Using DOSmemory under the Go32 extender

Because Free Pascalfor DOSis a 32 bit compiler, and uses a DOSextender, accessing DOS memory isn’t trivial. What follows is an attempt to an explanation of how to access and use DOSor real mode memory⁸.

In Protected Mode, memory is accessed through Selectors and Offsets. You can think of Selectors as the protected mode equivalents of segments.

In Free Pascal, a pointer is an offset into the DS selector, which points to the Data of your program.

To access the (real mode) DOSmemory, somehow you need a selector that points to the DOSmemory. The go32 unit provides you with such a selector: The DosMemSelector variable, as it is conveniently called.

You can also allocate memory in DOS’s memory space, using the global_dos_alloc function of the go32 unit. This function will allocate memory in a place where DOSsees it.

As an example, here is a function that returns memory in real mode DOSand returns a selector:offset pair for it.

procedure dosalloc(var selector : word;
                   var segment : word;
                   size : longint);

var result : longint;

begin
     result := global_dos_alloc(size);
     selector := word(result);
     segment := word(result shr 16);
end;

(You need to free this memory using the global_dos_free function.)

You can access any place in memory using a selector. You can get a selector using the function:

function allocate_ldt_descriptors(count : word) : word;

and then let this selector point to the physical memory you want using the function

function set_segment_base_address(d : word;s : longint) : boolean;

Its length can be set using the function:

function set_segment_limit(d : word;s : longint) : boolean;

You can manipulate the memory pointed to by the selector using the functions of the GO32 unit. For instance with the seg_fillchar function. After using the selector, you must free it again using the function:

function free_ldt_descriptor(d : word) : boolean;

More information on all this can be found in the Unit Reference, the chapter on the go32 unit.