Power-Programmierung/CD1.mdf/assemblr/library/sampler0/spies.doc

Text File | 1987-05-15 | 12KB | 203 lines

615 796 2640
This file, SPIES.DOC, is the documentation for the 1.0 versions of the two
programs SPY.COM, and SPY'.COM and also the accompanying source files SPY.ASM
and SPY'.ASM.

The programs were written for the TSR Seminar/Tutor section of the Microsoft
forum of CompuServe.  They are shareware programs with a $1 minimum requested
donation.  If you like the programs and/or the ideas presented in the source
code I would be pleased if you would send me a monetary note of thanks.  What
do you get for your $1?  Not much to be sure.  In fact each $1 that you send
will be a vote for an enhanced version of a SPY program that will be written
contingent upon the number of $1's received.  (It too would be shareware and
source code would be made available to those sending in payments).

You may send donations to:

   Charles Lazo III
   P. O. Box 2312
   Hohenwald, TN 38462

My CompuServe user ID is 72210,17.

The object of these programs is to demonstrate two methods that allow a program
to detect if it has been loaded as a TSR before and if so to optionally pass
parameters to the previous load.  In addition to making the programs as simple
as possible it was also desired to make them useful in their own right.
Whether I have been successful in this endeavor will be up to you to decide.

Both SPY.COM and SPY'.COM behave in exactly the same way as far as the user is
concerned (except, of course, for the copyright and syntax notices).  Running
the SPY program without parameters (or by making an error in syntax) you will
see:

SPY Copyright (C) 1987 by Charles Lazo III, v1.0

SPY syntax:  SPY xxxx:yyyy L zzz

Where xxxx, and yyyy are hexadecimal numbers up to four digits in length and
zzz is a one to three digit hexadecimal number.  SPY will monitor the segment-
offset region xxxx:yyyy for a length of zzz bytes and report to the user with a
number of beeps equal to the number of bytes changed in that region if and when
any are changed.

The method of parameter setting for SPY is the same as one of the methods of
specifying a range for DEBUG, i.e., segment:offset L size.  After the parameter
string you may optionally place comments on the same line by preceding them with
a semicolon (may be useful for SPY in batch files).  The parameter string may be
entered in any combination of upper and lower case, and tabs or spaces may be
sprinkled liberally within it except that hex numbers may not be broken up.

The length of the spy region has been arbitrarily limited to three hex digits.
This allows for up to 4095 bytes to be monitored at once for changes while SPY
is loaded.  This maximum value also determines how much memory that SPY will
reserve for the spy buffer, an area of memory that will contain a copy of the
region of memory to be spied upon.  Each time a change occurs in the spy region
chosen by the user on the command line SPY will increment a counter and update
its copy of the region which it uses for comparison.  This action takes place
in a modified interrupt 8 (clock) handler.  In the new int 8 routine if this
counter is non zero, then the speaker will be turned on and the counter will be
decremented.  If the speaker is on then it is turned off at the next int 8.
A delay counter is set after each beep and is decremented thereafter at each
int 8 tick.  Another beep will not occur until the delay counter is decremented
to zero.  Thus the beeps emitted by SPY occur at intervals which depend upon
the value set into the delay counter.  The maximum size of the spy region and
the value of the delay interval can be changed if the constants which control
them are altered in the SPY.ASM file and the program is reassembled, relinked
and passed through the EXE2BIN program.  One more thing that can be easily
changed is the frequency of the tone emitted by SPY.  The default frequency is
just about 2,000 hertz.

Setting SPY's sights upon a volatile region can become distracting so it is
welcome knowledge that you can shut it up merely by pointing it to another
region of memory.  I suggest pointing it to ROM, e.g., SPY F800:0 L1.  Thus
to be explicit about the nature of SPY's operation after being loaded once
before:  SPY detects a previous load of itself as a TSR, and if there were any
parameters on the command line of the current load, then the code in spy will
pass these parameters to the code of the previous load and they will be acted
upon as if SPY is being loaded for the first time.  (This is why SPY will save
a buffer of 4095 bytes [by default] even if you only ask for the monitoring of
a single byte.)

Keeping the program simple has lead to a number of compromises in its operation.
One undesirable result is segment wrap around.  If you were to run SPY as:

                           SPY 0:ff00 L 200,

then instead of monitoring the 512 bytes from ff0:0 to ff0:200 as you might
expect, you would instead be keeping tabs on the 256 bytes from ff0:0 to ff0:ff
and also the 256 bytes at the top of this segment from 0:0 to 0:ff.  Instead
enter this as:

                           SPY ff0:0 L 200.

While DEBUG will flag this sort of carelessness with an error, SPY will accept
it and wrap the segment without complaint.

