Q. Illustrate Header section of a device driver?

Header comprises information which allows DOS to identify the driver. It also comprises pointers which allow it to chain to other drivers loaded into the system.

Header section of a device driver is 18 bytes in length and comprises pointers and name of the driver.

Following structure of the header:

CHAIN DD -1    : link to next driver

ATTR DW 0    : driver attribute

STRT DW START: address of strategy

INTER DW INT: address if interrupt

DNAME DB 'MYDRIVER': driver name.

The first double word comprises a -1 which informs DOS this is the last driver in chain. If extra drivers are added DOS inserts a chain address in this double word as the segment and offset address. Chain address points to the subsequent driver in the chain. This enables additional drivers installed at any time.

The attribute word signifies the type of headers included for driver and the type of device the driver installs. It also signifies whether the driver control a character driver or a block device.