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Browse Prior Art Database

Reduction of Command Overhead in Hard Disk Drives

IP.com Disclosure Number: IPCOM000117453D
Original Publication Date: 1996-Feb-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 2 page(s) / 91K

Publishing Venue

IBM

Related People

Bipes, RA: AUTHOR [+5]

Abstract

Disclosed is a method of reducing the command overhead in a hard disk drive by precalculating several parameters and after a request for data, doing a limited number of comparisons in the order necessary for the shortest times needed to begin an access, after which the rest of the calculations are done concurrently with the access.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Reduction of Command Overhead in Hard Disk Drives

      Disclosed is a method of reducing the command overhead in a
hard disk drive by precalculating several parameters and after a
request for data, doing a limited number of comparisons in the order
necessary for the shortest times needed to begin an access, after
which the rest of the calculations are done concurrently with the
access.

      The full calculation of a target Logical Block Address (LBA)
into the zone, cylinder number, head number and sector number
requires a fraction of a millisecond, which directly adds to the
total access time  of a Hard Disk Drive (HDD).  Once an access is
started, the actuator is  normally driven in an open loop fashion
until the velocity intersects the distance-to-go velocity profile, at
which time the servo provides a  controlled descent to the correct
track.  The overhead calculation time  can be minimized by starting
the access on minimal information determined  in a much shorter time,
and then completing the calculations while the  access is in the open
loop part of the profile.

      One of a few quantized levels of initial actuator current is
selected by a few comparisons of the requested LBA to the
precalculated LBA values corresponding to different access lengths.
Although the head  number is not known at the start of the access, it
is known before the  controlled portion of the access profile is
started.  The method described here is to do as much of the required
calculations for an access before the command comes in.  Next is to
calculate only what is  needed to start the access, and complete the
calculations in the order  required, in a "just in time" way.  The
average time is also reduced by ordering the comparisons
probabilistically.

      The most important things to determine are if an access is
needed and which way the access will be, which will determine the
polarity of the current to the actuator.  A positive access is
required if the requested LBA is greater than the last LBA of the
present cylinder.  A negative access is required if the requested LBA
is less than the first sector of the present cylinder.  When one or
the other access direction is needed, then the actuator current can
be started.  A few more comparisons for approximate length of the
access, can determine whether the current is to be held down to a
lower value when only a few tracks will be moved, or if the current
is to be set to the maximum.  While t...