Debugging with GDB (February 2008)

Chapter 8: Examining Data 77
8.10 Registers
You can refer to machine register contents, in expressions, as variables with names
starting with ‘$’. The names of registers are different for each machine. Use info registers
to view the names used on your machine.
info registers
Print the names and values of all registers except floating-point registers (in
the selected stack frame).
info all-registers
Print the names and values of all registers, including floating-point registers.
info registers regname ...
Print the relativized value of each specified register regname. As discussed in
detail below, register values are normally relative to the selected stack frame.
regname may be any register name valid on the machine you are using, with or
without the initial ‘$’.
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
2
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
2
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 Section 11.4 [Returning from a function], page 99.