18.04.2020

Dev C++ Inline Assembly Tutorial

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How can i get inline assembly to work in dev-cpp in can get it to work in msvcpp but not in dev-cpp. Any help would. C Tutorial; 5 ways you can learn to program. Our main focus is to study the basic format and usage of (GCC) inline assembly functions. To declare inline assembly functions, we use the keyword asm. Inline assembly is important primarily because of its ability to operate and make its output visible on C variables. Because of this capability, 'asm' works as an interface between the assembly.

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The idea behind Inline Assembly is to embed assembler instructions in your C/C code, using the asm keyword, when there's no option but to use Assembly language. First, the authors describe basic usage syntax for inline assembly (inline asm) embedded within C and C programs. Then they explain intermediate concepts, such as addressing modes, the clobbers list, and branching stanzas, as well as more advanced topics, such as memory clobbers, the volatile attribute, and locks are discussed for those who want to use inline asm in multithreaded applications.

Microsoft Specific

Assembly language serves many purposes, such as improving program speed, reducing memory needs, and controlling hardware. You can use the inline assembler to embed assembly-language instructions directly in your C and C++ source programs without extra assembly and link steps. The inline assembler is built into the compiler, so you don't need a separate assembler such as the Microsoft Macro Assembler (MASM).

Note

Programs with inline assembler code are not fully portable to other hardware platforms. If you are designing for portability, avoid using inline assembler.

Dev c inline assembly tutorial video

Inline assembly is not supported on the ARM and x64 processors. The following topics explain how to use the Visual C/C++ inline assembler with x86 processors:

END Microsoft Specific

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Dev C Inline Assembly Tutorial For Kids

See also

Dev C++ Inline Assembly Tutorial Pdf

Compiler Intrinsics and Assembly Language
C++ Language Reference

Dev C++ Inline Assembly Tutorial 1

Documentation‎ > ‎Tutorials‎ > ‎

Inline ASM Basics


Please Note: this page is under development. Comments (via the Parallax forums) are welcome.
Introduction
Often times you want your code to run faster or with deterministic behavior. You can take control of exactly what the processor is doing by writing in the language of the machine: assembly.
This page gives an introduction of writing inline assembly for the Parallax Propeller with PropGCC. You should have some familiarity with PropellerGCC and Propeller assembly (PASM).
Basic Example

Here, we will make a function that measures the length of a C string character array. Here is the C version:
int strlenC(char * string){
int i = 0;
int t1;
//C version:
for(; string[i] != '0'; i++){
}
return i;
}
Here we have a simple C/C++ function. What we want to do is compute the length of the string. We do this by counting characters. You can see we declare the counter and a temporary variable (unused for now). We then return i for our result.
And here is the equivalent PASM version:
int strlenASM(char * string){
int i = 0;
int t1;
__asm__ (
'strloop: '
'rdbyte %[t1], %[string]nt'
'cmp %[t1], #0 wznt'
'if_nz add %[i], #1nt'
'if_nz add %[string], #1nt'
'if_nz brs #strloop'
: /*outputs (+inputs) */
[i] '+r' (i)
: /*inputs */
[string] 'r' (string),
[t1] 'r' (t1)
);
return i;
}
Lets break this down, starting with the C/C++ wrapper:
int strlenASM(char * string){
int i = 0;
int t1;
__asm__ (
..
);
return i;
}
First, we create our assembly section. This is denoted with __asm__, a volatile (optional, if you don't want the code messed with), and the () pair. At the end of each inline PASM block we must indicate the inputs and outputs.

For each __asm__ block, you need to tell the compiler the inputs and outputs. These are relative to the C/C++ code around it: it's not inputs or outputs that deal with hardware. The format is:
int strlenASM(char * string){
int i = 0;
int t1;
__asm__ (
..
: /*outputs (+inputs) */
[i] '+r' (i)
: /*inputs */
[string] 'r' (string),
[t1] 'r' (t1)
);
return i;
}
If you have an input that is also an output, then you indicate that by '+r'. If it's only an output, you use '=r'. Take a look at the GCC documentation for more information on modifiers (link).
Each of the input/output lines shares a similar format. The format is:
[<PASM name>] '<constraints>' (<C/C++ name>)
The PASM name is whatever you want, and does not have to match the C/C++ name. The constraints give the compiler information on the variable (more information here). You can have multiple inputs or outputs by separating each with a comma.
Next, we need to add the PASM code:
int strlenASM(char * string){
int i = 0;
int t1;
__asm__ (
'strloop: '
'rdbyte %[t1], %[string]nt'
'cmp %[t1], #0 wznt'
'if_nz add %[i], #1nt'
'if_nz add %[string], #1nt'
'if_nz brs #strloop'
: /*outputs (+inputs) */
[i] '+r' (i)
: /*inputs */
[string] 'r' (string),
[t1] 'r' (t1)
);
return i;
}
Here we have a few things:
1. Each PASM variable that we use must be enclosed in %[..]. Remember that this is just an alias for our C/C++ variables!
2. We branch to a label using the brs pseudo instruction for short jumps. We can use brw for absolute (long) jumps.
3. Each line with an instruction must end with the nt characters.
The final function:
int strlenASM(char * string){
int i = 0;
int t1;
__asm__ (
'strloop: '
'rdbyte %[t1], %[string]nt'
'cmp %[t1], #0 wznt'
'if_nz add %[i], #1nt'
'if_nz add %[string], #1nt'
'if_nz brs #strloop'
: /*outputs (+inputs) */
[i] '+r' (i)
: /*inputs */
[string] 'r' (string),
[t1] 'r' (t1)
);
return i;
}

So, now that we have our function, how does it stack up in execution speed? Not very well..
The example, compiled using -mcmm and -Os, to count 'Hello' we have:
strlen 3808 cycles

So, it's a bit slower with our handwritten ASM method. The moral of this exercise is that the GCC compiler is very good at generating extremely efficient code. You should carefully consider whether or not to hand write an assembly section of code, and test afterwards to ensure that your new version is actually faster.
References
For a good reference on using C/C++ variables ('operands') with inline assembly, see this page: http://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html
For a good introduction to inline ASM (x86 biased), look here: http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html
For more documentation on the Propeller specific features of the assembler, look under the Propeller section here: https://propgcc.googlecode.com/files/as.pdf
For complete documentation on constraints, take a look here: http://gcc.gnu.org/onlinedocs/gcc/Constraints.html