![]() |
|
![]() |
|
|
Thread Tools | Display Modes |
|
|
#1 |
|
Newbie
Join Date: Jun 2005
Posts: 18
Rep Power: 0
![]() |
Access variable within inline asm
Is there a way to access a local variable via inline assembly? I'm working on a Solaris machine with g++. What I'd like to do is this:
long myWrapper(long *value1, long value2)
{
int tmp;
asm(" ... "
"st %l7 tmp"
" ... ");
return tmp;
}So basically I'm going through this assembly algorithm, and the value I want to return from the function is in a particular register in the middle of the algorithm. Any thoughts? Thanks in advance. |
|
|
|
|
|
#2 |
|
Resident Grouch
![]() ![]() ![]() ![]() ![]() ![]() Join Date: Jun 2005
Posts: 6,453
Rep Power: 10
![]() |
This sort of thing will work:
#include <iostream>
using namespace std;
int yankAsmTooth ();
int main (int argc, char *argv [])
{
int regVal = yankAsmTooth ();
cout << regVal;
}
int yankAsmTooth ()
{
int a;
__asm
{
mov dword ptr [a],0Ah
}
return a;
}
__________________
Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers |
|
|
|
|
|
#3 |
|
Newbie
Join Date: Jun 2005
Posts: 18
Rep Power: 0
![]() |
Thanks DaWei. This is for SPARC assembly, so the syntax is different and I'm not sure I can reference variable names within inline asm with my compiler.
But I found a similar solution: register int tmp asm("l2");
asm(" ... "
"mov %l0, %l2"
" ... ");
return tmp;I guess this allocates my local variable to the contents of register %l2, which is a local register for SPARC. I can modify that register in the asm block and the change is made to the variable as well. ![]() |
|
|
|
|
|
#4 |
|
Resident Grouch
![]() ![]() ![]() ![]() ![]() ![]() Join Date: Jun 2005
Posts: 6,453
Rep Power: 10
![]() |
Whatever woiks, see my sig
. I judge you'd be interested in exploring the means by which your compiler passes values to, and returns values from, functions. It comes in handy in mixed-language programming. I can't really discern your level of familiarity from the posts thus far; if you'd like some material on a generic (but fairly common), stack-based method, I can probably dig something up.
__________________
Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers |
|
|
|
|
|
#5 |
|
Newbie
Join Date: Jun 2005
Posts: 18
Rep Power: 0
![]() |
Certainly. I've read a little bit about "function preludes/prologues" with regard to pushing parameters onto the stack, etc. Don't recall much about return values, other than maybe having dedicated registers (i.e., eax on Intel?) for return values...?
But anyways, yeah I'm always interested in building up the bookmark list with some good docs/tutorials for rainy days, if you happen to have anything around... Thanks. |
|
|
|
|
|
#6 |
|
Resident Grouch
![]() ![]() ![]() ![]() ![]() ![]() Join Date: Jun 2005
Posts: 6,453
Rep Power: 10
![]() |
Nothing so formal. If you've read that, it's probably old news. You'd be surprised, though, how many people don't really understand how parameters are passed or WHY local values disappear when a function returns. Of course, there's nothing in the language standards specifiying that a stack be used, but it's a very common implementation. Some stacks grow downwards, some upwards, some move the pointer before the storage, some after, and some micros don't have a stack at all.
You are correct about the return in the case of the x86 C/C++ platform. Some languages allow multiple returns of any type object, so those are usually stack-based, also. Anyway, here it is, maybe someone can get some insight from it. I hope it reproduces the spacing properly. GENERIC STACK-USAGE METHOD
Most microprocessors have an internal pointer (the stack pointer) which
references memory so that the micro can keep track of the point of execution
as it varies because of interrupts, function calls, and so forth. The stack
(memory to which it points) is also used by many systems (sometimes
unfortunately) as a storage place for local values, saved registers, and so
forth.
Just prior to a call, the stack pointer, which is much like any pointer one
defines, is pointing to some place in memory (designated by the programmer
or the operating system) for its use. When you call a function, it works
something like this (its usage varies somewhat from language to language --
even within one language).
stack pointer -->| orig position | In most systems, the stack pointer moves toward
| | lower addresses as you use it.
~ ~
At call: | orig position |
| arguments |
stack pointer -->| last argument |
| |
| |
| |
| |
~ ~
There may be zero or more arguments. They are pushed onto the stack in a
predetermined order. For C/C++, it is right-to-left. The stack pointer moves
with each push.
After call: | orig position |
| argument here |
| (maybe more)
stack pointer -->| ret addr here |
| |
| |
| |
~ ~
Into procedure | orig position |
Arguments avail | arguments... | When you modify the argument(s), you modify
for use | ret addr here | the value(s) stored here. If an argument
| saved regs, | is a reference or pointer you may use it to
| locals, etc. | modify the value pointed to elsewhere
| in this area | (in the calling procedure, say). If you write
stack pointer -->| | more data to one of the local variables than it
~ ~ can store, guess where the excess winds up.
The function does its work and unwinds the stack (locals, etc.)
Before return | orig position | Immediately before the return, after storage
| arguments... | for saved registers, locals, etc. has already
stack pointer -->| ret addr here | been recovered (and disappeared). The arguments
| | are still on the stack.
| |
| |
~ ~
After return: | orig position | Immediately after the return. The very first
stack pointer -->| arguments... | thing the machine is going to do next is destroy
| | the arguments, whether you've modified them or not.
| | Sometimes the arguments are removed by the called
| | function and the return address position adjusted
| | appropriately.
~ ~
stack pointer -->| orig position | And it's done; you are right back where you started,
| | bookkeeping wise, when you made the call. Any
| | changes you made to the arguments are history, for
~ ~ all practical purposes (they may persist until the
next stack operation).
If you passed an argument as a reference, any modifications you made to the
value it referred to are, of course, in force. If you modified the reference,
itself, to point to something else, you could modify that something else, also
(for example, subsequent bytes pointed to by a char *). The reference itself
disappears. If you pass an argument by value, that value is perfectly usable
to the called procedure; it could specify a length to use for some operation,
for example. If you modify the value, such modifications disappear when the
arguments disappear, immediately after the called procedure returns to the
caller. If you want lasting changes in the caller, you need to make them by
reference or RETURN a value from the called procedure.
__________________
Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers |
|
|
|
![]() |
| Bookmarks |
| Currently Active Users Viewing This Thread: 1 (0 members and 1 guests) | |
| Thread Tools | |
| Display Modes | |
|
|