Red Hat ENTERPRISE LINUX 4 - DEVELOPER TOOLS GUIDE, Руководство пользователя

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80 Chapter 10. Examining Data
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 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 (refer to Section 8.3 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 com-
piler. If some registers are not saved, or if gdb is unable to locate the saved registers, the selected stack
frame makes no difference.
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
; (refer to Section 16.4 Returning from a function .