1. Added comments to ASM files

2. Fixed a bug in 64-bit AsmDisablePaging64(), which may cause a #GP exception.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@2206 6f19259b-4bc3-4df7-8a09-765794883524

1 files changed, 14 insertions, 4 deletions

diff --git a/MdePkg/Library/BaseLib/Ia32/Thunk16.asm b/MdePkg/Library/BaseLib/Ia32/Thunk16.asm
index 5e35799127..5c9bc81a2b 100644
--- a/MdePkg/Library/BaseLib/Ia32/Thunk16.asm
+++ b/MdePkg/Library/BaseLib/Ia32/Thunk16.asm
@@ -75,6 +75,11 @@ SavedGdt    LABEL   FWORD
 ; by user code. It will be shadowed to somewhere in memory below 1MB.

 ;------------------------------------------------------------------------------

 _BackFromUserCode   PROC

+    ;

+    ; The order of saved registers on the stack matches the order they appears

+    ; in IA32_REGS structure. This facilitates wrapper function to extract them

+    ; into that structure.

+    ;

     push    ss

     push    cs

     DB      66h

@@ -104,6 +109,11 @@ _ThunkAttr  DD      ?
     mov     eax, ss

     DB      67h

     lea     bp, [esp + sizeof (IA32_REGS)]

+    ;

+    ; esi's in the following 2 instructions are indeed bp in 16-bit code. Fact

+    ; is "esi" in 32-bit addressing mode has the same encoding of "bp" in 16-

+    ; bit addressing mode.

+    ;

     mov     word ptr (IA32_REGS ptr [esi - sizeof (IA32_REGS)])._ESP, bp

     mov     ebx, (IA32_REGS ptr [esi - sizeof (IA32_REGS)])._EIP

     shl     ax, 4                       ; shl eax, 4

@@ -167,7 +177,7 @@ _ToUserCode PROC
     pop     fs

     pop     gs

     popf                                ; popfd

-    DB      66h

+    DB      66h                         ; Use 32-bit addressing for "retf" below

     retf                                ; transfer control to user code

 _ToUserCode ENDP

 

@@ -197,7 +207,7 @@ GdtEnd          LABEL   QWORD
 ;   );

 ;------------------------------------------------------------------------------

 InternalAsmThunk16  PROC    USES    ebp ebx esi edi ds  es  fs  gs

-    mov     esi, [esp + 36]             ; esi <- RegSet

+    mov     esi, [esp + 36]             ; esi <- RegSet, the 1st parameter

     movzx   edx, (IA32_REGS ptr [esi])._SS

     mov     edi, (IA32_REGS ptr [esi])._ESP

     add     edi, - (sizeof (IA32_REGS) + 4) ; reserve stack space

@@ -227,11 +237,11 @@ InternalAsmThunk16  PROC    USES    ebp ebx esi edi ds  es  fs  gs
     push    10h

     pop     ecx                         ; ecx <- selector for data segments

     lgdt    fword ptr [edx + (_16Gdtr - SavedCr0)]

-    pushfd

+    pushfd                              ; Save df/if indeed

     call    fword ptr [edx + (_EntryPoint - SavedCr0)]

     popfd

     lidt    fword ptr [esp + 36]        ; restore protected mode IDTR

-    lea     eax, [ebp - sizeof (IA32_REGS)]

+    lea     eax, [ebp - sizeof (IA32_REGS)] ; eax <- the address of IA32_REGS

     ret

 InternalAsmThunk16  ENDP