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Debugging with DDE
Abstract
The DDE link in Windows gives you a powerful way to exert control over your
debugging session. Here's how.
No emulator can be all things to all people. There are so many different requirements
that tool vendors are forced to cater to the needs of the many, sometimes unfortunately
neglecting the needs of the few.
With Windows and DDE (Dynamic Data Exchange) this problem disappears. Would
you like some sort of special feature - say, generate a 3-D graph of the value
of a variable in memory versus time? It's easy to create a program that communicates
over a DDE link with the debugger. Softaid's Source Level Debugger for Windows
(SLDW) supports DDE to let you create your own customized applications.
However, everyone knows how hard it is to write Windows apps... right? In fact,
everyone is wrong. Microsoft's Visual Basic is so easy that, quite literally,
we had a simple working Windows application running only an hour after opening
the box. Visual Basic is not appropriate for large programs because it is somewhat
slow, but it gives you a very fast way to build DDE-aware applications that
can control an emulator. HyperApp
To illustrate this, Softaid provides an application written in Visual Basic
on the SLDW distribution disks. We include the source code so you can use it
as a model. HyperApp is a moderate-size program that displays the values of
the I/O ports in certain high-integration processors (the Z180, 80188/186, and
others).
For example, the 80188 includes over a dozen ports dedicated just to interrupt
control. Though the emulator itself lets you issue I/O commands to view and
change these ports, it's a little awkward to remember the addresses and bit
positions of this information. HyperApp solves this problem. It's a separate
application that displays everything you need to know about each port and bit.
It communicates via the DDE link to SLDW to get the state of each port, and,
if you change a bit setting, to transmit the new port value back to the emulator.
DDE Made Easy
Visual Basic (VB) is the glue that makes DDE communication easy. Softaid's SLDW,
which is written in C, has a rather complex and large module needed to handle
DDE via the Windows SDK calls. By contrast, VB masks the details behind a few
simple commands.
VB is a true object-oriented language (in Basic! Who would have imagined!).
Every important action is associated with an object - in VB lingo, a control.
Each button on a screen is a control; every box, frame, and other screen element
is a control as well. So, you must define an object responsible for handling
the DDE communications. In HyperApp we call this txtLink, and give it attributes
that make the control invisible to the user. It's existence is for DDE only.
Softaid defined a DDE API for SLDW that includes a small set of commands you
can invoke over the DDE. The commands are:
MEMBS address
MEMWS address
IOWS address
IOBS address
ASSIGN address, value
MEMBS, IOWS, IOBS all let you read or set one memory or I/O (MEM Vs IO) word
or byte (W Vs B).
ASSIGN immediately sets an address to the value supplied. It uses the DDE Execute
method. ASSIGN is not used in the HyperApp program.
When SLDW receives any of these DDE commands it runs the address parameter through
its symbol processor. The result - you can specify case-sensitive symbolic names
from your program as addresses!
To set any VB application up as a DDE "client" (i.e., an application
requesting data), issue the following statements, all in this case tied to the
txtLink control:
txtLink.LinkTopic="SLDW|SLDWAPI"
txtLink.LinkMode=3
txtLink.LinkItem="BREAKADVISE"
These statements tell VB that the application will communicate with SLDW. LinkMode
3 means that whenever SLDW encounters a breakpoint (BREAKADVISE) it will cause
the txtLink control to go to the LinkNotify routine. LinkNotify is a standard
routine (called "Event" in VB parlance) for any object. Thus, if the
emulator hits a breakpoint LinkNotify in the VB application will be automatically
invoked via these commands.
To read data from SLDW issue one of the commands listed above over the DDE.
For example, to read a byte from address 00, use the following commands.
txtLink.LinkItem="IOBS 00"
txtLink.LinkRequest
Immediately after the LinkRequest the object (txtLink) will get the data in
string form in its Text property. Retrieve the value into sValue$ via:
sValue$=txtLink.Text
Sending data is just as easy. Put the string to send to SLDW in the object's
Text property. This example sends 11 to port 00:
txtLink.LinkItem="IOBS 00"
txtLink.Text="11"
txtLink.LinkPoke
Conclusion
HyperApp uses these simple DDE commands to extract and change any of the CPU's
internal I/O ports. There's no reason you couldn't extend this to include information
about your own I/O -- external to the CPU -- since all of the port and bit definitions
are in an ASCII file read by HyperApp on powerup.
HyperApp is included with each of Softaid's emulators. We supply it with the
complete Visual Basic source code.
Copyright 1994, Softaid, Inc.
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