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MMD - Mark's MegaDebugger.
==========================
Version 1.00. September 1994.
Copyright (c) 1993 Mark Thomas
NPS Software.
1 Contents
─────────────────────────────────────────────────────────────────────
1 Contents
2 Introduction
2.1 Features and Limitations
3 Legal Notice
4 Quick Start
5 Using MegaDebugger
5.1 Using the Window Mode
5.2 Using the Non-Window Modes
5.3 Screen Swapping
5.4 Expressions
6 Debugging a Program
6.1 Controlling Program Execution
6.2 Restarting the Program
7 Command Line Arguments
8 Command Reference
8.1 Command Parameters
9 Configuration
9.1 MMDCFG
10 Default Key Definitions
10.1 Default Microsoft Keys
10.2 Default Borland Keys
11 Operator Precedence Reference
11.1 Assembler Language Operators
11.2 C Language Operators
11.3 Pascal Language Operators
11.4 A86 Language Operators
12 Error Messages
12.1 Initialisation Errors
12.2 Command Errors
13 Known Bugs
14 Registering
15 Future Enhancements
2 Introduction
─────────────────────────────────────────────────────────────────────
MegaDebugger is a low level debugger for DOS programs that is
designed to provide powerful debugging functions in a more
modest amount of memory than traditional debuggers.
This manual assumes that you have a basic knowledge of the 80x86
processor's instruction set and registers and of the debugging
process. All key references in the manual are based on MMD's
default Microsoft configuration, a configuration file MMD_TD.CFG
is supplied for a Turbo Debugger setup.
2.1 Features and Limitations
■ Features:
- A windowed output that automatically displays the registers,
watches and unassembly information.
- Saves and restores MDA, CGA, EGA and VGA text and graphics
screens. Register level swapping means that MMD can handle
split screens and VGA mode X perfectly.
- Shows DOS memory allocation blocks, interrupt usage and open
files.
- VGA 25, 28, 43, and 50 line text screen support.
- Supports expressions using the operators and data types from C,
Pascal and Assembler/A86.
- Extremely customisable.
- Uses little memory (20k-80k) and works well on older 8086 and
286 systems with limited memory. Fast: No disk overlays and
quick conditional breakpoint and watchpoint evaluation.
- MMD can 'keep' the keyboard interrupt, allowing it to function
with programs that install their own Int09 handler.
■ Limitations:
- No support for numeric co-processor data types.
- No symbolic or source level debugging.
- No 386 support.
3 Legal Notice
─────────────────────────────────────────────────────────────────────
The MegaDebugger documentation and software are not public
domain. They are copyrighted, Copyright (C) 1994 Mark Thomas.
MegaDebugger is free for personal or private use. However some
contribution to the development of MMD would be appreciated.
This can be a registration, donation, supply of information (see
the final section) or simply postcard or Email saying that you
find MMD useful.
MegaDebugger is NOT free for use in a commercial, institutional
or governmental setting. Permission is given to use the
software free of charge for a thirty day evaluation period,
after which registration is required for continued use. A
single user registration costs AUS$50.00 and entitles you to a
free upgrade to the next version of MMD. For site licence
information contact me. Please send cheques or postal orders
(in Australian dollars please) to:
Mark Thomas
8 Herronbee Street
Toowoomba, Queensland
Australia 4350.
Please indicate a registration name and preferred disk size
when registering.
You are free to distribute COMPLETE and UNMODIFIED copies of all
the software and documentation via any means to any recipient
provided that no charge is made except a small and nominal fee
whose sole purpose is to recover reasonable distribution
expenses.
This software is supplied "AS IS", WITHOUT ANY WARRANTY; without
even the implied warranty of MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE. It is supplied as is, in the hope that
people will find it useful. The author is not responsible for
any damage caused by use or misuse of this software.
4 Quick Start
─────────────────────────────────────────────────────────────────────
If you are familiar with other debuggers such as DEBUG, SYMDEB,
or CodeView then you will find the default MMD configuration
provides a similar environment. However MMD is designed to be
configurable and MMD can easily be setup like Turbo Debugger
using the sample configuration file supplied.
Here is a quick introduction to using MMD:
■ An important feature of MegaDebugger is its dual cursor
system. Only the Assembler and Command windows can be made
active and both have separate cursors. When the Assembler
window is the current window then its cursor will be
highlighted. By holding the Shift key while using the scroll
keys the other window cursor can be controlled. All letters
typed will go to the command window, and commands such as
F9;Toggle Breakpoint will work regardless of which window is
current.
■ To change the size of the windows use Ctrl-G; Expand command
window and Ctrl-T; Shrink command window.
■ There are some useful shortcut keys for the Assembler Window:
Ctrl+O;Origin, Ctrl+N;New CS:IP, Ctrl+P;NOP Instruction
■ The commands used by MMD are a superset of those in DEBUG and
are more flexible in that expressions can be used in the
parameters. The H command or F1;Help will provide help on their
usage. F3;Replay Command can be used recall and edit previous
commands.
■ "/VR /VG /VP" provides the best screen swapping.
Enables register level swapping, complete graphics saves and
palette saves. The default swapping mode MMD uses is the
standard method used by most other programs which can corrupt
graphic screens, while register level swapping can restore
almost all screens.
│
│ Do NOT use register level swapping if the target program may use SVGA
│ screen modes. All Super VGA modes are controlled by extended set of
│ chipset specific registers, which MMD's register level swapping is
│ not aware of. Therefore when swapping between the target screen and
│ MMD only the standard VGA registers will be saved and restored,
│ leaving the extended registers in an undetermined state.
│
│ *** USING REGISTER LEVEL SWAPPING WITH SUPER VGA MODES MAY DAMAGE ***
│ *** YOUR MONITOR OR GRAPHICS CARD. ***
│
│ All other MMD swapping modes (Standard and VGA BIOS) are completely
│ safe to use in all screen modes.
│
■ "/H- /VN /VR" minimises the memory used by MMD.
Disables help and higher screen mode changes and enables
register level swapping. The standard swapping mode uses about
16k of memory for swapping, while register mode uses only 4k.
Help uses around 15k of memory and is rather limited so should
be disabled if it is not required. If there is still
insufficient conventional memory for the target to run then try
Screen (/S) or DOS (/D) output modes (Note: is difficult to use
LOADHI to load MMD into the HMA as about 128k of memory is used
during initialisation).
■ I also recommend using the A86 language support: it provides a
unambiguous method for specifying decimal and hexadecimal numbers
and has the same operators as TASM and MASM.
■ MMDCFG can be used to create a custom configuration for MMD.
5 Using MegaDebugger
─────────────────────────────────────────────────────────────────────
When MegaDebugger has successfully loaded it will display its
initial screen with a start up message and a hyphen (-) prompt.
MegaDebugger. Version 1.00
(c) 1994 Mark Thomas
CPU:80286
-
This means that the target program is ready for execution and
that MegaDebugger is ready to accept your commands. Each
command consists command name of one or two characters followed
by any parameters that are required. All input on the command
line is case insensitive, so input can be in upper or lower
case.
