20 Adapter Diagnostic21 Set Host Adapter Options*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 theCommand/Data Out I/O port (base + 1). Before each byte is written by the host to thehost adapter, the host must verify that the CDF bit (Status bit 3) is zero, indicatingthat the command port is ready for another byte of information. The host adapterusually 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. Inthis case, the host monitors the DF bit (Status bit 2) to determine when the hostadapter has placed a byte in the Data In I/O port for the host to read. The DF bit isreset 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 uponthe correct number of parameter bytes to be transferred during a command.All Adapter Commands except Return Installed devices, Start SCSI, and Start PCAT BIOS typically require less than 200 microseconds to complete. Return InstalledDevices will typically complete in less than three seconds. Start SCSI and Start PCAT 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. Noadditional information bytes are exchanged.Mailbox Initialization (Operation Code 01)This command is used to specify the number of mailbox locations used by the hostadapter 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 definitionof those four bytes is shown below.Byte Definition0 Mailbox count - Must be greater than zero.1 Mailbox address (MSB)2 Mailbox address3 Mailbox address (LSB)Mailbox address: Location of the first byte of the mailbox area.When the Mailbox Initialization command and parameters are received, the hostadapter will then assume that the specified number of Mailbox Out entries and thesame 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 theadaptec AHA-1740A/1742A/17445-2
