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The polling has three disadvantages:
A.
Cause SCSI bus busy
B.
Require host overhead
C.
The response time is delayed by the host polling interval
This delay could be as long as several hundred milli-seconds.
The ACB-5500 and ACB-5580 will queue the requests from the other
hosts and disconnect from the SCSI bus.
As soon as the first
host request is done, one of the queued requests is performed by
reconnecting to
the host.
This function
will
dramatically
increase the performance of a multi-host environment.
4.4
COMMAND STRUCTURE
4.4.1
COMMAND DESCRIPTOR BLOCK
(COB)
An I/O request to a device
is made by passing a
Command
Description Block (COB) to the Controller.
The first byte of the
COB is the command group and operation code.
The remaining bytes
specify the Logical Unit Number (LUN), starting block address,
control byte, and the number of blocks to transfer.
Commands are
categorized into two formats supported in Adaptec controllers:
o
Group 0:
6-Byte commands
o
Group 1:
10-Byte commands
Figures 4-3
and 4-4 show typical Group 0 and Group 1 command
descriptor block formats.
BIT
BYTE
07
06
05
04
03
02
01
00
------~~~~;-~l~~----I------I------6;~~~~-I------I----- --
01
02
03
04
05
Logical Unit Number!
(MBS)
Logical Block Address
Logical Block Address
Logical Block Address
(LSB)
Number of Blocks
Reserved
(0)
! Flag ! Link
Figure 4-3.
Group 0 Commands (6-Byte Commands)
500504-00
4-18
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