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
info registers regname ...
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
$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
or print the instruction to be executed next with
or add four to the stack pointer(3) 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,
registers displays the processor status register as
$psr but you
can also refer to it as
$ps; and on x86-based machines
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 (see section Selecting a frame). 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.
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