Cyclical Redundancy Check - HP Compaq SP750 Supplementary Manual

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The technological advances that allow Ultra ATA/66 drives to perform more efficiently are:
Decreased command turnaround overhead. Ultra ATA/66 minimizes the periods during
which the bus remains idle while waiting on the host PC to issue commands to the hard drive.
Because the effective data transfer rate equals the burst transfer rate minus the command
turnaround time, improvements in command turnaround overhead will increase overall
performance.
Minimized slipped revolutions. Slipped revolutions, or "slipped revs," occur when the drive
must rotate a second time to allow the host PC to drain a buffer. The Ultra ATA/66 higher
data rate minimizes slipped revs. As a result, the buffer does not accumulate data and no revs
are slipped.
Improved timing margins. Ultra ATA/66 improves timing margins by eliminating
propagation and data turnaround delays.
Hard Drive Sizes and Speeds for Ultra ATA
Ultra ATA/66 hard drives will be available in a wide range of sizes throughout 1999. Most drives
will be 10GB and larger, with some drives exceeding 20GB.
Ultra ATA hard drives are transitioning from 5400 rpm to 7200 rpm speeds. Just how suddenly
this transition occurs depends on how quickly 7200 rpm drive costs reach 5400 rpm costs.
Compaq recommends business users invest the extra dollars in converting to 7200 rpm drives in
1999. The combination of faster spin speed and other advantages of 7200 rpm class drives will
generally improve overall drive performance by 20%. Additionally, customers who choose to
standardize on longer lasting technology will want to invest in 7200 rpm drives considering 5400
rpm drives will phase out more quickly.
Customer Benefits
Faster and larger hard drives with continued improvement in hard drive and controller
technologies.
Wide range of fixed storage technologies now available allows customers to select a system
that best meets their needs.
High speed and economy with readily available Ultra ATA/66 controllers.
Cyclical Redundancy Check (CRC)
Ultra3 SCSI and Ultra ATA/66 drives implement a significant feature called Cyclical
Redundancy Check (CRC), which provides data transfer verification. CRC is calculated on a per-
burst basis by both the host and the drive and is stored in the respective CRC registers. At the
termination of each burst, the host sends the contents of its CRC register to the drive, which
compares the burst data against the register contents to ensure integrity. For even greater
integrity, the protocol can be used at speeds slower than its maximum 66 MB/s. The slower the
drive' s transfer speeds, the greater the integrity margins.
CRC is used by Compaq Intelligent Manageability solutions to provide Integrity Monitoring
functions, yielding proactive notification of hard drive data transmission problems and
recommendations for enhancing system performance.
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Compaq Professional Workstation - Key Technologies