gdb.info: Registers

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Registers

You can refer to machine register contents, in expressions, as variables
with names starting with `$'.  The names of registers are different for
each machine; use `info registers' to see the names used on your
machine.
`info registers'
     Print the names and values of all registers except floating-point
     and vector registers (in the selected stack frame).
`info all-registers'
     Print the names and values of all registers, including
     floating-point and vector registers (in the selected stack frame).
`info registers REGNAME ...'
     Print the "relativized" value of each specified register REGNAME.
     As discussed in detail below, register values are normally
     relative to the selected stack frame.  REGNAME may be any register
     name valid on the machine you are using, with or without the
     initial `$'.
   GDB has four "standard" register names that are available (in
expressions) on most machines--whenever they do not conflict with an
architecture's canonical mnemonics for registers.  The register names
`$pc' and `$sp' are used for the program counter register and the stack
pointer.  `$fp' is used for a register that contains a pointer to the
current stack frame, and `$ps' is used for a register that contains the
processor status.  For example, you could print the program counter in
hex with

p/x $pc

or print the instruction to be executed next with

x/i $pc

or add four to the stack pointer(1) with

set $sp += 4

   Whenever possible, these four standard register names are available
on your machine even though the machine has different canonical
mnemonics, so long as there is no conflict.  The `info registers'
command shows the canonical names.  For example, on the SPARC, `info
registers' displays the processor status register as `$psr' but you can
also refer to it as `$ps'; and on x86-based machines `$ps' is an alias
for the EFLAGS register.
   GDB always considers the contents of an ordinary register as an
integer when the register is examined in this way.  Some machines have
special registers which can hold nothing but floating point; these
registers are considered to have floating point values.  There is no way
to refer to the contents of an ordinary register as floating point value
(although you can _print_ it as a floating point value with `print/f
$REGNAME').
   Some registers have distinct "raw" and "virtual" data formats.  This
means that the data format in which the register contents are saved by
the operating system is not the same one that your program normally
sees.  For example, the registers of the 68881 floating point
coprocessor are always saved in "extended" (raw) format, but all C
programs expect to work with "double" (virtual) format.  In such cases,
GDB normally works with the virtual format only (the format that makes
sense for your program), but the `info registers' command prints the
data in both formats.
   Normally, register values are relative to the selected stack frame
(*note Selecting a frame: Selection.).  This means that you get the
value that the register would contain if all stack frames farther in
were exited and their saved registers restored.  In order to see the
true contents of hardware registers, you must select the innermost
frame (with `frame 0').
   However, GDB must deduce where registers are saved, from the machine
code generated by your compiler.  If some registers are not saved, or if
GDB is unable to locate the saved registers, the selected stack frame
makes no difference.

---------- Footnotes ----------

   (1) This is a way of removing one word from the stack, on machines
where stacks grow downward in memory (most machines, nowadays).  This
assumes that the innermost stack frame is selected; setting `$sp' is
not allowed when other stack frames are selected.  To pop entire frames
off the stack, regardless of machine architecture, use `return'; see
*Note Returning from a function: Returning.