MS DOS Device Drivers

• Sources of Information • Device driver basics • Structure and internal routines

Sources of information

John Angermeyer and Kevin Jaeger, MS-DOS Developer’s Guide , Howard W. Sams, Indianapolis, 1986 Ray Duncan, Advanced MS-DOS , Microsoft Press, Redmond, Washington, 1986 Robert Lai, Writing MS-DOS Device Drivers , Addison Wesley, Reading, Massachusetts, 1992

Device driver basics

• Operating System modules that control hw • Isolate higher levels from hw specifics • Standard interface with OS • Installable at boot time as a “chain” (CONFIG.SYS) • Not necessary to modify the OS • Similar to Unix/Xenix/Linux device drivers • Two types: Block and Character

General Structure

Device Header Driver Data Storage Strategy Routine Interrupt Entry Command Handlers Interrupt Service Routine Initialization Code and Driver Data Buffers

Device Header

• 18-byte block at beginning of every device driver 00H Link to next driver, offset 02H Link to next driver, segment 04H Device attribute word 06H Strategy entry point, offset 08H Interrupt entry point, offset 0AH Logical name (8 bytes) if character device. Number of units (1 byte) if block device (other 7 bytes reserved)

Strategy Routine

• Called by MS-DOS when driver is first loaded • Called by MS-DOS whenever I/O request is issued to device • Request call includes a pointer to a request header; driver saves pointer and returns to MS-DOS • Request header includes command code and other information

Interrupt Routine

• Called after the strategy routine • Implements the device driver proper • Performs the actual I/O operations • Collection of subroutines to implement various functions (read, write, …) • Centralized entry routine saves registers and sets up driver ops • Centralized exit routine restores registers and sets status and error codes

Sample Command Codes

• 00H: Driver Initialization • 01H: Media Check • 04H: Read • 08H: Write • 0DH: Open • 0EH: Close