The Programmer's Guide to TRSDOS Version 6 - Tim Mann's Home ...

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| Bit-7: Disk File character I/O |
| Bit-3: NIL device - no I/O |
| Bit-4: ROUTE to DCB or FCB |
| Bit-5: LINK to 2nd DCB |
| Bit-6: FILTERed DCB or filter |
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Figure 3-9: DCB Hierarchy
3.4.8 Device Chain Summary
The preceding discussion should shed a great deal of light on the handling of device I/O
by the operating system. You should also understand that in order to accomplish this
device independence and flexible handling of character I/O, the programmer of device
drivers and filters must adhere to a strict protocol of handshaking the modules with the
operating system. The next section will explore device I/O looked at from the standpoint
of the modules and drivers. Once you grasp these requirements, you will be in total
control of filters and device drivers.
3.5 DEVICE DRIVER/FILTER TEMPLATE
The system contains command level procedures that provide easy access to device
references so that modifications may be made to the way in which devices are treated by
the system. All devices require some type of driving program (a device driver) that is
used to handshake the device with the system and cater to the special features and
requirements of the device hardware. Some drivers are already implemented within the
operating system to handle standard devices. For instance, drivers for handshaking the
keyboard, video display, parallel printer port, and RS-232 serial port are included with
the system.
Some devices are completely supported with the existing drivers in the total DOS
environment. Other devices may need a little more support. The characteristics of a
driver may be modified by the introduction of a FILTER. For instance, suppose your
printer required a line feed upon receipt of a carriage return to advance the paper. The
printer driver does not provide this function. Instead of writing a completely new
printer driver, only a filter need be included to add that single function (the FORMS/FLT
filter which incorporates this function is usually provided with the system).
The DOS provides two commands to aid in interfacing drivers and filters. The SET command
is used to define a new device, re-define an existing device, or install a filter module
while assigning it a device name. FILTER is used to place the installed filter into an
existing device chain.
The SET command takes the device specification from the command line "SET *XY to
filespec" and searches the Device Control Block tables for a matching device name. If the
requested device is not defined in your configuration, SET establishes a Device Control
Block for the new device. Control then passes to the DRIVER or FILTER with register pair
DE containing the address of the Device Control Block record assigned to the "SET"
device.
Register pair HL points to the command line character separating the DRIVER/FILTER
program filespec and optional parameters. This provides the module initialization
routines with the opportunity of parsing a parameter string by using a parameter table
and the @PARAM SuperVisor Call. SET provides a default file extension of /FLT since the
function of adding filters to the system is the more usual case.
The SET and FILTER commands are designed such that the DRIVER or FILTER program should
first load into the User Program Region (starting at X'3000'). After parsing any options
or parameters, the module initialization routine automatically relocates the resident
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