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

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358 Appendix E. The GDB Agent Expression Mechanism
If attempting to access memory at
addr
would cause a processor exception of some sort, termi-
nate with an error.
ref_float
(0x1b):
addr
=>
d
ref_double
(0x1c):
addr
=>
d
ref_long_double
(0x1d):
addr
=>
d
l_to_d
(0x1e):
a
=>
d
d_to_l
(0x1f):
d
=>
a
Not implemented yet.
dup
(0x28):
a
=
i
a a
Push another copy of the stack’s top element.
swap
(0x2b):
a b
=
i
b a
Exchange the top two items on the stack.
pop
(0x29):
a
=
i
Discard the top value on the stack.
if_goto
(0x20)
offset
:
a
=>
Pop an integer off the stack; if it is non-zero, branch to the given offset in the bytecode string.
Otherwise, continue to the next instruction in the bytecode stream. In other words, if
a
is non-
zero, set the
pc
register to
start
+
offset
. Thus, an offset of zero denotes the beginning of the
expression.
The
offset
is stored as a sixteen-bit unsigned value, stored immediately following the
if_goto
bytecode. It is always stored most significant byte first, regardless of the target’s normal endian-
ness. The offset is not guaranteed to fall at any particular alignment within the bytecode stream;
thus, on machines where fetching a 16-bit on an unaligned address raises an exception, you
should fetch the offset one byte at a time.
goto
(0x21)
offset
: =>
Branch unconditionally to
offset
; in other words, set the
pc
register to
start
+
offset
.
The offset is stored in the same way as for the
if_goto
bytecode.
const8
(0x22)
n
: =>
n
const16
(0x23)
n
: =>
n
const32
(0x24)
n
: =>
n
const64
(0x25)
n
: =>
n
Push the integer constant
n
on the stack, without sign extension. To produce a small negative
value, push a small twos-complement value, and then sign-extend it using the
ext
bytecode.
The constant
n
is stored in the appropriate number of bytes following the
constb
bytecode.
The constant
n
is always stored most significant byte first, regardless of the target’s normal en-
dianness. The constant is not guaranteed to fall at any particular alignment within the bytecode
stream; thus, on machines where fetching a 16-bit on an unaligned address raises an exception,
you should fetch
n
one byte at a time.
reg
(0x26)
n
: =>
a
Push the value of register number
n
, without sign extension. The registers are numbered follow-
ing GDB’s conventions.
The register number
n
is encoded as a 16-bit unsigned integer immediately following the
reg
bytecode. It is always stored most significant byte first, regardless of the target’s normal endian-