102 lines
2.2 KiB
ObjectPascal
102 lines
2.2 KiB
ObjectPascal
unit gdt;
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interface
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uses
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util,
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types;
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procedure create(base : dword; limit : dword; flags : byte);
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procedure init();
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type
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TSegmentDescriptor = bitpacked record
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limit_low : WORD;
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base_low : WORD;
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access : Byte;
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limit_n_flags : Byte;
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base_high : Byte;
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end;
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PSegmentDescriptor = ^TSegmentDescriptor;
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implementation
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var
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GDTarr : array [0..5] of TSegmentDescriptor;
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GDTptr : PSegmentDescriptor;
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GDT_length : integer = 0;
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procedure create(base : dword; limit : dword; flags : byte);
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var
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descriptor : array[0..8] of Byte;
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descriptor_ptr : PByte;
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s_descriptor : TSegmentDescriptor;
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s_descriptor_ptr : PSegmentDescriptor;
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begin
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descriptor_ptr:= @descriptor[0];
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s_descriptor_ptr:= @s_descriptor;
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if limit <= 65536 then begin
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descriptor[6] := $40
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end else begin
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if (limit and $FFF) <> $FFF then begin
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limit := (limit SHR 12) -1
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end else begin
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limit := limit SHR 12;
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end;
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end;
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descriptor[6] := $C0;
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descriptor[0] := limit and $FF;
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descriptor[1] := (limit shr 8) and $FF;
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descriptor[6] := limit or ((limit shr 16) and $F);
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descriptor[2] := base and $FF;
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descriptor[3] := (base shr 8) and $FF;
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descriptor[4] := (base shr 16) and $FF;
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descriptor[7] := (base shr 24) and $FF;
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descriptor[5] := flags;
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asm
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MOV EAX, descriptor_ptr
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MOV s_descriptor_ptr, EAX
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end;
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descriptor_ptr:= @descriptor[4];
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asm
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MOV EAX, descriptor_ptr
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MOV EBX, s_descriptor_ptr
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ADD EBX, 1
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MOV [EBX], EAX
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end;
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GDTarr[GDT_length] := s_descriptor;
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GDT_length := GDT_length + 1;
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end;
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procedure init();
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var
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data_addr : dword;
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begin
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GDTptr := @GDTarr[0];
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data_addr := (@GDTarr[2] - @GDTarr[0]) shr 16;
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create($0000, $0000, $00); //null segment / GDT pointer
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create($0000, $FFFF, $9A); //code descriptor
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create($0000, $FFFF, $92); //data descriptor
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asm
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LGDT GDTptr
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MOV EAX, data_addr
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MOV DS, AX
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MOV SS, AX
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MOV ES, AX
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MOV FS, AX
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MOV GS, AX
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end;
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end;
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end.
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