As mentioned above, the SCSI adapters contain ROM modules with the compatibility BIOS (CBIOS), the advanced BIOS (ABIOS), and the power-on self test (POST) routines for the SCSI subsystem. Each of these BIOS functions is explained in the following sections.
Compatibility BIOS (CBIOS)
The CBIOS interface supports SCSI device and adapter functions for the single tasking environment, as implemented by PC DOS. The SCSI CBIOS intercepts and replaces parts of the Interrupt 13h (fixed disk) and Interrupt 15h (system services) functions. See Figure "SCSI BIOS Interface Block Diagram" for a functional overview of the BIOS interface. It also adds the new Interrupt 4Bh (advanced services) support to provide a generic interface to SCSI devices other than disk drives. This new interface uses the carry flag and the AH register to report status information.
The CBIOS interface makes it possible for programs in a DOS environment to use both SCSI and other types of disks via INT 13h, with the help of new device drivers and INT 4Bh services. The CBIOS interface also supports other types of SCSI devices.
Advanced BIOS (ABIOS)
The ABIOS interface shown in Figure "SCSI BIOS Interface Block Diagram" provides new and enhanced device ID 02h (fixed disk) support. ABIOS used by operating systems such as OS/2 V2.0, which use the protected mode of the Intel microprocessors. The enhanced functions enable support of both SCSI and other types of disk without an upgrade of operating system software. The new functions enable a more effective use of SCSI disk devices by operating systems through new and enhanced device drivers. The new ABIOS functions are:
These two functions enable or disable the intelligent buffer capability for all subsequent commands to this device. If the function is not supported, an error code is returned.
Returns the status of the intelligent buffer. Status may be enabled, disabled or not supported.
This function programs the adapter with the supplied pacing value. The value specifies what percentage (25% to 100%) of the DMA transfer bandwidth the adapter is allowed to use. All devices on the adapter are affected.
This function returns the current pacing factor for the adapter.
This function programs the adapter to process a Subsystem Control Block
(SCB). SCBs are described further in Subsystem
Control Block Architecture.
Power-On Self Test (POST)
The POST routines will automatically configure the SCSI subsystem at power-on and system reset. The SCSI subsystem supports the coexistence of other types of fixed-disk adapters, and if more than one SCSI adapter is installed in a system, the BIOS ROM modules on the extra adapters will be disabled by the POST routines.
POST will also issue the Inquiry command to all 56 possible combinations of physical unit number/logical unit number (PUN/LUN), until 15 logical devices (logical device IDs 0 to 14) are assigned. The SCSI adapter is logical device 15.