5.1 Using the Window Mode
The default output mode for MegaDebugger is window mode. This
presents the debugging information on the screen in a series of
windows. This is the most powerful mode of operation because
all the information from the Register, Unassembly and Watch
commands is automatically displayed and updated on the screen at
the same time.
Register Windows
There are two register windows in MegaDebugger, one across the
very top of the screen known as the Horizontal Register window,
which will be closed initially. The second, the Vertical
Register window, runs along the right hand side of the screen
and is open by default (the initial settings can be changed with
MMDCFG). The reason for providing two windows is that the
output in the command window, particularly that of Dump Memory
command, can sometimes be obscured by the Vertical Register
window.
The Vertical Register can be open and closed with F2;Toggle
Vertical Register Window, Ctrl-F2;Toggle Horizontal Register
Window performs the same function for the Horizontal Register
window.
Watch Window
The Watch window displays any watches that are active. If there
are no watches then it will be closed, and remain so until a
watch is added using one of the Watch commands. Each time a
watch is added or deleted this window will resize itself to
display all the current watches.
Assembler Window
In the centre of the screen is the Assembler window, which
displays an area of memory in disassembled processor
instructions. A large blue highlight bar indicates the current
address of CS:IP. As each instruction is executed, and the
values of CS:IP change the bar moves through the displayed code
to represent the flow of execution. Also in the window is a
solid non-blinking cursor, which is the Assembler window cursor.
An important feature of MegaDebugger is the dual cursor system.
At any time there are two independent cursors on the screen, the
first is the traditional hardware blinking cursor in the Command
window. The second is a simulated cursor in the Assembler
window. One of the windows is known as the current window, the
window that responds to the scrolling keys (Up, Down, PgUp, and
PgDn) and the other will be the secondary window. MegaDebugger
provides a way to access the secondary window without having to
use F6;Change Window.
By default the Assembler window is the current window. Using
any of the window scroll keys will scroll the Assembler window.
By pressing and holding the Shift key while using the scroll
keys the Command window cursor can be controlled. This method
is faster than continually pressing F6;Change Window to move
between windows. It also means that a breakpoint can be set at
the address of the Assembler window cursor using the F9;Toggle
Breakpoint key at any time regardless of which is the current
window.
The converse is also true, if the Command window is the
current window then the scroll keys will move the Command
window cursor and the Shift and scroll key combinations will
address the Assembler window.
When the Assembler window is made the current window then its
cursor will become highlighted. You can determine which is the
current window simply by looking at the colour of the Assembler
window cursor.
Scrolling the Assembler window backwards will cause the screen
to flip backwards. This is because it is very difficult for MMD
to determine the length of a previous instruction, as each byte
may have a valid interpretation. Because it is possible for the
disassembly to become out of step with the actual code this way
the offset address can be adjusted one byte at a time using the
Ctrl-Right and Ctrl-Left keys.
Command Window
At the very bottom of the screen is the Command window, where
the MegaDebugger user prompt (-) is displayed and your commands
are entered. The current input position is indicated by the
blinking cursor. This is where the characters you type will be
displayed. All keyboard input goes to this window regardless of
which is the current window. Previous commands can be recalled
and edited using F3; Replay Command.
The initial height of the window is only five lines. Any lines
that are printed by MMD will force the top lines to scroll
upward and disappear behind the Assembler window. When the
window scrolls up the blinking cursor will disappear from the
screen, but it will still record any characters you enter.
The height of the Command window (and consequently the Assembler
window) can be changed using the Ctrl-G and Ctrl-T keys.
5.2 Using the Non Window Modes
When either Screen or DOS output mode is specified on the
MegaDebugger command line the window functions are disabled and
the output can be thought of as if the Command window has been
expanded to fill the entire screen. Instead of updating the
register window after a section of the target program has been
executed, MMD will dump the contents of the registers to the
screen.
The function keys will continue to work in Screen output mode,
except those that are specifically for window mode. In DOS
output mode none of the function keys are available so the
equivalent MegaDebugger commands must be used.
Non window modes are usually only used to save memory or for a
primitive form of remote debugging (by redirecting the DOS
output to a COM port and displayed by a terminal program on the
local computer).
5.3 Screen Swapping
Because MMD output occupies the entire screen in Window and
Screen modes it leaves no space for the target program to
display its output. To overcome this, a technique called screen
swapping is used to save and restore the output screen each time
a section of the target screen executes. This means that all
the target program's output will be displayed normally on the
screen, and when MMD becomes active it will save a copy of this
screen and replace it with its own. Then when returning to the
target program the output screen is restored. MMD has three
swapping modes Smart, On and Off. In most cases only smart
swapping mode will be the only mode required. It swaps the
screen only when it considers necessary:
- An instruction reads or writes to BIOS video data or screen
memory.
- An IN or OUT (and INS or OUTS) instruction is executed.
- During Go, Proceed or Goto Cursor.
The "On" swapping mode swaps all the time, "Off" will disable
all swapping (losing any target output and corrupting the MMD
screen). The View Output command or F4;View Output Screen can
be used to view the output screen from MMD at any time.
5.4 Expressions
MegaDebugger can evaluate expressions using operators from one
of four languages supported: C, Pascal, Assembler and A86. By
default Assembler operators are used, but this can be modified
as a command option or with MMDCFG. An expression can be used
in any command parameter instead of providing a constant
address.
An expression consists of an ordered mixture of constants and
operators from the current language.
■ Constants
There are three types of constant values accepted by
MegaDebugger:
- Numeric Constants: Numbers are interpreted according to the
current radix and language. MMD has a several extensions to
the standard radix types for some languages, this is detailed
in Section 11.
- Registers: The name of any of processor registers, both 8 and
16 bit, can be specified in an expression. MMD will determine
the current value of the register and use it in the
expression.
- Quoted Character Constants: A single character enclosed in
single quotation marks (') will be used in the expression as
the ASCII character equivalent. To specify a single quote as
the character constant use three consecutive quote characters
(''').
■ Operators
MMD supports all simple unary and binary operators and
parentheses for each language, see Section 11. It does not
support any assignment operators, array and structure operators,
or pointer and type casting operators. In evaluating an
expression all arithmetic is done in 16-bit unsigned integer
math, if the result exceeds that which can be stored in 16 bits
then it will simply wrap around to zero, with no warning.
6 Debugging a Program
─────────────────────────────────────────────────────────────────────
6.1 Controlling Program Execution
MegaDebugger provides a number of ways for you to control the
execution of the target program.
Trace.
In single step mode, control returns to MMD after each processor
instruction is executed. The Trace command or F8;Trace function
key can be used to trace through the target program.
Breakpoint.
A breakpoint defines an address in memory which, when executed,
will return control to MMD. MegaDebugger provides 10 user
breakpoints for this purpose. Breakpoints can be set with the
Set Breakpoint command, F9;Toggle Breakpoint function key or
with the Go command.
Proceed.
