Borland Turbo C++ 3.0 32-bit I/O port read and write functions

IO32.CPP

#include <dos.h>
#include <stdio.h>

// outport32(): Performs a 32-bit OUT I/O instruction in Borland Turbo C++.
// This corresponds to "out dx, eax" command in 386 protected mode assembly.
// port: 16-bit I/O port address to write to.
// value: a 32-bit value to write.
void outport32(unsigned int port, unsigned long value) {
  // Problem: Borland Turbo C++ is a 16-bit real-mode compiler, so does
  // not have a 32-bit outport() C API, and its assembler does not
  // recognize 32-bit EAX register or 32-bit instructions.

  // However x86 assembly allows a single-byte instruction prefix 66h
  // to be prepended to many instructions to make these instructions
  // perform a wider 32-bit operation instead.
  asm {
    db 0x66
    mov ax, word ptr value  // Actually 'mov eax, dword ptr value'

    mov dx, port            // Load dx with the 16-bit port to write to

    db 0x66
    out dx, ax              // Actually 'out dx, eax'
  }
}

// inport32(): Performs a 32-bit IN I/O instruction in Borland Turbo C++.
// This corresponds to "in eax, dx" command in 386 protected mode assembly.
// port: the 16-bit port address to read from.
// return: the 32-bit value read from the port.
unsigned long inport32(unsigned int port) {
  unsigned long result;
  asm {
    mov dx, port            // Load dx with the 16-bit port to read from

    db 0x66
    in ax, dx               // Actually 'in eax, dx'

    db 0x66
    mov word ptr result, ax // Actually 'mov dword ptr result, eax'
  }
  return result;
}

int main()
{
  // Example code to enable a bit in Rendition Verite V2200 PCI I/O space.
  printf("Before: 0x%08lX\n", inport32(0x60C0));
  outport32(0x60C0, inport32(0x60C0) | (1ul << 19));
  printf("After: 0x%08lX\n", inport32(0x60C0));
  return 0;
}