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git-svn-id: https://spexeah.com:8443/svn/Asuro@17 6dbc8c32-bb84-406f-8558-d1cf31a0ab0c

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This commit is contained in:
aaron 2017-05-15 18:55:45 +00:00
parent a4ed035c93
commit e33d8ad9eb
1 changed files with 93 additions and 0 deletions
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src/gdt.pas Normal file
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unit gdt;
interface
uses
util;
type
TSegementDescriptor = bitpacked record
limit_low : WORD;
base_low : WORD;
access : Byte;
limit_n_flags : Byte;
base_high : Byte;
end;
var
GDTarr : array[0..3] of TSegementDescriptor;
GDTptr : ^TSegementDescriptor;
GDT_length : integer = 0;
procedure init();
procedure create(base : dword; limit : dword; flags : byte);
implementation
procedure init();
var
data_addr : dword;
begin
GDTptr := @GDTarr[0];
data_addr := (@GDTarr[2] - @GDTarr[0]) shr 16;
create($0000, $0000, $00); //null segment / GDT pointer
create($0000, $FFFF, $9A); //code descriptor
create($0000, $FFFF, $92); //data descriptor
asm
LGDT GDTptr
MOV AX, data_addr
MOV DS, AX
MOV SS, AX
MOV ES, AX
MOV FS, AX
MOV GS, AX
end;
end;
procedure create(base : dword; limit : dword; flags : byte);
var
descriptor : array[0..8] of Byte;
descriptor_ptr : ^Byte = @descriptor[0];
s_descriptor : TSegementDescriptor;
s_descriptor_ptr : ^TSegementDescriptor = @s_descriptor;
begin
if limit <= 65536 then
descriptor[6] := $40
else
if (limit and $FFF) <> $FFF then
limit := (limit >> 12) -1
else
limit := limit >> 12;
descriptor[6] := $C0;
descriptor[0] := limit and $FF;
descriptor[1] := (limit shr 8) and $FF;
descriptor[6] := limit or ((limit shr 16) and $F);
descriptor[2] := base and $FF;
descriptor[3] := (base shr 8) and $FF;
descriptor[4] := (base shr 16) and $FF;
descriptor[7] := (base shr 24) and $FF;
descriptor[5] := flags;
asm
MOV EAX, descriptor_ptr
MOV s_descriptor_ptr, EAX
end;
descriptor_ptr = @descriptor[4];
asm
MOV EAX, descriptor_ptr
MOV EBX, s_descriptor_ptr
ADD EBX, 1
MOV (EBX), EAX
end;
GDTarr[GDT_length] := s_descriptor;
GDT_length := GDT_length + 1;
end;
end.