Proceed is similar to the trace function however it will step
over LOOP, CALL and repeated string instructions. The Proceed
command or F10;Proceed function key can be used to proceed
through the target program.
Go.
When the Go function is used the target program is allowed to
execute freely until it terminates, or is interrupted by a
breakpoint or keyboard break. The Go command or the F5;Go
function key can be used to run the target program.
Keyboard Break Interrupt.
When the target program is running MMD will monitor the keyboard
for the Ctrl-Break keyboard combination. When these keys are
pressed it will force an immediate entry into the debugger
screen. This method is different to the normal DOS Ctrl-Break
routine, however if you wish to force a DOS break then use the
Ctrl-C key combination. MMD will not normally interrupt DOS
while it is handling a software INT 21 instruction, instead MMD
will wait until DOS completes processing and the break will
occur immediately after the INT 21 instruction.
WatchPoint (Global Breakpoint)
Setting a watchpoint and executing the target will cause MMD to
single step through each instruction and evaluate the expression
for each watchpoint at each step. If the result is TRUE then
MMD will interrupt execution. This slows the execution speed
greatly.
Go To Cursor.
When using Window mode there is an additional way to run the
target program, F7; Go To Cursor. This will place a temporary
breakpoint at the address represented by the current cursor
position in the Assembler window and then run the target program
using the Go command.
6.2 Examining and Modifying the Program State
Each time MMD returns from executing a section of the target
program it will update the display of the program state and
display the prompt to allow debugger commands to be entered.
The program can be altered at this stage by changing the values
of flags and registers (including CS:IP), or by modifying the
program memory with the Enter or Fill memory commands.
6.3 Restarting the Program
When the target program runs to completion MegaDebugger will
display the program termination messages:
Program Terminated Normally (xx)
or
Program Terminated Resident (xx)
The value in brackets is the return code, or error level that
the target program returned. No further instructions can be
executed by MMD until the target program is reloaded using the
Reload Target File command. If you try to use one of the
execution commands (Go, Trace or Proceed) or their equivalent
function keys MMD will display a message advising you of this:
Program Terminated: Restart
7 Command Line Arguments
─────────────────────────────────────────────────────────────────────
The command line format for MegaDebugger is:
MMD [options] [target [arguments]]
■ options
/?, /H Command line help.
/C<file> Use configuration file.
Forces a specific configuration file to be loaded. MMD will
automatically load a configuration file called 'MMD.CFG' in the
default directory, but other files with other names must be
directly specified.
/D DOS redirectable output.
Specifying DOS output mode will disable the screen swapping and
use DOS functions for both input and output. These DOS
functions do not support function keys so all input will be
through the command line. The primary advantage of this mode is
that the input and output of MMD can be redirected using the DOS
redirection commands. MMD uses around 10k less memory because
the screen saving buffers are not required.
/DO, /2 Dual Monitor output.
Redirects all MMD output to the monochrome screen on a system
with two monitors.
│ Untested: I did not have the necessary equipment to actually
│ try this out but have tested it as far as possible, could
│ someone let me know if they can use it with their system.
/Hxx Set screen height: 25,28,43,50.
This switch is relevant only for EGA and VGA systems, where the
number of lines in a text screen can be varied. By default
MegaDebugger will use the same number of screen lines that the
current text screen has, but this switch allows you to override
this and use a specific screen size.
Increasing the size of the screen also increases the size of the
buffer required by MegaDebugger for screen swapping. You can
view the difference by examining the size of the debugger memory
block with the Memory Information command.
In DOS output mode this switch has no effect.
/H[±] Include or exclude online help.
Disables loading of the MMD help file. Although compressed is
still quite large and this option allows loading of the help
file to be disabled, decreasing the amount of memory required by
MMD. Help will still be available through the command line
?;Help command.
/I[±] Allow DOS process ID switching.
MMD attempts to have as little to do with DOS as possible,
however when writing files from MMD it is wise (though not
necessary) to enable PSP switching.
│ Note: MMD does not have a critical error handler so any
│ any errors occurring when writing a file with
│ or W will be passed to the target's handler, if it has one.
/KD[±] Keep keyboard interrupt thru DOS.
Many programs install a new keyboard interrupt (Int09) meaning
that the MMD keyboard handler cannot interrupt the target
program. MMD will attempt to 'keep' the keyboard interrupt by
intercepting any DOS Set Interrupt Vector (Int21/AH=25h) calls
that would change the Int09 vector and installs its own handler
which chains to the requested new handler.
/KP[xx][±] Keep keyboard interrupt thru PIC.
This option attempts to keep the keyboard by reprogramming the
Programmable Interrupt Controller (PIC) to generate an interrupt
other than Int09 when a key is pressed. A program installing a
new handler for interrupt 09h will be unaware that MMD is using
a different set of interrupts. As MMD controls these new
interrupts it can check for a keyboard break before chaining to
the target's Int09 handler. This method ensures that a
keyboard break can always be detected as long as the keyboard
IRQ is enabled. However reprogramming the PIC has two
drawbacks, firstly a series of 8 consecutive interrupts must be
available for the new interrupts. In most systems there is
usually such a range between interrupts 78h - EFh. The default
interrupt range is D8h-DFh, but MMDCFG should be used to verify
that this or any other range is free before it is used. The
second problem is that redirecting the interrupts may interfere
with a protected mode environment which has usually already
shifted the interrupts. If VCPI or DPMI is detected then MMD
will notify them of the change.
/Kx Key definitions; B:Borland, M:Microsoft.
By default MegaDebugger uses CodeView key definitions, but if
you are more familiar with Turbo Debugger or any of the Borland
Integrated Development Environments then this switch will
redefine the function keys (although there are some differences
between individual Borland and Microsoft products, see Section
10 for MMD key definitions).
/S Screen swapping, no windows.
This switch will disable the Window output mode of MMD and
produce line orientated output similar to the DOS output mode.
However screen swapping and function keys will still be active.
This conserves memory by not having to load the window code and
buffers.
/Lx Expression language; A:Assembler, P:Pascal, C:C, 8:A86.
MegaDebugger supports expression operators from four different
languages: C, Pascal, Assembler and A86. By default Assembler
operators will be used.
/Vx Screen swapping type; S:Standard, B:VGABIOS, R:Register.
Sets the method MMD uses for swapping screens. Standard mode
uses several BIOS functions and adaptor registers to swap
screens. VGABIOS mode uses the VGA BIOS Save and Restore state
functions, it was written only out of curiosity and was
extremely slow on my system. Register level is the most
powerful method, it swaps modes by reading and writing all the
registers directly, so can support mode-x and split screens but
not SVGA modes. Register level swapping is supported on MDA,
CGA and VGA systems.
│
│ Do NOT use register level swapping if the target program may use SVGA
│ screen modes. All SuperVGA modes are controlled by extended set of
│ chipset specific registers, which MMD's register level swapping is
│ not aware of. Therefore when swapping between the target screen and
│ MMD only the standard VGA registers will be saved and restored,
│ leaving the extended registers in an undetermined state.
