Inline Assembly Language in C

Whexy /
November 14, 2020

Writing assembly code is hard and boring! However, if you want to set regisiters, read memories, sometimes you must do the "dirty work".

Luckily we have GCC's help. GCC provides a keyword asm, which allows you to embed assembler instructions within C code. C language definitely saves our life.

Basic Asm

A basic asm statement has the following syntax:

asm asm-qualifiers ( Assembler Instructions );

For example, in PMU settings , we want to read the system register PMCR_EL0, to make sure whether the PMU is set up successfully. The corresponding assembler instruction is MRS x0, PMCR_EL0. In the C code, we can write:

asm ("mrs x0, PMCR_EL0");

Notice that the asm-qualifiers are omitted. All basic asm blocks are implicitly "volatile".

You should be warned that GCC doesn't parse the assembler instructions and have no idea about what they mean or even whether they are valid or not.

Extended Asm

Basic asm knows nothing about the C syntax around. For example, if you want to put a integer stored in C variable int_a into a register, you cannot use basic asm.

With extended asm you can read and write C variables from assembler and perform jumps from assembler codes to C labels. The extended asm has the following syntax:

asm qualifiers (AssemblerTemplate
                 : output_constraint (C lvalue)
                 : input_constraint (C expression)
                 : Clobbers)

Some assembler instructions will have side-effect, and we should explicitly use qualifier volatile to tell GCC don't optimize our code (yes, GCC is really smart and sometimes we need to tell it try not be so smart).


Constraint is kind of confusing. We only need to know one common constraint we are likely to use when configuring Ninja.

The constraint is r marks the related calculation result can be stored in a general register, often used in input constraint. On the other hand, =r means the same thing, with write-only register, often used in output constraint.

For example, the following code change the value in system register.

asm volatile("msr pmintenset_el1, %0" : : "r" ((u64)(0 << 31)));

It marks the C expression (u64) (0<<31) to be stored in any general register. While running, GCC will assign a general register to fill the %0 in the assembler template, and fill it with the calculation result of the expression.

Another example of print the value stored in system register.

long long f;
asm("mrs %0, PMCR_EL0" : "=r" (f));
printk(KERN_INFO "PMCR_EL0 = %llu\n", f);

It marks the C variable (long long) f to be handled as any general register. While running, GCC will assign a general register to fill the %0 in the assembler template, and bind it with variable f.

Clobber List

Extended asm is designed to have a C expression as input and have a C left value written with the output. But sometimes our assembler code have side effects, like we will change the stored value in another register.

Unfortunately, GCC don't know the effect, since it cannot understand assembler instuctions (still remember GCC is designed to understand C code?). So we must explicitly list the registers that is affected, that's why we have clobber list.

Here's an example

asm( "movl %0,%%eax;\n\tmovl %1,%%ecx;\n\tcall _foo"
        : /*no outputs*/
        : "g" (from), "g" (to)
        : "eax", "ecx"

That is an x86 example, and feel no worry that you cannot understand because so am I. The only thing we care is that we mark eax and ecx as clobber, so GCC won't depend on these two register to maintain other things.