How can I perform 64-bit division with a 32-bit divide instruction?

Question

This is (AFAIK) a specific question within this general topic.

Here\'s the situation:

I have an embedded system (a video game console) based on a 32-bit RISC

Answer 1

If your dividend is unsigned 64 bits, your divisor is unsigned 32 bits, the architecture is i386 (x86), the div assembly instruction can help you with some preparation:

#include <stdint.h>
/* Returns *a % b, and sets *a = *a_old / b; */
uint32_t UInt64DivAndGetMod(uint64_t *a, uint32_t b) {
#ifdef __i386__  /* u64 / u32 division with little i386 machine code. */
  uint32_t upper = ((uint32_t*)a)[1], r;
  ((uint32_t*)a)[1] = 0;
  if (upper >= b) {   
    ((uint32_t*)a)[1] = upper / b;
    upper %= b;
  }
  __asm__("divl %2" : "=a" (((uint32_t*)a)[0]), "=d" (r) :
      "rm" (b), "0" (((uint32_t*)a)[0]), "1" (upper));
  return r;
#else
  const uint64_t q = *a / b;  /* Calls __udivdi3 in libgcc. */
  const uint32_t r = *a - b * q;  /* `r = *a % b' would use __umoddi3. */
  *a = q;
  return r;
#endif
}

If the line above with __udivdi3 doesn't compile for you, use the __div64_32 function from the Linux kernel: https://github.com/torvalds/linux/blob/master/lib/div64.c

Answer 2

GCC has such a routine for many processors, named _divdi3 (usually implemented using a common divmod call). Here's one. Some Unix kernels have an implementation too, e.g. FreeBSD.