│
│ *** USING REGISTER LEVEL SWAPPING WITH SVGA MODES MAY DAMAGE YOUR ***
│ *** MONITOR OR GRAPHICS CARD. ***
│
│ All other MMD swapping modes (Standard and VGA BIOS) are completely
│ safe to use in all screen modes.
│
/VG[±] Enable complete EGA/VGA graphics save.
By default MMD will not save and restore video memory used for
the character set. In standard text modes and some graphics
modes this is acceptable but in graphics modes and some custom
text modes it will corrupt the screen. Enabling a complete
graphics save requires 4k more memory.
/VN[±] Disable higher mode sets.
MMD can allow its screen height to be changed from the command
prompt, however this requires additional buffers to save the
largest screen size possible, even if it is not used.
/VP[±] Enable VGA palette saving.
MMD saves and restores the palette information. Uses about 1k
extra memory.
■ target
This specifies the name of the file you wish to debug. An
extension is not required for the file name if the file is .COM
or .EXE type because MMD will try both extensions when searching
for the target file. The first attempt will be for a .COM
extension and then .EXE, which is the same order that DOS
searches for an executable file. In window mode the file name
and path of the target file are displayed at the top of the
screen. MMD does not allow the target file to be changed once
it is loaded.
■ arguments
This is the command line you wish to run the target program with
when MegaDebugger loads it. The command line can be changed
with L;Reload Target command.
8 Command Reference
─────────────────────────────────────────────────────────────────────
Command Parameters
- Address: An address can consist of just an offset, in which
case the command will use a default value (either DS or CS
depending on which command) for the segment, or both the
segment and offset values. In this case the segment and
offset must be separated by the segment override character (a
colon ':').
- Byte: A byte value is an 8-bit expression.
- Command line: The command line is simply a string of
characters which will be passed to the target program as its
command line argument.
- Comment: A comment string is simply a string of characters.
- Count: A count is simply a expression that will evaluate to a
word.
- Expression: An expression can be supplied as defined in the
Expressions section, however the segment override operator is
also allowed.
- Format: A format character defines how the output of the
command is to be displayed. MegaDebugger uses the following
format characters:
A ASCII string
B Byte
C Character
D Double Word
I Decimal integer
L Long Integer
U Unsigned decimal integer
W Word
- List: A list is a set of numbers between 0 and 9, or * to
specify all numbers from 0-9.
- Port: A port address is the number of one of 65,355 I/O ports
available on PC computers.
- Range: A range consists of two addresses, or a single address
and a length specifier. Only the first address may have a
segment specified. In some commands the range is optional and
only the address may have to be supplied. To specify an
entire segment in a range a length of 0 should be given.
- Register: Any 16-bit register name, as well as the flags (FL)
register.
- Values: A set of bytes and strings separated by spaces. The
strings must be delimited by matching (') or (") characters.
To specify one of these characters as an element of the string
use either double delimiters 'it''s', or use the other type of
delimiter to delimit the string, "it's". The ASCII values of
the characters in the string are used as byte values.
Current Location
──────────────────────────────────────────────────────────────
.
This command will return the Assembler window highlight cursor
bar, which represents the address of CS:IP, to the top of the
screen.
SeeAlso: Ctrl-O;Origin.
Help
──────────────────────────────────────────────────────────────
?, H
Displays a brief summary of all MMD commands and their
parameters. Unlike the help window this command is always
available, in all output modes and when the context sensitive
online help window is not included.
SeeAlso: F1;Help.
Evaluate Expression
──────────────────────────────────────────────────────────────
? expression[,format]
This is essentially a calculator, any expression given will be
evaluated and displayed. The optional format character allows
the result to be displayed in a format other than the default,
which is a hexadecimal word.
SeeAlso: Configuration;Default Expression Format.
Assemble Instructions
──────────────────────────────────────────────────────────────
A [address]
Assembles instructions directly into memory. This is similar to
the DEBUG assembler with the following enhancements:
- Inline overrides are supported, eg: REP MOVSB and MOV
AL,ES:[SI].
- Most real mode 286 instructions are available.
- Directives ORG, DB and DW.
The default segment is the current CS.
Math coprocessor instructions are not supported.
SeeAlso: Ctrl-P;NOP Instruction.
Set a Breakpoint
──────────────────────────────────────────────────────────────
BP [address] [count][,expression]
MegaDebugger supports 10 user breakpoints. These can be set at
any address in memory, except that of another breakpoint. If at
any point the computer executes the instruction at the address
of a breakpoint the program will be interrupted and MMD will
regain control.
If no address is specified then it will default to the current
CS:IP.
The optional count allows the breakpoint to be passed a number
of times before it will interrupt the program's execution. This
is particularly useful when the breakpoint is placed inside a
loop. Each time the breakpoint is executed the pass count is
decremented, until it reaches 1 when the breakpoint becomes the
same as an ordinary breakpoint with no pass count. The only way
to clear a pass count is by manually clearing and resetting the
breakpoint.
SeeAlso: F9;Toggle Breakpoint.
Clear Breakpoints
──────────────────────────────────────────────────────────────
BC list
Clearing a breakpoint will remove it from the list of
breakpoints that MMD maintains and discards it.
Multiple breakpoint numbers may be specified in the list
parameter and all will be removed.
SeeAlso: F9;Toggle Breakpoint.
Disable Breakpoints
──────────────────────────────────────────────────────────────
BD list
Disabling a breakpoint removes their effect but still keeps them
in the list of breakpoints. This allows then to be reenabled at
a later point, which is faster and more convenient then
clearing and resetting a breakpoint.
Disabled breakpoints will be displayed in the Breakpoint List
command with the letter D beside its entry. In the Assembler
window a disabled breakpoint will be the same colour as the
text, and not highlighted.
Enable Breakpoints
──────────────────────────────────────────────────────────────
BE list
After having disabled a breakpoint you may reactivate it using
this command. Enabling a breakpoint that is already enabled
will have no effect on that breakpoint.
List all Breakpoints
──────────────────────────────────────────────────────────────
BL
The list of breakpoints displays the address, status, and pass
count for all current breakpoints.
The pass count is displayed as a decimal integer rather than a
hexadecimal number.
Compare Memory
──────────────────────────────────────────────────────────────
C range address
Compares all bytes in the range to those starting at address,
and displays any of the differences. For each difference found
MMD will display the address and the bytes that differ.
If the two areas are identical then there will be no display.
Dump Memory
──────────────────────────────────────────────────────────────
D[format] [range]
Dumps memory in the range specified to the screen in the format
specified. If no range is specified then the dump will continue
from the last address previously displayed.
There should be no space between the command name and the format
character, or the format will be interpreted as part of the
range address. The range should also be separated from the
command name by a space, to prevent the address being
interpreted as a format character.
The character format, 'C' is not supported in the Dump Memory
command, instead the byte format will display an ASCII character
translation of each byte it displays.
Enter
──────────────────────────────────────────────────────────────
E address [values]
This command enters bytes directly into the memory at address.
