No Operation (Operation Code 00); Mailbox Initialization (Operation Code 01) - Adaptec AHA-1740A Technical Reference Manual

adaptec
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*This command can be issued when the host adapter is executing a SCSI command.
All adapter commands except Start SCSI (02) and Enable Mailbox Out Available In-
terrupt (05) must be executed only when the IDLE bit (Status bit 4) is one. Many com-
mands require additional parameter bytes which are then written to the
Command/Data Out I/O port (base + 1). Before each byte is written by the host to the
host adapter, the host must verify that the CDF bit (Status bit 3) is zero, indicating
that the command port is ready for another byte of information. The host adapter
usually clears the Command/Data Out Port within 100 microseconds. Some com-
mands require information bytes to be returned from the host adapter to the host. In
this case, the host monitors the DF bit (Status bit 2) to determine when the host
adapter has placed a byte in the Data In I/O port for the host to read. The DF bit is
reset automatically when the host reads the byte. The format of each adapter com-
mand is strictly defined, so the host adapter and host system can always agree upon
the correct number of parameter bytes to be transferred during a command.
All Adapter Commands except Return Installed devices, Start SCSI, and Start PC
AT BIOS typically require less than 200 microseconds to complete. Return Installed
Devices will typically complete in less than three seconds. Start SCSI and Start PC
AT commands completion times will vary with the SCSI device and the command is-
sued.

No Operation (Operation Code 00)

No host adapter action is taken, but HACC is set indicating command completion. No
additional information bytes are exchanged.

Mailbox Initialization (Operation Code 01)

This command is used to specify the number of mailbox locations used by the host
adapter and to specify the base memory location of the mailbox area. The host adapt-
er requires that four bytes of outbound data follow the command byte. The definition
of those four bytes is shown below.
Byte
0
1
2
3
Mailbox address: Location of the first byte of the mailbox area.
When the Mailbox Initialization command and parameters are received, the host
adapter will then assume that the specified number of Mailbox Out entries and the
same number of Mailbox In entries will be assigned beginning at the Mailbox ad-
dress. The total number of bytes reserved for the mailbox area will be eight times the
5-2
Adapter Diagnostic
Set Host Adapter Options
Definition
Mailbox count - Must be greater than zero.
Mailbox address (MSB)
Mailbox address
Mailbox address (LSB)
AHA-1740A/1742A/1744