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{$mode delphi}
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.
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