15.6. Example: Operating System Usage

The following code segment illustrates the algorithm to be used when calling the Query System Address Map function. This is an implementation example and uses non-standard mechanisms:

E820Present = FALSE;
Reg.ebx = 0;
do {
  Reg.eax = 0xE820;
  Reg.es = SEGMENT (&Descriptor);
  Reg.di = OFFSET (&Descriptor);
  Reg.ecx = sizeof (Descriptor);
  Reg.edx = 'SMAP';

   \_int( 15, regs );

  if ((Regs.eflags & EFLAG_CARRY) \|\| Regs.eax != 'SMAP') {
    break;
  }

  if (Regs.ecx < 20 \|\| reg.ecx > sizeof (Descriptor) ) {
    // bug in bios - all returned descriptors must be
    // at least 20 bytes long, and cannot be larger then
    // the input buffer.
    break;
  }

  E820Present = TRUE;
  .
  .
  .
  Add address range Descriptor.BaseAddress through
  Descriptor.BaseAddress + Descriptor.Length
  as type Descriptor.Type
  .
  .
  .

} while (Regs.ebx != 0);

if (!E820Present) {
  .
  .
  .
  call INT-15 88 and/or INT-15 E801 to obtain old style memory information
  .
  .
  .
}