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ID:XD XDI Specification for DESQview
Quarterdeck Technical Note
by Daniel Travison
Q: How can my device driver/TSR work better with DESQview?
DESQview's External Device Interface
DESQview version 2.26 (and above), can issue informative messages to third
party software in order for that software to monitor certain DESQview
operations such as starting a task or swapping out a process. These
messages can be useful for determining when to allocate/deallocate
resources (e.g., memory) on a process by process basis, communicating with
tasks running within DESQview, rescheduling tasks, tracking DESQview's task
switching, etc.
DESQview provides this information through the External Device Interface
(XDI) - Quarterdeck's specification for communicating with external
drivers.
An XDI driver may be implemented either as a DOS device driver (loaded by
CONFIG.SYS), a Terminate and Stay Resident program (TSR) loaded before or
after DESQview, or as a DESQview shared program (shared among several
processes). For the rest of this documentation, the term 'driver' will
normally refer to a program that uses the XDI irrespective of its method of
implementation (device driver, TSR or shared program). Also, any number of
XDI drivers can be loaded. When DESQview starts, it simply attaches itself
to the XDI chain and starts sending XDI messages to to previously loaded
XDI drivers.
Within the XDI, there are 15 subfunctions (messages). Fourteen of these
messages inform the driver what action DESQview has just performed or
is about to perform. Your driver can choose to ignore the message and pass
it on to the next driver in the chain, perform/schedule work based on the
message, or save information for later use (e.g., the current mapping
context).
XDI_CHECK_PRESENCE Check for XDI driver presence
XDI_RESERVED_SUBFUNC XDI driver custom subfunction
XDI_START_DV DV system initialization complete
XDI_END_DV DV system termination
XDI_START_PROC DV process creation
XDI_END_PROC DV process termination
XDI_START_TASK Task creation
XDI_END_TASK Task termination
XDI_SAVE_STATE Task state save
XDI_RESTORE_STATE Task state restore
XDI_KEYBOARD Change of keyboard focus
XDI_PROCESS_DVP Processing of DVP file complete
XDI_SWAPPING_OUT Swap out of DV process
XDI_SWAPPED_IN Swap in of DV process
XDI_FAILED_DVP DV process creation failure
An XDI driver can schedule second level handlers to make API calls, collect
data, transfer data, dispatch tasks, perform initialization/termination
when DESQview is started and quit.
The fifteenth subfunction, XDI_RESERVED_SUBFUNC provides a method for
applications to communicate with your driver without needing to grab an
additional interrupt vector. Additionally, this interface does not require
DESQview to be loaded. In other words, if DESQview is not loaded, the
application and XDI driver will still be able to communicate.
Q: When would I want to use the XDI specification?
If your driver needs to write into an application's address space then you
need to ensure that the address is valid when you perform the write. (e.g.,
The DOS SETDTA call requires the caller provide a pointer to a buffer that
will be filled in at a later time. A communications handler might use a
similar interface but perform the update at interrupt time.) With
DESQview, this buffer may get mapped in and out to accomodate running
multiple programs. If your driver attempts to write into this buffer when
DESQview has mapped some other application into it (e.g., at interrupt
time) then you will be corrupting the unknown application. The XDI allows
your driver to determine when the write operation is safe or specifically
request that DESQview map the application in to allow safe access to the
application's address space.
Another example is the use of Multiplex boards. Multiplex boards have two
important features: One, they provide access to multiple external devices
(e.g., modems) using a single IRQ and two, they provide a software
interface for applications. The software interface removes the need for
the application to manage the hardware directly. In the case of a
multiplex board providing additional serial ports, the user will often want
to run more than one program at a time to take advantage extra serial
ports. (e.g., A number of BBS systems provide the option for running
multiple copies at once and also support drivers for multiplex boards.)
The programmer writing the driver will need to accomodate simultaneous
access to the driver as well as provide a reliable software interface.
Other issues addressed by the XDI:
1) Allocation of a software interrupt to allow the application to
communicate with the driver. With more and more device drivers and
TSR's available, it is important to prevent possible conflicts with
other software. The XDI specification uses the multiplex interrupt
(INT 2Fh) as a basis for sharing a single interrupt using a well
defined interface. Once the application has 'logged' itself with the
driver (XDI_RESERVED_SUBFUNC), the driver might provide an ID for
future use or simply manage it internally based on which process
DESQview has mapped in when the software interrupt is called. This
portion of the interface functions in the same manner whether
DESQview is loaded or not. When DESQview is not present, the driver
does not need to be concerned with the mapping context.
2) Allocation of memory/application that needs to be accessible at any
given time. This might be a buffer for incoming data. Normally, the
driver would need to allocate all of its memory needs during its
initialization code. DESQview allows an XDI driver to allocate
COMMON memory (memory that does not get mapped out) during an XDI
call. This allows the driver to allocate only the memory necessary
when a new application starts. It DESQview is not present, the
application would allocate a buffer out of its own data space. A
check for DESQview would be all that is necessary for the application
to determine whether it needs to allocate a buffer or allow the
driver to allocate it from COMMON memory.
3) Preventing reentrancy during non-reentrant sections of code. In a
perfect world, all of your driver's code would be reentrant. Since
this may not be practical without unreasonble code or CPU overhead,
the XDI driver can temporarily suspend multitasking to perform its
critical work. This does not mean that the XDI allows you to write
non-reentrant handlers but it can solve some sticky issues that arise
when you need to support simultaneous access.
4) The application can not keep up with the data rate. There will be
times when the application can not empty the driver's buffer faster
than the driver can fill it. The user may have started up a few
extra applications and the CPU is too slow to give everyone enough
time. The XDI driver could consider some percentage of the buffer as
a threshold. When this threshold is reached the driver can
temporarily override DESQview's dispatcher and force the
application's 'buffer management' routine to execute at the next
context switch.
5) The driver needs to know when the application exits to allow proper
cleanup. Normally, the application itself would call the driver to
initiate cleanup. Occasionally this will not occur (e.g., the user
shut down the application via DESQview's Close Window menu
selection). DESQview notifies the XDI driver when a process ends to
allow the driver to determine if it needs to perform any cleanup for
the process.
There are other areas where an XDI driver would be of use; resource
tracking on a process by process basis, modifying the .DVP when each
process starts, allocating additional system memory for the process at
startup, tracking CPU usage, or even displacing DESQview's dispatcher.
Q: How do I find out more about the XDI?
DESQview's XDI is fully documented in Quarterdeck's API Reference Manual
(versions 1.20 and later). Included is a sample XDI driver (POKEXDI.ASM)
that can be used as a template for designing your own. Contact Quarterdeck
for information on obtaining a copy of our API Reference Manual or upgrading
and older API Reference Manual.
Copyright (C) 1990 by Quarterdeck Office Systems
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