Collection of Education/collectionofeducationcarat1997.iso/COMPUSCI/D86V322.ZIP/D05.DOC

Text File | 1988-05-20 | 13KB | 264 lines

CHAPTER 5   COMMAND LANGUAGE

In addition to immediate-execution assembly language commands,
there is a set of commands recognized by the debugger.  They are
identified by the first keyword on the line being a single letter
(i.e., the second character of the line is a non-letter, usually
a comma or ENTER).


General Operands to Debugger Commands

Most of the debugger commands consist of their single-letter
identifier, followed by a comma, followed by one or more general
operands, separated by commas.  General operands can be one of
the following:

   a. a numeric constant, whose format is just as in the assembly
      language (leading zero means default hex, otherwise default
      decimal)

   b. a register name

   c. a user symbol from the assembly language program being
      debugged.


Format of Debugger Command Examples

Many of the examples given below will be given in double quotes.
Note that the double quotes are not part of the command.  You are
encouraged to try out the example on the debugger, by typing the
string within the quotes, not including the quotes, and always
followed by the ENTER key. Note further that the double-quoted
string may be broken across two lines of this manual, but that
does not mean you should type an ENTER where the string is broken
--debugger commands always consist of a single line, always
terminated by ENTER.


The Debugger Command Set

Following is a description of the debugger commands recognized:

B  sets and clears the fixed breakpoints of the program.  The
   debugger has four breakpoints.  Two are transitory; they are
   automatically cleared after each return from the program to
   the debugger.  They can be set by the G command. The other two
   are fixed-- they will remain in effect until you explicitly
   clear them.  The fixed breakpoints are controlled by this B
   command.

   You follow the B with zero, one, or two general operands.  If
   there are zero operands (the B is followed immediately by an
   ENTER), then both fixed breakpoints are cleared.  If there are
   one or two operands, then the fixed breakpoints are set to the
   operands.
                                                              5-2

   Note that previously-set breakpoints can be implicitly
   cleared, by overwriting them with other breakpoints.  If your
   B command has one operand, and there was one breakpoint
   previously set, the debugger sets the unused breakpoint, so
   that both remain in effect.  If your B command has one
   operand, and both breakpoints were previously set, the most
   recently set breakpoint is saved, and the older breakpoint is
   overwritten.

   The status screen, displayed by typing Ctrl-S, shows you the
   B-command breakpoints in effect.

   Examples: if you type "b,numout", the debugger will set a
   breakpoint at location NUMOUT, which should be a label in the
   program being debugged. You may start and stop the program
   many times, and the breakpoint will stay there.  You may even
   allow the program to stop at NUMOUT repeatedly; the breakpoint
   is not cleared even if the program stops there.  If you
   subsequently type the command "b,01000", then there will be
   breakpoints at both NUMOUT and location hex 01000.  If you
   then type "b,01200", the first breakpoint NUMOUT is
   overwritten; the two breakpoints now in effect are 01000 and
   01200.  The 01000 breakpoint will be next in line to be
   overwritten.  You may clear both breakpoints by typing "b".
   There is no way to clear one breakpoint at a time.


D  sets or clears a data breakpoint.  This command is available
   only to registered D86 users running on a 386-based machine. A
   data breakpoint causes the program to trap to the debugger
   whenever a specified memory location is accessed.  The trap
   occurs after the instruction causing the access, so you should
   press the Up-arrow key to see the instruction.

   You follow the "D" with a comma, followed by a specification
   parameter.  The parameter consists of up to three characters,
   at most one each from the following categories:

   1. A letter giving the size of the memory element being
      checked: B for byte, W for word, D for doubleword, or a
      minus sign if you are clearing the breakpoint.  Default is
      B.

   2. The letter R if you wish to trap if the memory location is
      either written to or read from.  If you leave the R out,
      the trap will occur only if the memory location is written
      to.

   3. A digit, (0,1,2, or 3) giving the number of the 386
      breakpoint register you are using to set the trap.  Default
      is 0.
                                                              5-3

   You terminate the specification parameter with a comma, then
   provide one or two numbers to specify the memory location you
   are trapping. If you provide two numbers, the first is the
   segment register value and the second is the offset.  If you
   provide only one number, it is the offset-- the value of DS is
   used as the segment register value. As with all value
   parameters in D86, you can give a register name or a label
   instead of a number.  You can also leave out the address
   entirely, to preserve the previous address setting of that
   breakpoint register.

   Note that the 386 requires Word and Doubleword breakpoints to
   be aligned in memory.  If you provide an odd address for a
   Word breakpoint, the 386 will ignore the bottom bit of your
   address.  Similarly, the 386 will ignore the bottom two bits
   of your address for a Doubleword breakpoint.

   Examples: D,R1,ES,0400 sets a byte data read-or-write
   breakpoint, using the 386's register number 1, at memory
   location ES:0400.  D,-1 would clear that breakpoint.  D,R1
   would set it again with its previous value.  D,W,MY_VAR sets a
   Word breakpoint, using the 386's register number 0, at
   location DS:MY_VAR-- the trap will occur if either byte of the
   variable MY_VAR is written to (but MY_VAR should be aligned to
   an even address for this to work).

   If the D command is enabled, you'll get a one-line display of
   the data breakpoint registers in the status window (invoked
   via Ctrl-S).  The registers are presented in order: 0,1,2,3.
   The breakpoint type is given, followed by the 5-digit absolute
   memory address of the breakpoint.


F  finds a string of memory bytes.  The memory to be searched
   starts at the current CS:IP location.  The string being sought
   is contained in memory at the CS:IP location marked with the
   last Shift-F7 command.  The number of bytes in the target
   string is given as the first operand to the F command.  For
   example, "F,1" finds the next instance of a single byte value
   after the current CS:IP.  If the marked location points to a
   NOP, "F,1" will find the next NOP code.

   If you provide a second operand to F, it is a "retreat
   number".  For example, "F 2,10" assumes that you are looking
   for a 2-byte sequence, and you have retreated 10 bytes from
   the starting location for your search.  When the string is
   found, F will retreat 10 bytes from that string.  That way you
   can view the instructions that preceded the found string.  I
   use this feature when I am searching for BIOS and DOS
   interrupt calls in a program.  I want to retreat before the
   calls, to see what function numbers were loaded into
   registers.  I can use the F3 key to repeat the searches,
   giving me a sequence of disassembly displays with the
   interrupt in the middle.
                                                              5-4

   F with no operands returns CS:IP to the marked location, in
   case you want to use F7 to deposit another string to be
   searched.

   If you have never pressed Shift-F7 in this session, the marked
   location is 0C000 of the program's starting segment.  That's
   often a good "scratchpad" area for small programs, far from
   both the program and the stack.


G  starts the user progra