I've been looking through questions on here and the internet for a while now and I cannot seem to find out whether or not it is possible to do inline assembly with GCC using something other than GAS. I am trying to find if I can avoid using not only GAS's AT&T syntax (though, I know how to use Intel syntax with GAS) but the extended asm format. While this is not for a project or anything other than my own curiosity, I would really appreciate any help I can get (this is actually my first question here because I could not find an answer about it)! Also, if this makes any difference, I'm currently using DevC++ (for C code, not C++) on Windows.
Thanks,
Tom
You can link the output from an assembler (a ".o" or ".obj" file) with your C or C++ program. Put your assembler code in a text file. Your IDE or makefile will assemble it just as it would any c source file. The only tricky bit is learning how to interface between the two different systems.
You cannot use another inline assembly syntax with GCC. inline assembly is implemented by GCC literally including the assembly you write inline with its own (textual) assembly output, which it then sends to gas to be assembled. Since GCC doesn't know how to change the format of its own output to feed to another assembler, you can't change the inline assembly, either.
Related
I'm learning assembly for 80x86 this semester. A typical asm file I write looks something like
.model tiny
.486
.data
#initializations
.code
.startup
#actual code
.exit
end
I was expecting a similar format when I created a .s file for a simple hello world. But I don't see any of the segments with their proper names and it's all very different. I compile using g++ -S -O0 hello.c
Why is the assembly for c so different than the assembly they make us write in class? Is the assembly I'm learning used by a different programming language? If I want to get the assembly version (that I'm used to) of hello world from some higher-level code, how do I do that?
The code does not match your command line. That is neither C (file name) nor C++ code (command line). That is assembly language.
Assembly language varies by tool (masm, tasm, nasm, gas, etc), and is not expected to be compatible nor standard in any way. Not talking about just intel vs at&t, all of the code, and this applies to all targets not just x86, easily seen with ARM and others.
You should try to use the assembler not a C nor C++ compiler as that creates yet another assembly language even though gcc for example will pass the assembly language on to gas it can pre-process it through the C preprocessor creating yet another programming language that is incompatible with the gnu assembler it is fed to.
x86 is the last if ever assembly language/instruction set you want to learn, if you are going to learn it then starting with the 8086/88 is IMO the preferred way, much more understandable despite the nuances. Since this appears to be a class you are stuck with this ISA and cannot chose a better first instruction set. (first, second, third...)
Very much within the x86 world, but also for any other target, expect that the language is incompatible between tools and if it happens to work or mostly work that is a bonus. Likewise there is no reason to assume that any tool will have a "masm compatible" or other mode, simply stating intel vs at&t is only a fraction of the language problem and is in no way expected to make the code port between tools.
Re-write the code for the assembly language used for the assembler is the bottom line.
I want to write some inline assembly in a DOS program which is compiled using Turbo C 2.01. When I write
asm {
nop
}
the compiler claims that in-line assembly is not allowed in function .... See:
Any ideas?
See the Turbo C user manual page 430:
Inline assembly not allowed
Your source file contains inline assembly language statements and you are compiling it from within the
Integrated Environment. You must use the TCC command to compile this
source file.
I believe that you need also to pass the -B option to TCC (page 455).
Alternatively you can use __emit__ (page 103) for relatively simple code entered as machine code rather than assembler mnemonics.
It seems an odd restriction to not allow inline assembly in the IDE. You might consider "upgrading" to Turbo C++ 3.0 which I believe does allow it. I would imagine that TC++ will compile C code when presented with a .c file, or that the IDE can be set to compile C explicitly. There's a manual for that too.
Turbo C converts C code directly into machine code without using an assembler phase, and thus cannot include assembly language source within a program. What it can do, however, is use the __emit directive to insert machine code. The cleanest way to use that is probably to use a separate assembler (or perhaps DEBUG) to process the code of interest by itself into a COM file, and then enter the byte values therein into an __emit directive. Parameters are stored in ascending order left to right, starting at either BP+4 (in tiny, small, or compact model) or BP+6 (medium, large, or huge). Local variables are stored at addresses below BP.
