Hex the H function
Prior to Windows, DOS was booted from a floppy disk and the entire
screen was the display. The DOS interpreter called COMMAND.COM was
automatically run and instructions were taken from the keyboard.
DEBUG.EXE was one of the programs that could be run under the DOS (
Disk Operating System ).
Once Windows is called, the DOS functions are different than before
Windows is run. These differences will be mentioned occasionally and
need to be taken into consideration. This example is run under Windows
emulated DOS.
This is a Debug program example. We will open Debug from the Command
Interpreter rather than calling it directoy from Windows. We can do
this in several ways.
Double clicking on the
resident COMMAND.EXE icon or its shortcut is one way.
Entering COMMAND in
the START-RUN edit box and clicking on OK is another.
Windows emulated DOS will open in the DOS PROMPT console window.
In either case,
the Debug program can be run by entering Debug after the DOS prompt and
pressing ENTER.
The Debug display will result giving us the minus sign prompt as a
prompt.
On the minus sign prompt, enter the character h, space, the
first number, the
second number and press ENTER. Two values will be returned to the
display.
The first will be the sum of the two numbers and the second will be the
difference. All the numbers in Debug are in hexidecimal.
One of the most useful uses for this function is to provide the
difference between two memory locations. The length of a data block
might be important as well. There will be a need for this function in
later examples.
Here are a few examples of some of the results as seen on the Debug
console
display.
In the Windows Debug console,
the entire text fore-ground and back-ground colors are selectable.
In the example;
however, the prompt is the minus sign..... the command is red..... the
return is green....remarks are in black.
-h 7
2 Enter
7 and 2 and ENTER.
0009
0005
the sum is 9 and the difference is 5.
-
-h 2 7
0009
FFFB
The sum is still 9 but the difference is minus 5.. or FFFB
-
-h FFFF
1
The sum is
0000, but the carry goes into the bit bucked.
0000
FFFE The
difference is FFFE or minus 1 in 16 bit two's compliment.
-
This function is entirely passive, in as much as it does not
alter the contents of memory or the data on the disk. Nothing will be
harmed by experimenting with this command. It is a calculator function
who's input is normally the keyboard and the output is normally the
display. Experimenting with the H function is quite safe. There are
other functions that can alter the state of the
computer and of the disk contents. Those functions should be used with
care.
Summary
It is suggested that you become familiar with calling,
exercising and
termination Debug. As described above, Debug may be entered from the
Window's desktop useing the RUN edit box or clicking on the Debug,exe
icon or its shortcut. It can be exited with the Q function and program
control will return to Windows.
It can also be opened from the DOS PROMPT by opening COMMAND.COM and
typing in Debug on the DOS prompt. When Debug is exited with the
Q function, program execution returns to Command.Com rather than to the
Windows desktop. Command.Com is then terminated the same way other
windows are closed. Keep in mind that the Debug program was intended to
be run from the Command Interpreter ( Command.Com ) rather than from
the Windows program directly. This difference will become important in
later examples.