Red Hat ENTERPRISE LINUX 4 - DEVELOPER TOOLS GUIDE, Manual

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Appendix E. The GDB Agent Expression Mechanism 355
reg 1
Push the value of register 1 (presumably holding
x
) onto the stack.
reg 2
Push the value of register 2 (holding
y
).
const32
address of z
Push the address of
z
onto the stack.
ref32
Fetch a 32-bit word from the address at the top of the stack; replace the address on the stack with
the value. Thus, we replace the address of
z
with
z
’s value.
ext 32
Sign-extend the value on the top of the stack from 32 bits to full length. This is necessary because
z
is a signed integer.
mul
Pop the top two numbers on the stack, multiply them, and push their product. Now the top of the
stack contains the value of the expression
y * z
.
add
Pop the top two numbers, add them, and push the sum. Now the top of the stack contains the
value of
x + y * z
.
end
Stop executing; the value left on the stack top is the value to be recorded.
E.2. Bytecode Descriptions
Each bytecode description has the following form:
add
(0x02):
a b
=>
a+b
Pop the top two stack items,
a
and
b
, as integers; push their sum, as an integer.
In this example,
add
is the name of the bytecode, and
(0x02)
is the one-byte value used to encode
the bytecode, in hexidecimal. The phrase "
a b
=>
a+b
" shows the stack before and after the bytecode
executes. Beforehand, the stack must contain at least two values,
a
and
b
; since the top of the stack
is to the right,
b
is on the top of the stack, and
a
is underneath it. After execution, the bytecode will
have popped
a
and
b
from the stack, and replaced them with a single value,
a+b
. There may be other
values on the stack below those shown, but the bytecode affects only those shown.
Here is another example:
const8
(0x22)
n
: =>
n
Push the 8-bit integer constant
n
on the stack, without sign extension.
In this example, the bytecode
const8
takes an operand
n
directly from the bytecode stream; the
operand follows the
const8
bytecode itself. We write any such operands immediately after the name
of the bytecode, before the colon, and describe the exact encoding of the operand in the bytecode
stream in the body of the bytecode description.