If no values are specified then an interactive mode is started.
In this mode you will be prompted with the current address and
the current value for that byte. You then have four choices:
- Enter a new value for the byte
- Press the space bar to advance to the next byte.
- Press the minus (-) key to move back to the preceding address.
- Press Enter which will terminate the interactive mode.
Fill Memory
──────────────────────────────────────────────────────────────
F range values
Fills the memory within the range with the values specified.
If the range is larger than the size of the values then the
values will be repeated until they fill the range. When the
size if the range is smaller than the values then the values
will be truncated to fill only the range specified.
Go
──────────────────────────────────────────────────────────────
G [address] {address}
This command will execute the target program, which will
continue to run until it terminates, executes a breakpoint or is
interrupted by a keyboard break.
If the address is specified then a breakpoint will be set at
that address, which is in the current code segment by default.
This breakpoint is only temporary, and is not added to the user
breakpoint list, and it will be lost when the control returns to
MMD.
SeeAlso: F5;Run.
Set Screen Height
──────────────────────────────────────────────────────────────
H25, H28, H43, H50
Sets the MMD screen height, if allowed by the video card and
command line options.
SeeAlso: /VN command line switch
Input Byte from Port
──────────────────────────────────────────────────────────────
I port
This command will read the current value from the I/O port
specified and displays it.
Reload Target
──────────────────────────────────────────────────────────────
L [arguments]
Reloading the program will terminate the program that is
currently executing and reload the target file from disk. It
will then be ready to start execution again from the beginning
of the program.
If no command line is specified then MMD will reuse the current
command line. To specify no command line type a single space
character after the command name.
Memory Information
──────────────────────────────────────────────────────────────
M[ | A | I | H | V ]
The output of this command is similar to the DOS MEM command.
There are five options available:
- The default is a list of all the system and program memory
blocks. Environment blocks will not be displayed.
- A All memory information will be displayed, including
interrupts and file information.
- I Interrupt information will be displayed for the main memory
blocks.
- H Provides information about any open files for program memory
blocks.
- V Verbose mode displays all memory blocks.
File information provides information about files opened with
file handles (not FCBs) and relies on undocumented DOS
structures to find the names of the files, and so it is
sensitive to changes between DOS versions.
Move Memory
──────────────────────────────────────────────────────────────
M range address
This command will copy the block of memory in the range to the
destination address.
If the blocks overlap then MMD will ensure that the move is
performed without losing any of the data.
Set Radix
──────────────────────────────────────────────────────────────
N2, N8, N10, N16
Changes the current radix for input values.
SeeAlso: Configuration;Default Radix.
Output Byte to Port
──────────────────────────────────────────────────────────────
O port byte
This command will output the byte value to the I/O port.
Set Option
──────────────────────────────────────────────────────────────
O[B | U][+ | -]
Sets an option:
- OB enables or disables byte decoding display in the Assembler
window.
- OU toggles between upper and lower case display.
SeeAlso: Configuration;Byte Decoding, Configuration;Unassembly Case.
Proceed
──────────────────────────────────────────────────────────────
P [count]
The proceed command is similar to the trace command, however it
will step over loops, calls and repeated string instructions.
When the optional count is used then proceed will execute count
number of times before returning to user input. The debugger
display will be updated for each instruction executed during the
count.
The proceed command will not work in ROM.
The following instructions will be stepped over by this command:
- CALL
- LOOP, LOOPZ, LOOPNZ
- REPx SCASx, REPx LODSx, REPx STOSx, REPx CMPSx, REPx MOVSx
- REPx INSx, REPx OUTSx
Quit
──────────────────────────────────────────────────────────────
Q
This command will terminate the target program, and quit from
MegaDebugger to DOS.
Register
──────────────────────────────────────────────────────────────
R [register] [expression]
The Register command allows you to set new values for any of the
processor register and flags.
If no expression is given for the register then MMD will display
the current value and prompt you for a new one. You may enter a
new value for the string or ignore it and leave the contents of
the register unchanged by pressing Enter.
If no parameters are supplied to this command then it will dump
the current values for the registers to the screen.
To modify the flags the FL register name should be given, again
you may either specify the new value directly or be prompted
with a display of the current flag settings.
Search Memory
──────────────────────────────────────────────────────────────
S range values
Searches all the memory in the range for the sequence of values
specified. For each match MMD will display the address of that
match.
If the values are not found then there will be no display.
Set Swapping Mode
──────────────────────────────────────────────────────────────
S[ | S | + | - ]
This command controls the screen swapping. There are three
options:
- The default is to display the current status of the screen
swapping: on, off, or smart.
- S Sets smart swapping.
- + Will turn screen swapping on.
- - Will turn screen swapping off.
SeeAlso: Alt-S;Toggle Swapping.
Trace
──────────────────────────────────────────────────────────────
T [count]
Single steps one instruction in the target.
SeeAlso: F8;Trace.
Unassemble
──────────────────────────────────────────────────────────────
U [address]
This command will disassemble the program memory at the
specified address and display it as processor instructions. MMD
can disassemble all 8086 instructions, most real mode 286
instructions and all non-32 bit 387 co-processor instructions
When in window mode all the output will be in the Unassemble
window.
The default segment for the address is the current code segment.
View Output Screen
──────────────────────────────────────────────────────────────
V, \
In screen swapping modes this command will switch to the output
screen. MMD will then wait for a key to be pressed before
returning to the debugger display.
SeeAlso: F4;View Output Screen.
List Watch
──────────────────────────────────────────────────────────────
W
The List Watch command will display all the current watches,
their results and any comments that are defined for them.
Write File
──────────────────────────────────────────────────────────────
W "filename" range
Writes the memory in the range to the specified file.
Watch Memory
──────────────────────────────────────────────────────────────
W[format] range [; comment]
This will add an area of memory to the watch list. The same
comments apply to the format and range as in the Dump Memory
command. The default dump size is one byte and the default
format is a hexadecimal word.
The comment is an optional section of text that can be used as a
simple description of the memory region being watched to serve
as a reminder to you.
SeeAlso: Y;DeleteWatch
Watch an Expression
──────────────────────────────────────────────────────────────
W? expression[,format] [; comment]
This command will add an expression to the Watch list. The
style of this expression is identical to that of the Evaluate
Expression command.
If the expression cannot be evaluated by MMD then it will not be
added to the watches.
The comment is an optional line of text that can be used as a
simple description of the expression being watched to serve as a
reminder.
SeeAlso: Y;Delete Watch
Write Listing
──────────────────────────────────────────────────────────────
WL "filename" range
Writes the unassembly listing of the memory in the range to a
text file. This will be a text file with the same formatting as
the current unassembly options.
SeeAlso: OB;Byte Decoding OC; Unassembly case.
Set WatchPoint (Global Breakpoint)
──────────────────────────────────────────────────────────────
WP? expression[,format] [; comment]
Sets a watchpoint, when the program is running it is put into
single step mode and the expression is evaluated on each
instruction executed. When it becomes true the program will be
interrupted. Another use of the watchpoint command is to slow
execution of the target.