When using Turbo Pascal, it's possible to use a handy program called "inline assembler" to convert assembly-language source into a Turbo Pascal literal-code directive. Turbo Pascal's directive is formatted differently from C's (I like Pascal's better) and can accommodate labels in ways Turbo C's cannot. Still, using __emit may have far less impact on build times than trying to use inline assembly code.
Recently I was told to look at how C functions are compiled into LLVM bytecode, and then how the LLVM bytecode is translated into x86 ASM. As a regular GNU/gcc user, I have some questions about this. To put it mildly.
Does GNU/gcc compile to bytecode, too? Can it? I was under the impression that gcc compiles directly into ASM. If not, is there a way to view the bytecode intermediary as there is with the clang command?
~$ clang ~/prog_name.c -S -emit-llvm -o - <== will show bytecode for prog_name.c.
Also, I find bytecode to be rather byzantine. By contrast, it makes assembly language seem like light reading. In other words: I have little idea what it is saying.
Does anyone have any advice or references for vaguely deciphering the information that the bytecode gives? Currently I compare and contrast with actual ASM, so to say it is slow going is a compliment.
Perhaps this is all comically naive, but I find it quite challenging to break through the surface of this.
Perhaps try taking a look at the language reference.
As far as I know, GCC does have an IR as well known as GIMPLE (another reference here).
If you mean that you would rather analyze the assembly output instead of the IR, you can take a look at this question which describes how to output an assembly file.
I'm writing some methods in C that must run very efficiently, so I'd like to manually edit some of the assembly code that is automatically generated.
I know how to read the assembly code using gdb or "objdump -d myfile.o", but can anyone tell me how to edit this code? (Will probably just be minor tweaks.)
gcc have -S switch, which stops compilation on after assembly generation phase. Then you cound edit resulting file and manually call assembly (with gas, for example)
Sure. It's called "inline assembly", and most compilers support it.
Here's an example using GCC:
http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html
There's no way you can do it.
Compiler generates code directly, there is no assembler involved; assembly listing is an aux product that can be generated, can be not.
You can take asm listing and assemble it yourself, however.
If you want to improve efficiency of your code, the best way to follow is, as stated before, inline asm; or write an assembly proc, make an object file and link it.
I am looking at lots of assembly language code that is compiled along with c. They are using simple #define assembly without any headers in boot.s code. How does this work ?
Typically .s files are processed by an assembler. Without knowing any other details, there's nothing more to say. .s file goes in, .o file comes out.
Many assemblers provide some kind of include directive to allow use of headers, which would also be in assembly language.
Ah, the code you linked is for use by the GNU as assembler. If you're on Linux or Mac, do man as to learn about it. If you're on Windows, install MinGW or Cygwin.
Compilers can frequently include in-line assembly, but I believe it is compiler specific.
I don't remember the precise details, but I think its something like:
void myFunc(void)
{
int myNum; /* plain old C */
__asm /* Assembly */
{
mov ax,bx;
xor cx,cx;
}
myNum = 5; /* more C */
}
Research your specific compiler for details.
The link you post in your comment is an assembly language source file that is meant to be first run through a c-preprocessor. It's just a programming convenience, but lots of assembly language compilers support similar constructs anyway, so I'm not sure why they went the c-preprocessor route.
If you have "main proc" inside of your code, you are using x86 architecture and your file ends with .asm you con use for compilation:
tasm fileName.asm
In result you will get your fileName.obj file. After that you need to link it and for
that you can use tlink filename.obj
To run, just enter the filename.exe on the command line
If you need to link more than one file use tlink filename1.obj filename2.obj and so on
during the compilation and linking is not necessary to specify the file extension like .obj or .asm. Using just filename should be fine.