Specifications

Writing Device-Driver Tables

4.2 Driver Dispatch Table

• Mount veriﬁcation routine

• Cloned UCB routine

• Start-I/O routine for a driver employing the kernel process services

In addition, the DPTAB macro allows you to specify the length of any diagnostic

buffer or error message buffer.

See the description of the DDTAB macro in OpenVMS AXP Device Support:

Reference. for additional information.

4.3 Function Decision Table

A function decision table (FDT) is a structure within a driver that consists of two

substructures:

• A quadword bitmap known as the buffered function mask

• A 64-element array of longwords known as the FDT action routine vector

Each bit in the buffered function mask represents an I/O function that is

serviced by the driver by means of an intermediate system buffer.

Each 32-bit slot of the FDT action routine vector corresponds to

the symbolic value of an I/O function deﬁned by the $IODEF macro in

SYS$LIBRARY:STARLET.MLB. Those vector slots that relate to functions

serviced by a driver contain the procedure value of an upper-level FDT action

routine that initiates driver-speciﬁc preprocessing of the function. Those slots

that represent functions the driver does not support contain the procedure value

of a system upper-level FDT action routine that processes illegal I/O functions

(EXE$ILLIOFUNC). When a $QIO is issued to a device driver, specifying an I/O

function code the driver does not support, EXE$ILLIOFUNC executes, calling the

FDT completion routine EXE_STD$ABORTIO. EXE_STD$ABORTIO terminates

the I/O request and passes SS$_ILLIOFUNC status back to the $QIO caller.

Figure 4–1 depicts the layout of the FDT.

Figure 4–1 Layout of Function Decision Table (FDT)

Buffered Function Mask

Procedure value of upper−level FDT action routine for

I/O function 0 or EXE$ILLIOFUNC

I/O function 1 or EXE$ILLIOFUNC

I/O function 63 or EXE$ILLIOFUNC

FDT action routine vector slots 2 through 62

Procedure value of upper−level FDT action routine for

FDT action routine

vector

A driver uses the following three macros in sequence to create and initialize its

FDT:

• FDT_INI—Creates, labels, and initializes an FDT

4–4