SeeAlso: Y;Delete Watch
Delete Watch
──────────────────────────────────────────────────────────────
Y list
Deletes any watches from the current watches
9 Configuration
─────────────────────────────────────────────────────────────────────
9.1 MMDCFG
MMDCFG can be used to configuration MMD.
Each project (if they are in separate directories) can have
different default configurations as appropriate.
During initialising MMD will search for a configuration file
called 'MMD.CFG', a different configuration file can be given
with the /C command line option. When initialising MMD
determines its configuration in the following order of priority:
Options given on the MMD command line will override any default
of configuration file settings.
- A specific configuration file given with the /C command line
option.
- A CFG file in the current directory.
- A CFG file in the same directory as MMD.EXE.
- If no configuration file is found then MMD will use the
options saved inside its EXE file.
MMDCFG menus:
■ Debugging
Keyboard Keeper: None, DOS, PIC
SeeAlso: /KP, /KD.
PIC Keeper IRQ Base
This option displays the current interrupt vectors for the
chosen new IRQ base. All entries in the selected range must be
marked Unused, MMD will allow you to select any range but if the
interrupts are being used by other software then conflicts will
occur.
SeeAlso: /KP.
Switch DOS PID: On, Off.
SeeAlso: /I
Expression Language: Assembler, A86, C, Pascal
SeeAlso: /L
Default Radix: 2, 8, 10, 16
SeeAlso: N;Set Radix
Default Expression Format: Hex, Unsigned, Decimal
Sets the default output format for the ?; Evaluate expression
command.
■ Screen
Screen Height: Default, 25, 28, 43 ,50
SeeAlso: /H, H;Set Screen Height.
Restrict Height changes: On,Off
SeeAlso: /VN
Default Output Mode: Window, Screen, DOS
SeeAlso: /S, /D
Default Swapping Mode: Smart, On, Off
SeeAlso: S;Set Swapping Mode
Default Swapping Type: Standard, Register, VGABIOS, Dual
SeeAlso: /VS, /VR, /VR, /VB, /Do, /2
VGA Palette Saving: On,Off
SeeAlso: /VP
EGA/VGA Complete Save: On,Off
SeeAlso: /VG
Byte Decoding: On, Off
SeeAlso: OB;Set Option: Byte Decoding.
Unassembly Case: Upper, Lower
SeeAlso: OU;Set Option: Unassembly Case.
Horizontal Register Window: Open, Close
SeeAlso: Ctrl+F2;Toggle Horizontal Register Window.
Vertical Register Window: Open, Close
SeeAlso: F2;Toggle Vertical Register Window.
Online Help: Include, Exclude
SeeAlso: /H
■ Keys
Key Definitions: Microsoft, Borland, Custom
The keys menu allows you to set the default MMD key definitions
to the Microsoft, Borland or a Custom setup.
SeeAlso: /KM, /KB.
Break key
Sets the MMD keyboard break combination. By default this will
be Ctrl-Break (Note that MMD distinguishes between Ctrl-Break
and Ctrl-C, this allows Ctrl-C to be used when a DOS break is
specifically required). Any key combination can be used as the
MMD keyboard break, if the key has a non standard scan code
(such as enhanced keyboards which have extra keys such as Macro)
then MMDCFG will display the hexadecimal scan code it received.
Customise Keys
Selecting the Customise keys displays a screen with a list of
all MMD functions. To change a key definition select the
function you wish to assign a new key to and then press the
desired key. If the key does not change then it has already
assigned or is key that has a fixed definition. Customise Key
will initially display the current key selection for
modification which is Microsoft, to base the modifications on
Borland key definitions select Borland keys before enterring
Customise Keys.
■ Colours
Allows the MMD screen colours to be set.
■ Directory
This is only required if MMD is running under DOS version 2.0 or
below, with later versions of DOS MMD is able to determine the
directory it was loaded from and search for a CFG file there.
Alternatively this option can be used to specify any directory
that MMD should search for a CFG file.
■ Save
Saves the changes to a CFG file or into MMD.EXE. Note that if
the options are written to the MMD executable they cannot be
reloaded, so if they require further editing they should also be
saved to a separate CFG file.
10 Default Key Definitions
─────────────────────────────────────────────────────────────────────
10.1 Default Microsoft Keys
Key Definition
──────────────────────────────────────────────────────────────
Command Line Editing.
Escape Clear command line
Left
Right
F3 Replay Command
Screen Keys.
F4 View Output Screen
Alt+S Toggle Screen Swapping
Window Keys.
Up Cursor up
Down Cursor down
PgUp Scroll up one screen.
PgDn Scroll down one screen.
Shift+Up Move secondary window up one line
Shift+Down Move secondary window down one line
Shift+PgUp Scroll secondary window up one screen
Shift+PgDn Scroll secondary window down one screen
Ctrl+Home Cursor to Top
Ctrl+End Cursor to Bottom
Window Control.
F2 Toggle Vertical Register Window
F6 Change Window
Ctrl+F2 Toggle Horizontal Register Window
Ctrl+G Expand Command Window
Ctrl+T Shrink Command Window
Assembler Window.
Ctrl+Right Byte Adjust Up
Ctrl+Left Byte Adjust Down
Ctrl+N New CS:IP
Ctrl+O Origin
Ctrl+P NOP Instruction
Debugging.
F5 Go
F7 Go To Cursor
F8 Trace
F9 Toggle Breakpoint
F10 Proceed
Help.
F1 Help
Tab Next Topic Link
Alt+B Sequential Next Topic
Alt+F Sequential Previous Topic
Alt+F1 Previous Topic
Ctrl+F1 Next Topic
──────────────────────────────────────────────────────────────
10.2 Default Borland Keys
Key Definition
──────────────────────────────────────────────────────────────
Command Line Editing.
Escape Clear command line
Left
Right
F3 Replay Command
Screen Keys.
Alt+S Toggle Screen Swapping
Alt+F5 View Output Screen
Window Keys.
Up Cursor up
Down Cursor down
PgUp Scroll up one screen.
PgDn Scroll down one screen.
Shift+Up Move secondary window up one line
Shift+Down Move secondary window down one line
Shift+PgUp Scroll secondary window up one screen
Shift+PgDn Scroll secondary window down one screen
Ctrl+Home Cursor to Top
Ctrl+End Cursor to Bottom
Window Control.
F6 Change Window
Ctrl+F2 Toggle Horizontal Register Window.
Alt+F2 Toggle Vertical Register Window.
Ctrl+G Expand Command Window
Ctrl+T Shrink Command Window
Assembler Window.
Ctrl+Right Byte Adjust Up
Ctrl+Left Byte Adjust Down
Ctrl+O Origin
Ctrl+N New CS:IP
Ctrl+P NOP Instruction
Debugging.
F2 Toggle Breakpoint
F4 Go To Cursor
F7 Trace
F8 Proceed
F9 Run
Help.
