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Bits-5, 4, 3, and 2 are reserved and must be zero.
Bits-I,
and 0 specify a logical unit number (disk drive address)
address in the ACB- 550 0.
4.3
FUNCTIONAL DESCRIPTION OF SCSI COMMANDS
By defining a fixed block structure using a simple,
logical
address scheme, the I/O interface can support device indepen-
dence. The same code can be used to support both the ACB-5500 SMD
Disk Controller and the ACB-5500 ST506 Disk Controller.
In
addition,
by including the logical block address as a component
of the command structure,
implicit operations (such as SEEK and
Retry) can be performed by the basic READ and WRITE commands.
This interface,
despite its simplicity,
is capable of providing
the high level of performance required in a mUlti-host/multi-task
environment.
Functional examples of SCSI command and data inter-
actions are detailed below.
4.3.1
SINGLE COMMAND EXAMPLE
A typical operation for the ACB-5500 is a READ of disk data.
The Initiator has an active state and a set of stored states
(representing active disconnected devices).
The Initiator sets
up the active state for the operation requested by the host
system, arbitrates for the SCSI bus, and selects the ACB-5500.
The ACB-5500 then assumes control of the operation.
The ACB-5500 checks to see if ATTN is present, indicating that
the Initiator is hoping to send an Identify message.
The ACB-
5500 obtains the Identify message and uses it to determine which
logical unit (SMD drive) is being addressed.
The ACB-5500 then
obtains the command descriptor block, 6 or 10 bytes of command
information, and determines that a Read of certain logical blocks
is desired by the Initiator.
The ACB-5500 performs all the disk
control and data transfer operations necessary to transmit the
logical blocks to the Initiator.
All seeks,
retries, defect
skipping,
and error correction are performed to recover the
logical blocks as quickly as possible and with no management from
the Initiator.
After the read data has been transferred to the Initiator, the
ACB-5500 presents ending status and a Command Complete message to
inform the Initiator that the operation was completed success-
fully.
The bus is then freed for further operations.
500504-00
4-16
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