with DOS/4GW? - Open Watcom

DOS Programming Guide
5.4 How do I access the first megabyte in the extended
DOS environment?
Many programmers require access to the first megabyte of memory in order to look at key low
memory addresses. Under DOS, it was standard practice to use a far pointer, with the far
pointer set to the segmented address of the memory area that was being inspected. Under
DOS extenders, this practice is not so standard. Each DOS extender provides its own method
for accessing the first megabyte of memory.
5.4.1 Accessing the First Megabyte under Tenberry Software
DOS/4GW
Under DOS/4GW, the first megabyte of physical memory - the real memory - is mapped as a
shared linear address space. This allows your application to access the first megabyte of
memory using a near pointer set to the linear address. The following program demonstrates
this method. This example is similar to the screen memory access example.
/*
KEYSTAT.C - This example shows how to get the keyboard
status under DOS/4GW by looking at the ROM BIOS
keyboard status byte in low memory.
Compile & Link: wcl386 /l=dos4g keystat
*/
#include
#include
/*
Under DOS, the keyboard status byte has a segmented
address of 0x0040:0x0017. This corresponds to a
linear address of 0x417.
*/
#define LOW AREA 0x417
void main()
{
/* Only need a near pointer in the flat model */
char *ptr;
/* Set pointer to linear address of the first
status byte */
ptr = (char *)LOW AREA;
28 How do I access the first megabyte in the extended DOS environment?