New Disk Device Driver

A new disk device driver interface has been defined for use by OS/2 V2.0 file systems. The request-passing mechanism employs a request list of prioritized commands which the device driver may reorder to optimize access. Read and write operations use scatter/gather descriptors (as used by SCSI adapters) allowing data transfer to and from discontiguous memory buffers. The interface is designed to be data-structure compatible with future versions of OS/2 in order to minimize work at the time of migration.

With this extended device driver support, both the standard OS/2 interface (using ABIOS read/write) and the new high performance interface (using the ABIOS Transfer SCB) allow both OS/2 file systems and the OS/2 kernel to access the disk.

The following is a summary of the device driver changes:

The disk device driver records the information indicating the type of DASD (ESDI/ST506 or SCSI) and the level of caching support for each logical drive (LID)

Support is provided for the new device command 1Dh - GetDeviceSupport. The device driver will return addresses to two structures in the request packet:

The Driver Capabilities Structure (DCS) indicates the specific features that are supported by the device driver
Examples are disk mirroring, disk duplexing and whether the device supports more than 16MB of memory.
The Volume Characteristics Structure (VCS) for the device identified by the unit code in the device command
Examples of characteristics reported in this structure are read-only, removable media, average seek time, outboard caching supported, SCB protocol supported and read prefetch supported.

1Ch - ExecuteChain

1Eh, 1Fh, and 20h - Read, Write and WriteVerify

21h - Prefetch

To optimize the I/O operation, all requests are sorted in logical block number sequence. If a specific flag is on in the request list indicating that the execution order is critical, then the list will not be sorted.

The possible priorities are:

Priority

Meaning

00h

Prefetch requests

01h

Low priority request - (lazy-write)

02h

Read ahead, low priority pager I/O

04h

Background synchronous user I/O

08h

Foreground synchronous user I/O

10h

High priority pager I/O

80h

Urgent request - used by kernel in critical situations.

ExecuteChain()

The strategy routine instead returns control to the caller after the I/O is initiated or queued.

Upon I/O completion, the device driver calls the notification routine specified in the request list.

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