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https://gitlab.com/freepascal.org/fpc/source.git
synced 2025-04-21 00:29:24 +02:00
91 lines
2.8 KiB
ObjectPascal
91 lines
2.8 KiB
ObjectPascal
{$mode delphi}
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uses
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crt,
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go32;
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const
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maxx = 80;
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maxy = 25;
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bytespercell = 2;
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screensize = maxx * maxy * bytespercell;
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linB8000 = $B800 * 16;
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type
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string80 = string[80];
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var
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text_save : array[0..screensize-1] of byte;
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text_oldx, text_oldy : Word;
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text_sel : Word;
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procedure status(s : string80);
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begin
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gotoxy(1, 1); clreol; write(s); readkey;
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end;
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procedure selinfo(sel : Word);
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begin
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gotoxy(1, 24);
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clreol; writeln('Descriptor base address : $',
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hexstr(get_segment_base_address(sel), 8));
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clreol; write('Descriptor limit : ', get_segment_limit(sel));
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end;
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function makechar(ch : char; color : byte) : Word;
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begin
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result := byte(ch) or (color shl 8);
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end;
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begin
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seg_move(dosmemselector, linB8000, get_ds, longint(@text_save),
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screensize);
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text_oldx := wherex; text_oldy := wherey;
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seg_fillword(dosmemselector, linB8000, screensize div 2,
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makechar(' ', Black or (Black shl 4)));
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status('Creating selector ''text_sel'' to a part of ' +
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'text screen memory');
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text_sel := allocate_ldt_descriptors(1);
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set_segment_base_address(text_sel,
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linB8000 + bytespercell * maxx * 1);
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set_segment_limit(text_sel, screensize - 1 - bytespercell *
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maxx * 3);
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selinfo(text_sel);
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status('and clearing entire memory selected by ''text_sel''' +
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' descriptor');
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seg_fillword(text_sel, 0, (get_segment_limit(text_sel)+1) div 2,
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makechar(' ', LightBlue shl 4));
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status('Notice that only the memory described by the' +
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' descriptor changed, nothing else');
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status('Now reducing it''s limit and base and setting it''s ' +
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'described memory');
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set_segment_base_address(text_sel,
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get_segment_base_address(text_sel) + bytespercell * maxx);
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set_segment_limit(text_sel,
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get_segment_limit(text_sel) - bytespercell * maxx * 2);
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selinfo(text_sel);
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status('Notice that the base addr increased by one line but ' +
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'the limit decreased by 2 lines');
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status('This should give you the hint that the limit is ' +
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'relative to the base');
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seg_fillword(text_sel, 0, (get_segment_limit(text_sel)+1) div 2,
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makechar(#176, LightMagenta or Brown shl 4));
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status('Now let''s get crazy and copy 10 lines of data from ' +
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'the previously saved screen');
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seg_move(get_ds, longint(@text_save), text_sel,
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maxx * bytespercell * 2, maxx * bytespercell * 10);
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status('At last freeing the descriptor and restoring the old '+
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' screen contents..');
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status('I hope this little program may give you some hints on '+
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'working with descriptors');
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free_ldt_descriptor(text_sel);
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seg_move(get_ds, longint(@text_save), dosmemselector,
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linB8000, screensize);
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gotoxy(text_oldx, text_oldy);
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end. |