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Allocate a number of descriptors
Source position: go32.pp line 87
function allocate_ldt_descriptors( |
count: Word |
):Word; |
Allocates a number of new descriptors.
Parameters:
Return value: The base selector.
Remark: | 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 the int31error variable.
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Free a descriptor |
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Return selector increment value |
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Map segment address to descriptor |
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Create new descriptor from existing descriptor |
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Set descriptor limit |
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Set descriptor's base address |
{$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.