F1 Help
Tab Next Topic Link
Ctrl+F1 Next Topic
Alt+F1 Previous Topic
Alt+B Sequential Next Topic
Alt+F Sequential Previous Topic
──────────────────────────────────────────────────────────────
11 Operator Precedence Reference
─────────────────────────────────────────────────────────────────────
These tables summarise the various operators supported by
MegaDebugger. These operators are listed in decreasing
precedence, and operators that are grouped together have the
same precedence. All the languages support the use of
parentheses. None of the memory reference operators are
supported by MMD (assembler [] or C * ).
11.1 Assembler Language Operators
Operator Description
──────────────────────────────────────────────────────────────
- Unary Minus
+ Unary Plus
NOT Bitwise NOT
───────────────────────────────────────────────
* Multiplication
/ Division
MOD Modulus
SHL Left shift
SHR Right shift
───────────────────────────────────────────────
+ Addition
- Subtraction
───────────────────────────────────────────────
EQ Equality
NE Inequality
LT Less than
LE Less than or equal to
GT Greater than
GE Greater than or equal to
───────────────────────────────────────────────
AND Bitwise AND
───────────────────────────────────────────────
XOR Bitwise XOR
OR Bitwise OR
───────────────────────────────────────────────
The NOT operator in both MASM and TASM is defined to have the
same precedence as the AND operator. However the debugging
assumes that all unary operators have a higher precedence than
binary operators.
All logical Assembler operators return 0FFFFh for a true result
and 0 for false.
digitsH Hexadecimal
digitsD Decimal digits
digitsB Binary digits
digitsO Octal digits
digitsQ Octal
11.2 C Language Operators
Operator Description
──────────────────────────────────────────────────────────────
- Unary Minus
! Logical Negation
───────────────────────────────────────────────
* Multiplication
/ Division
% Modulus
───────────────────────────────────────────────
+ Addition
- Subtraction
───────────────────────────────────────────────
<< Left shift
>> Right shift
───────────────────────────────────────────────
< Less than
<= Less than or equal to
> Greater than
>= Greater than or equal to
───────────────────────────────────────────────
== Equality
!= Inequality
───────────────────────────────────────────────
& Bitwise AND
───────────────────────────────────────────────
^ Bitwise XOR
───────────────────────────────────────────────
| Bitwise OR
───────────────────────────────────────────────
&& Logical AND
───────────────────────────────────────────────
|| Logical OR
───────────────────────────────────────────────
No assignment operators are supported by MMD including the C
auto assignment operators (++ and --).
0Xdigits Hexadecimal
0Ndigits Decimal
0digits Octal
11.3 Pascal Language Operators
Operator Description
──────────────────────────────────────────────────────────────
- Unary Minus
+ Unary plus
NOT Bitwise NOT
───────────────────────────────────────────────
* Multiplication
/ Division
DIV Division
MOD Modulus
AND Bitwise AND
SHL Left shift
SHR Right shift
───────────────────────────────────────────────
+ Addition
- Subtraction
OR Bitwise OR
XOR Bitwise XOR
───────────────────────────────────────────────
< Less than
<= Less than or equal to
> Greater than
>= Greater than or equal to
= Equality
<> Inequality
───────────────────────────────────────────────
MMD supports only the INTEGER forms for each operator, therefore
there are no equivalents for the BOOLEAN logical and and or
operators.
Although the type of the / division operators result is defined
as REAL in Pascal, MegaDebugger does not support this data type
and so the result will be integer.
$digits Hexadecimal
#digits Decimal
11.4 A86 Language Operators
Operator Description
──────────────────────────────────────────────────────────────
- Unary Minus
+ Unary Plus
NOT Bitwise NOT
! Logical NOT
BIT Bit Number
───────────────────────────────────────────────
* Multiplication
/ Division
MOD Modulus
SHL Left shift
SHR Right shift
───────────────────────────────────────────────
+ Addition
- Subtraction
───────────────────────────────────────────────
EQ Equality
NE Inequality
LT Less than
LE Less than or equal to
GT Greater than
GE Greater than or equal to
───────────────────────────────────────────────
AND Bitwise AND
───────────────────────────────────────────────
XOR Bitwise XOR
OR Bitwise OR
───────────────────────────────────────────────
The NOT operator in A86 is defined to have the same precedence
as the AND operator. However MMD assumes that all unary
operators have a higher precedence than binary operators.
All logical Assembler operators return 0FFFFh for a true result
and 0 for false.
0digits Hexadecimal
digitsH Hexadecimal
digitsD Decimal
digitsB Binary
digitsO Octal
digitsQ Octal
digitsXH Hexadecimal
digitsXD Decimal
digitsXB Binary
digitsXO Octal
digitsXQ Octal
The underscore '_' character can be used to separate digits.
12 Error Messages
─────────────────────────────────────────────────────────────────────
12.1 Initialisation Errors
All error messages MMD displays on the output screen are
prefixed with the '<MMD>' string to distinguish them from any
possible target output.
■ Cannot find target file : "xxx"
This means that MMD cannot find the target file you specified on
the command line. Check that path and file name and try again.
■ Conflicting command line switches.
When processing the command line options you supplied
MegaDebugger found two switches that conflicted. For example
specifying Screen mode and DOS mode together (/S /D) or two
language specifiers.
■ Failure loading target file in DOS LOADEXEC.
DOS could not run the target program. The most common cause of
this is that the program is too large to fit into the remaining
free memory. Other causes are that the file is not a valid EXE
or COM file, the memory allocation chain is corrupted or MMD has
insufficient access rights to execute the file.
■ Internal memory allocation failure.
This error message should never appear, as MMD makes only one
call to the DOS memory block. If this message does appear it is
likely that your system memory has been corrupted (not unusual
when debugging an errant program), reboot and try again.
■ Invalid PIC base interrupt : "xx"
The valid range for the new PIC base interrupts are from D8-F0h.
■ Missing or invalid configuration file : "xxx"
The configuration file specified with the /C command line option
could not be found or was not a valid MMD configuration file.
■ PIC has already been reprogrammed by another VCPI client.
When EMM386 or a similar memory manager is running MMD is
required to notify the protected mode manager when it reprograms
the PIC controller to the new interrupt base. However if
another program has already changed the settings then MMD is not
allowed to make any further changes.
■ Target is not an executable file : "xxx"
The target file specified in the command line is not a file.
■ Unknown language type specified.
Only four languages are supported by MMD /LA; Assembler, /LC; C,
/LP; Pascal and /L8; A86.
■ Unknown screen height specified : "xx"
MMD supports only 25 and 43 line modes on EGA cards and 25,28,43
and 50 with VGA. MDA and CGA support only 25 line modes.
■ Unknown switch : "xx"
Use MMD /? for a list of valid command line switches.
■ VCPI did not allow PIC to be reprogrammed.
For some reason the call to notify the VCPI protected mode
manager of the changes to the hardware interrupts failed.
12.2 Command Errors
■ Bad Expression
The expression that you have entered cannot be evaluated by MMD.
