I/O Port Definitions

To control an advanced function bus master adapter, the SCB architecture defines a set of I/O ports. These ports are assigned as a sequence of register addresses in the I/O address space. Since multiple adapters of the same or different types may be used in the system, the base address for the I/O space of each adapter must be defined during system setup. The IBM SCSI adapters support eight address ranges. However, ABIOS supports only four. The ports used by the IBM SCSI adapters are shown indexed from the base address in Figure "IBM SCSI Adapter I/O Ports". The IBM SCSI adapter implements a subset of the ports defined in the SCB architecture.

The following are general descriptions of the I/O space control ports:

Request Ports

There are four types of request ports associated with sending requests to a device. The first of these is the Command Interface Port (CIP), which consists of four 8-bit read/write registers used to transfer either a 32-bit immediate command or a Subsystem Control Block address from the system to the adapter. The immediate commands are typically device-directed and control-oriented.

The second is the Attention Port (AP). This is an 8-bit read/write register used by the system to request an adapter operation. The register contains two pieces of information. The high-order four bits define the operation request code and the low-order four bits define the logical device that should be selected for the operation.

The third, the Interrupt Status Port (ISP), is an 8-bit read-only register used by the subsystem adapter to return command completion information, if allowed by the interrupt-enable bit in the Subsystem Control Port. The adapter stores the logical device ID in the four low-order bits and an interrupt ID in the high-order bits. A hardware interrupt is then generated.

The fourth port is the Command Busy/Status Port (CB/SP). It is an 8-bit read-only register that is used by the adapter to serialize access to the shared logic of the control block delivery service. It contains the following indicators:

Busy - indicates that the adapter is busy (using the shared logic). Commands submitted while the busy indicator is on are ignored by the feature adapter.
Interrupt Valid - indicates that the content of the ISP (Interrupt Status Port) is valid, and that the feature adapter has requested an interrupt on behalf of one of its entities. This indicator will be set even if interrupts are not allowed by the system and will be reset by the EOI (End Of Interrupt) request. This allows an adapter to operate without presenting hardware interrupts to the system.
Reject - indicates that the feature adapter has rejected a request. A Reset Reject request is needed to clear the reject signal and allow the adapter to resume accepting requests.
Status - three bit indicator giving the reason for rejection.

Subsystem Control Port

The Subsystem Control Port (SCP) is an 8-bit read/write register used for direct hardware control of the subsystem adapter. This type of control cannot typically be handled by requests to the adapter management via immediate commands through the Command Interface Port (CIP). The SCP contains the following control indicators:

Enable interrupt - indicates that interrupts to the system unit should be enabled or disabled for all devices attached to the adapter.
Enable DMA - indicates that DMA operations should be enabled or disabled for the subsystem.
Reset reject - indicates that a reset of the reject state for the subsystem should be performed (see Reject under the description of Command Busy/Status Port above).
Hardware reset - indicates that a controlled reset of the adapter and attached devices should be performed.

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