Before I begin comment on the details of the source code that allows sensing a
previous program load I would like to mention a few of the discoveries that I
have made snooping around with SPY.  There is a 16 byte region in the DOS data
section of memory called the ICA (Intra-Application Communications Area).  Not
many programs use this region and one good application of SPY is to monitor it
to determine which programs do use it.  I was surprised to discover that BRIEF,
the editor which I am using to put this stuff together, uses this region.  Why
I don't know.  To monitor this region enter SPY 0:4f0L10.  Another good place to
point SPY is the interrupt table:  SPY 0:0l400.  I have noticed while monitoring
this region that even though interrupt vectors are four bytes long I sometimes
count a number of changes which is not divisible by four.  I dono why.  The most
interesting discovery so far, however, has been the location of a byte in a
section of memory used by DOS.  I suppose it is in the non transient portion of
COMMAND.  The location is 291:1a7 in my PC DOS ver 3.20.  What is interesting is
that while SPY is pointed at this single byte and DOS is at the DOS prompt every
keypress that places a printable character on the screen also elicits a beep
from SPY.  The byte is changed very many times for a long display such as a
directory listing or a file typed to the screen.

SPY.ASM and SPY'.ASM differ in only one respect except for the near universal
replacement of SPY with SPY' in the SPY'.ASM program.  This is the method that
the two programs use to detect previous loads of themselves.  The detection done
by SPY is carried out entirely in the routine named set_es while for SPY' it is
done in both set_es and newint16h (a routine chained into the ROM BIOS keyboard
interrupt vector).  Both programs use a set of three data values, key1, key2,
and key3 that are words which are set into specific offsets when the program
loads for the first time and becomes a TSR.  The idea of detection in each case
is to search locations in memory for the three key values in the relative
locations in which they are expected to be found.  SPY uses a brute force method
that looks at every paragraph prior to the current value of the code segment for
the presence of the keys.  The value of key1 is the two byte op code for the
instruction int 20h which is found at offset 0 of every PSP.  It was chosen
because if all of the keys were at offsets following location 100h in the code
segment, then SPY could fail in its attempt to find itself in memory because it
might mistake a set of data in a cache or in the buffers of DOS as a prior load
if itself.  SPY' uses the routine newint16h to add a new function to the ROM
BIOS keyboard interrupt int 16h.  The set_es routine invokes the int 16h with
the function number ah = 77h.  When SPY' has not been previously loaded the
interrupt returns with nothing changed.  However, if SPY' has been previously
loaded, then the installed newint16h will service the 77h function by checking
to see if the three keys are found in their respective locations (if not found,
then the 77h function call must be due to another program using the same method
and function number so is ignored by newint16h).  If the keys are found in the
right locations, then key1 is returned in ax and key2 is returned in bx while
the value of cs for the newint16h routine is passed back in es.  Thus set_es
compares ax to key1 and bx to key2.  If both equal are equal, then it can be
assumed with fair confidence that es now contains the code segment of SPY' when
it was first loaded as a TSR.

I have thought of a few enhancements for a 2.0 version of SPY:

     (1) The ability to report the currently spied region of memory.

     (2) Allow the use of multiple spy channels (two or more spied regions at
         once using different tones for reporting changes in each channel).

     (3) Allow the use of the second type of DEBUG range specification.  I.e.,
         SPY seg1:off1 seg2:off2.

     (4) Preventing the wrap of a segment (a problem mentioned earlier).

     (5) Implementing SPY OFF and SPY ON.

     (6) Allowing command line setting of the delay count, maximum spy buffer
         size, and frequency of tone.

     (7) The ability to make calls to SPY from within another program.  (This
         could be helpful in monitoring stack usage.)

     (8) Optional use of a popup window for on-the-fly change or monitoring of
         SPY parameters.

Vote for any of the above or make your own suggestions when sending in your $1
minimum contribution.

                                   REFERENCES

I found the idea used by SPY to find a previous TSR load of itself while taking
apart the QEGA.COM program which was supplied by the QUADRAM corporation with
their QUAD EGA+ EGA card.  As I recall this program used only two keys and did
not use the int 20h instruction as a key.  I dono how it avoids making the
mistake that can be caused by cache/buffers.

The idea for adding another function to the int 16h keyboard routine as used in
the SPY' program I got from the program PUSHDIR (used also in the companion
program POPDIR) which is presented in the May 27, 1986 issue of PC Magazine.
The programs' author is John Friend.  John uses the same function number 77h (or
rather I am using the same number as John).  The reason I chose to use the same
function number was so that it can be demonstrated that no conflict arrises in
the use of SPY' and PUSHDIR-POPDIR together in the same machine.  Other than
this similarity the programs SPY' and PUSHDIR-POPDIR are very different in their
inner workings.  PUSHDIR-POPDIR does not use keys, but instead compares the
copyright notice in a candidate for a prior load.


                                   DISCLAIMER

This software SPY.COM, SPY'.COM, SPY.ASM, and SPY'.ASM and the accompanying
documentation SPIES.DOC are not warranted for any purpose either expressed or
implied including but not limited to merchantability or fitness of purpose.  The
author of the software shall have no liability or responsibility to any user
of the software with respect to any liability, loss or damage directly or
indirectly arising out of the use of the software, including, but not limited to
any loss of business or other incidental or consequential damages.  By using the
software you agree to this.