Function allocate_ldt_descriptors (count : Word) : Word;
Allocates a number of new descriptors.
Parameters:
Return value: Base selector
Notes: The descriptors allocated must be initialized by the application with other function calls. This function returns descriptors with a limit and size value set to zero. If more than one descriptor was requested, the function returns a base selector referencing the first of a contiguous array of descriptors. The selector values for subsequent descriptors in the array can be calculated by adding the value returned by the get_next_selector_increment_value() function.
Check int31error variable
free_ldt_descriptor, get_next_selector_increment_value, segment_to_descriptor, create_code_segment_alias_descriptor, set_segment_limit, set_segment_base_address
Program sel_des;
uses crt,
go32;
const maxx = 80;
maxy = 25;
bytespercell = 2;
screensize = maxx * maxy * bytespercell;
linB8000 = $B800 * 16;
type string80 = string[80];
var
text_save : array[0..screensize-1] of byte;
text_oldx, text_oldy : Word;
text_sel : Word;
procedure status(s : string80);
begin
gotoxy(1, 1);
clreol;
write(s);
readkey;
end;
procedure selinfo(sel : Word);
begin
gotoxy(1, 24);
clreol;
writeln('Descriptor base address : $',
hexstr(get_segment_base_address(sel), 8));
clreol;
write('Descriptor limit : ',
get_segment_limit(sel));
end;
function makechar(ch : char; color : byte) : Word;
begin
result := byte(ch) or (color shl 8);
end;
begin
seg_move(dosmemselector, linB8000,
get_ds, longint(@text_save), screensize);
text_oldx := wherex; text_oldy := wherey;
seg_fillword(dosmemselector, linB8000,
screensize div 2,
makechar(' ', Black or (Black shl 4)));
status('Creating selector ' +
'''text_sel'' to a part of text screen memory');
text_sel := allocate_ldt_descriptors(1);
set_segment_base_address(text_sel, linB8000
+ bytespercell * maxx * 1);
set_segment_limit(text_sel,
screensize-1-bytespercell*maxx*3);
selinfo(text_sel);
status('and clearing entire memory ' +
'selected by ''text_sel'' descriptor');
seg_fillword(text_sel, 0,
(get_segment_limit(text_sel)+1) div 2,
makechar(' ', LightBlue shl 4));
status('Notice that only the memory described'+
' by the descriptor changed, nothing else');
status('Now reducing it''s limit and base and '+
'setting it''s described memory');
set_segment_base_address(text_sel,
get_segment_base_address(text_sel)
+ bytespercell * maxx);
set_segment_limit(text_sel,
get_segment_limit(text_sel)
- bytespercell * maxx * 2);
selinfo(text_sel);
status('Notice that the base addr increased by '+
'one line but the limit decreased by 2 lines');
status('This should give you the hint that the '+
'limit is relative to the base');
seg_fillword(text_sel, 0,
(get_segment_limit(text_sel)+1) div 2,
makechar(#176, LightMagenta or Brown shl 4));
status('Now let''s get crazy and copy 10 lines'+
' of data from the previously saved screen');
seg_move(get_ds, longint(@text_save),
text_sel, maxx * bytespercell * 2,
maxx * bytespercell * 10);
status('At last freeing the descriptor and '+
'restoring the old screen contents..');
status('I hope this little program may give '+
'you some hints on working with descriptors');
free_ldt_descriptor(text_sel);
seg_move(get_ds, longint(@text_save),
dosmemselector, linB8000, screensize);
gotoxy(text_oldx, text_oldy);
end.