User`s manual

Overview

2-8 Single Board Computers SCSI Software UserÕs Manual

After the command request completes, the MPU code updates some fields in

the command structure (siop_struc), updates Firmware queues, and then calls

the user's return status routine. At this point, the siop_struc is back in the

control of the user. The user may immediately call the Firmware through

siop_cmd() to send a new command request; however, it is recommended that

the user wait until the Firmware returns control of the MPU via the

siop_cmd() return before sending any more command requests. If the user

calls siop_cmd() each time the return status routine is invoked then the stack

eventually overflows. This happens because the Firmware will not have

returned (unstacked) from any of the calls to siop_cmd().

Upon return from the user's return status routine, control of the MPU is

returned to the user. The user should now check the status in the status.allstat

field of the command structure (siop_struc) to determine if any recovery

actions are needed.

Interrupt Mechanism

Interrupts from the SIOP are generated in response to various hardware

conditions or are programmed interrupts generated by the SCRIPTS INT

instruction. The SIOP halts SCRIPTS execution whenever an interrupt occurs.

The following is the list of SIOP interrupts.

SCSI Bus Reset

This interrupt is generated when the SIOP detects a SCSI bus

reset. The Firmware terminates all outstanding commands

and returns status for each.

Phase Mismatch

This interrupt is generated when a target changes SCSI phases

before the SIOP data counter register has exhausted its count.

This interrupt occurs when an intermediate disconnect is

pending or a data underrun has occurred.

Selection Timeout

This interrupt is generated when a device at a selected SCSI

address fails to respond within 250 milliseconds after the start

of the SELECTION phase.

Unexpected Disconnect

This interrupt is generated when a target device unexpectedly

transitions to the BUS FREE phase. The Firmware determines

if the disconnect resulted from an intentional action initiated

by the Firmware (i.e., device reset message).