This is usually caused when an operator is misplaced or a
register name is misspelt in the expression.
■ Bad Flag
There are two possible causes for this error:
- The flag string you supplied to the Register command contained
a value that was not the name of a flag.
- The string contained conflicting values for the same flag.
■ Bad Format Character
The character supplied after an expression or in the Dump
command is not one of the format characters MegaDebugger
recognises.
■ Bad Register
The register name given as a parameter to the Register command
was not a valid 16-bit register name.
■ Breakpoint Error
This error occurs when using the Set Breakpoint command, and you
try to set a new breakpoint at the same address as another
breakpoint
■ Closing Quote Character is Missing
When evaluating either a quoted character constant in an
expression, or in a set of values MMD reached the end of input
string before it found a closing quote character.
■ Divide by Zero
The value derived for the right hand side of a division or
modulus operator was equal to zero. The result of a division by
zero is mathematically undefined.
■ Extra Characters on Line
MMD has fulfilled the parameters for a particular command but
there are still excess characters remaining on the line.
■ File Error
This error occurs when MMD cannot open, write to, or close the
file given for the W;Write File or WL;Write Listing commands.
■ MCBs Corrupted
This occurs when the Memory Information command detects an error
in the structure of the DOS memory allocation system. This is a
serious error and it is unlikely that you will be able to exit
without the system crashing.
■ No Free Watches
MegaDebugger limits the number of watches you can define to 10,
this error is caused if you attempt to add another watch after
this limit has been reached.
■ Syntax Error
A syntax error occurs when MMD cannot convert the parameters of
a command into the required format. There are four conditions
that will cause this error:
- The start address in a range is larger than the final address.
- One of the items in the list cannot be converted, lists can
contain only values between 0 and 9.
- One of the items in a set of values cannot be converted, or
the value exceeds one maximum number that can be stored in one
byte; 255.
- An incorrect number of parameters was supplied, usually when a
complete range is not given.
■ Unbalanced Parenthesis
While evaluating an expression the end of the string was reached
and there were unclosed parenthesis. There must be a matching
closing parenthesis ')' for each opening parenthesis '(' and
vice versa.
■ Unknown Command
The first non space character on the command line did not match
any of MMD's command names.
13 Known Bugs
─────────────────────────────────────────────────────────────────────
/KP does have occasional problems with memory manages such as
EMM386 and QEMM.
Dual monitor support has not been used with two actual monitors,
but has been thoroughly tested as far as possible on a single
monitor.
MMD does not install its own critical error handler so when
writing files from MMD the target's handler will be invoked if a
DOS error occurs.
Swapping does not support Hercules graphics mode and cannot
properly handle text modes which use custom (non-BIOS) fonts.
Not all 286 instructions are support by the A; Assemble command,
namely the extended shift instructions, 3 operand MUL and all
protected mode instructions. Maths coprocessor instructions are
also not supported.
MegaDebugger does not restore the entire hardware state. If the
target program is using any critical system interrupts when the
Reload Target Program or Quit commands are used then these
interrupts will be left containing addresses that point to
unused memory locations that will be overwritten when next file
is loaded. In order to avoid crashing the computer when using
this command always ensure that either:
- The target program has terminated or has not begun execution.
When the target program exits it should always return the
system to a safe state.
- If the target program is still executing then ensure that it
is not using any of the critical interrupts. This can be done
using the Memory Information command, if any block that is
owned by the target program is using any of the interrupts
between 00f and 2Fh then it is unsafe to use the Reload Target
Program or Quit commands.
When comparing MMD to commercial debuggers (TurboDebugger 3.00
and CodeView 2.00) it was interesting to see how poorly they
handle fundamental debugging operations:
- MMD can handle breakpoints at normalised addresses (same
physical address, but different segment:offset address). Both
CodeView and TurboDebugger will crash if a breakpoint is
executed when set a normalised address, MMD does all its
internal address comparisons with 32 bit physical address
values.
- MMD handles graphics modes much better then TD and CV,
especially ModeX. TurboDebugger has a stupid 'smart' screen
swapping which will swap unnecessarily. CodeView does not
have smart swapping.
- MMD handles the interaction between WatchPoints and all forms
of breakpoint consistently. For example: CodeView will not
update breakpoints with pass counts when watchpoints are set;
TurboDebugger will not acknowledge a true watchpoint if a
breakpoint exists on the line which makes the watchpoint true.
14 Registering
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I really want MMD to be the most useful debugger tool available
and so would very much like to hear all comments, suggestions,
criticisms you have.
You can contact me at the following addresses:
Internet:
n1360906@student.fit.qut.edu.au [until December 1994]
No Net Address:
Mark Thomas
8 Herronbee Street
Toowoomba, Queensland
Australia 4350.
Other software by the same author:
2OBJ110
2Obj v1.10: Converts binary files directly to OBJ files. Can
decode GIF, PCX, LBM and CEL files and their palettes. Freeware.
DIV0
Divide by Zero: the first demo by Sivris.
DNC100
DOOM Network Copier: Copies DOOM files from a single computer to
up to four other machines simultaneously.
MCD100
MegaChangeDirectory: Replacement for Norton's CD. Can create
NCD compatible TreeInfo files. Configurable. Extended directory
matching. To Be Released Soon.
15 Future Enhancements
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While this version of MMD is complete there is still a great
deal I wish to add, which will require almost a complete rewrite
of all the current code. This list is in approximate order of
priority:
- 386 support. The first priority is simply adding 386
instruction and register support. Of course adding 386
debugging register and v86 mode support is very important.
(I have only recently upgraded from a 286)
- Commands: I have a long list of commands to be included in the
next version. The main areas are searching (forward and
backward global searches, case-insensitive string searches,
instruction searches) and debugging commands (Breakpoint on
entry or exit into interrupts, Go to RET, Trace into
Interrupt).
- Improved memory management. Most important is allowing MMD to
use high memory to free more memory under 640k but also to use
XMS and EMS.
- Remote debugging. I have already written the communications
drivers for serial, parallel, NETBIOS and IPX links but all
the MMD commands will have to be re-written to work with the
remote link interface.
- Better interface: pull down menus, mouse support, resizing etc
(and stack and memory and output screen windows). I
personally prefer to have a command line interface for
debugging so it will still be available.
- Symbolic Debugging. The original idea of MMD was to provide a
alternative to the large symbolic debuggers but now it seems a
waste not to include it. MMD will still be primarily a
low-level debugger.
I am looking for information on the following:
- The EXE debug information format used by CodeView and
TurboDebugger (and Watcom, Symantec etc) and details of any
debugging hooks placed in executables.
- Detecting and using 8 bit interrupt driven parallel I/O.
- Interfacing with the NMI, particularly about building a cheap
and safe NMI breakout switch that plugs into the bus.
- System Management Mode. One of the most interesting ideas I
have seen is to use the SMM of 386SL and 486SL for a debugger.
SL chips are used mainly in laptops but apparently all new
i486 CPUs have this mode.
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