Talk:Debug (command)
From Wikipedia, the free encyclopedia
[edit] Syntax section
The syntax section, IMHO, ought to contain descriptions of the command-line invocation parameters for DEBUG; and using DEBUG with DOS redirection ought to be discussed separately, as should running DEBUG from batch files.
(Calling DEBUG's commands "subcommands" is unnecessarily pedantic.)
As an example of running DEBUG with redirection (implemented by COMMAND.COM or CMD.EXE), consider a text file containing the following two lines:
? q
This file can be given any name (and perhaps extension), for example, DEBUG-IN.TXT.
Then, invoking DEBUG with:
DEBUG < DEBUG-IN.TXT >> DEBUGOUT.TXT
will invoke DEBUG, taking its input not from "standard in", but from the file, DEBUG-IN.TXT, and sending its output, not to "standard out", but to the file DEBUGOUT.TXT, appending it to the original file, if it exists, or creating it otherwise. This is a feature of DOS's COMMAND.COM called redirection, and is supported by DEBUG, which can indeed be very useful, but is peripheral to DEBUG's syntax. In this example, DEBUGOUT.TXT will contain the output of DEBUG's "?" command, a brief listing of its commands (along with an "echo" of its prompts and input) as follows:
-? assemble A [address] compare C range address dump D [range] enter E address [list] fill F range list go G [=address] [addresses] hex H value1 value2 input I port load L [address] [drive] [firstsector] [number] move M range address name N [pathname] [arglist] output O port byte proceed P [=address] [number] quit Q register R [register] search S range list trace T [=address] [value] unassemble U [range] write W [address] [drive] [firstsector] [number] allocate expanded memory XA [#pages] deallocate expanded memory XD [handle] map expanded memory pages XM [Lpage] [Ppage] [handle] display expanded memory status XS -q
The "trick" of RCX isn't a trick at all; it's simply DEBUG's way of working nicely without separators when they can be inferred. It's equivalent to R CX, as implied above in the output of the "?" command. Here's the output of DEBUG < DEBUG-IN.TXT when the input file has "R" on the first line, followed by "Q" on the next:
-R AX=0000 BX=0000 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000 DS=1671 ES=1671 SS=1671 CS=1671 IP=0100 NV UP EI PL NZ NA PO NC 1671:0100 65 DB 65 -Q
This is DEBUG's register display. Each of the strings containing an equals sign gives the value of a register. Those ending in "S" are segment registers. Those ending in X are general-purpose registers, CS and IP stand for Code Segment and Instruction Pointer, respectively, and the others are Stack Pointer, Byte Pointer, Source Index, and Destination Index, if memory serves me. They all have their peculiarities.
The general purpose registers can be addressed in instructions in two halves. For example "AH" indicates the higher (first) two nibbles of the AX register, "AL" indicates the lower two nibbles of the same, and similarly for BX, CX, and DX.
The final four pairs of letters on the second line DEBUG output indicate the states of the bits in the flag register.
The last line gives the "current" instruction, located at CS:IP (shown here as 1671:0100), which consists here of the hex digits "65", which, since they don't correspond to any instruction DEBUG recognizes, are interpreted as a "data byte", which is shown "disassembled" as DB 65.
Any of the register abbreviations can be used with the "R" command, for example, RIP (or R IP), RCS, or RAX. "R" entered without a register abbreviation causes DEBUG to issue a colon prompt ":", at which time a new value for the register can be entered. If no input is given (by pressing ENTER, or in a script file, by an empty line), the hyphen prompt returns and no register is changed.
This is all easily discoverable by playing with DEBUG.
Hope it helps, somehow.
