers/designers optimize the interleave in the first place?
On Thu, 3 Apr 2003, ben franchuk wrote:
> It depends on your disk hardware and the speed of your computer.
> 1) Computers got faster 2) Disk i/o got dma rather than poiled
> i/o.

It also depends on what kind of processinbg is being done to it during
loading. For example, dumping file contents into RAM can be done a lot
faster than loading a document into a word-processor.

The fastest interleave occurs if the system can handle the next sector
immediately after the previous. In that case, the fastest sequence is
plain sequential, and a track can be read in a single revolution. But
when you add extra processing of the data during reading, (such as a word
processor loading a document), you could end up with NOT being ready for
the next sector in time, resulting in the read taking as many revolutions
of the disk as there are sectors per track. Alternating sectors might
reduce that to two revolutions (or skipping 2 sectors, etc.), but that can
easily be twice as long as necessary for a task that did NOT need the
extraneous processing. Most system software developers DO think that
they have optimized it. But they have optimized it for the type of file
access that they EXPECT you to be doing (usually whatever THEY do most).

BTW, it can be done either by placing the physical sectors in various
orders (such as 1,3,5,7,9,2,4,6,8,10) OR can be done by a translation
between logical sector and physical sector (physical sectors numbered
1,2,3,4, . . . , but data that doesn't finish in sector 1 continues in
sector 3, etc.)
Received on Thu Apr 03 2003 - 16:54:00 BST

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