Sample Code to Inline assembly for x86 in Linux

http://www.ibm.com/developerworks/linux/library/l-ia.html is a great introduction to inline assembly code in C. But its code examples are not ready to run. So I write the following code to help me understand the article.

 

#include <stdio.h>
#include <unistd.h>

#define rdtscll(val) \
     __asm__ __volatile__ ("rdtsc" : "=A" (val))

void test_rdtsc() {
  unsigned long long abc;
  abc = 0;
  rdtscll(abc);
  printf("%#llX\n", abc);
}

void test_r_constraint() {
  int x = 10, y;
  asm ("movl %1, %%eax;"
      "movl %%eax, %0;"
      :[output] "=r"(y) /* y is output operand */
      :[input] "r"(x) /* x is input operand */
      :"%eax"); /* %eax is clobbered register */
  printf("x=%d, y=%d\n", x, y);
}

void test_symbolic_name() {
  int x = 10, y;
  asm ("movl %[input], %[output];"
      :[output] "=r"(y) /* y is output operand */
      :[input] "r"(x) /* x is input operand */
      :"%eax"); /* %eax is clobbered register */
  printf("x=%d, y=%d\n", x, y);
}

void test_a() {
  int var = 100;
  asm ("incl %0" :"=a"(var):"0"(var));
  printf("var: %d\n", var);
}

void test_m() {
  int x = 100;
  asm("incl %0\n" : :"m"(x));
  __asm__("incl %0\n" : :"m"(x));
  printf("x=%d\n", x);
}

void test_controlName() {
  static int foo asm("myfoo") = 2;
}

void test_read_write() {
  int foo, bar;
  foo = 1;
  bar = 2;
  asm("movl %1, %%eax;"
      "movl %2, %%ebx;"
      "addl %%ebx, %%eax;"
      "movl %%eax, %0;"
      :"=r" (foo)
      :"0" (foo), "g" (bar)
      :"%eax", "%ebx");
  printf("sum: %d\n", foo);
}

void test_cpuid() {
  unsigned op;
  unsigned _eax;
  unsigned _ebx;
  unsigned _ecx;
  unsigned _edx;
  op = 0;
  asm("cpuid"
      : "=a" (_eax), "=b" (_ebx), "=c" (_ecx), "=d" (_edx)
      : "a" (op));
  printf("eax=%#X, ebx=%#X, ecx=%#X, edx=%#X\n", _eax, _ebx, _ecx, _edx);
}

void asm_strcpy(char *dest, char *src, int count) {
  asm("cld\n\t"
      "rep movsb"
      :
      : "S" (src), "D" (dest), "c" (count));
}

void test_asm_strcpy() {
  char *src = "abc";
  char dest[10];
  asm_strcpy(dest, src, 4);
  printf("dest: %s\n", dest);
}

void test_syscall() {
  int sys_number = 1; /* syscall number for exit */
  int status = 100;
  asm("int $0x80"
      : 
      : "a" (sys_number), "b" (status));
}

void test_asm_memcpy() {
  char *src = "123";
  char dst[4];
  int num = 4;
  asm("movl %[count], %%ecx;\n"
      "up: lodsl;\n\t"
      "stosl;\n\t"
      "loop up;"
      :
      : "S" (src), "D" (dst), [count] "r" (num)
      : "%ecx", "%eax");
  printf("dst: %s\n", dst);
}

int main(void) {
  test_rdtsc();
  test_r_constraint();
  test_symbolic_name();
  test_a();
  test_m();
  test_controlName();
  test_read_write();
  test_asm_strcpy();
  test_cpuid();
  test_syscall();
  test_asm_memcpy();
}