MdePkg BaseLib NASM Thunk16: Use bits 16 for 16-bit code

By using 'bits 16', we can write code for 16-bit use the actual
assembly syntax rather than 'DB' and sometimes writing code with
seemingly incorrect operands because we know it will run correctly
when the processor is running in 16-bit mode.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16023 6f19259b-4bc3-4df7-8a09-765794883524

1 files changed, 32 insertions, 49 deletions

diff --git a/MdePkg/Library/BaseLib/Ia32/Thunk16.nasm b/MdePkg/Library/BaseLib/Ia32/Thunk16.nasm
index c00e3ab078..a1093e0107 100644
--- a/MdePkg/Library/BaseLib/Ia32/Thunk16.nasm
+++ b/MdePkg/Library/BaseLib/Ia32/Thunk16.nasm
@@ -82,23 +82,24 @@ _BackFromUserCode:
     ; in IA32_REGS structure. This facilitates wrapper function to extract them

     ; into that structure.

     ;

+BITS    16

     push    ss

     push    cs

-    DB      66h

-    call    .Base                       ; push eip

+o32 call    dword .Base                 ; push eip

 .Base:

-    pushfw                              ; pushfd actually

+    pushfd

     cli                                 ; disable interrupts

     push    gs

     push    fs

     push    es

     push    ds

-    pushaw                              ; pushad actually

+    pushad

     DB      66h, 0bah                   ; mov edx, imm32

 .ThunkAttr: dd   0

     test    dl, THUNK_ATTRIBUTE_DISABLE_A20_MASK_INT_15

     jz      .1

-    mov     eax, 15cd2401h              ; mov ax, 2401h & int 15h

+    mov     ax, 2401h

+    int     15h

     cli                                 ; disable interrupts

     jnc     .2

 .1:

@@ -108,24 +109,17 @@ _BackFromUserCode:
     or      al, 2

     out     92h, al                     ; deactivate A20M#

 .2:

-    xor     ax, ax                      ; xor eax, eax

-    mov     eax, ss                     ; mov ax, ss

-    DB      67h

-    lea     bp, [esp + IA32_REGS.size]

-    ;

-    ; 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     [esi - IA32_REGS.size + IA32_REGS._ESP], bp

-    mov     ebx, [esi - IA32_REGS.size + IA32_REGS._EIP]

-    shl     ax, 4                       ; shl eax, 4

-    add     bp, ax                      ; add ebp, eax

+    xor     eax, eax

+    mov     ax, ss

+    lea     ebp, [esp + IA32_REGS.size]

+    mov     [bp - IA32_REGS.size + IA32_REGS._ESP], ebp

+    mov     bx, [bp - IA32_REGS.size + IA32_REGS._EIP]

+    shl     eax, 4                      ; shl eax, 4

+    add     ebp, eax                    ; add ebp, eax

     DB      66h, 0b8h                   ; mov eax, imm32

 .SavedCr4:  DD      0

     mov     cr4, eax

-    DB      66h

-    lgdt    [cs:edi + (SavedGdt - .Base)]

+o32 lgdt [cs:bx + (SavedGdt - .Base)]

     DB      66h, 0b8h                   ; mov eax, imm32

 .SavedCr0:  DD      0

     mov     cr0, eax

@@ -134,8 +128,7 @@ _BackFromUserCode:
     mov     ss, eax

     DB      66h, 0bch                   ; mov esp, imm32

 .SavedEsp   DD      0

-    DB      66h

-    retf                                ; return to protected mode

+o32 retf                                ; return to protected mode

 

 _EntryPoint:

         DD      _ToUserCode - ASM_PFX(m16Start)

@@ -153,45 +146,34 @@ _16GdtrBase:
 ; It will be shadowed to somewhere in memory below 1MB.

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

 _ToUserCode:

-    mov     edx, ss

-    mov     ss, ecx                     ; set new segment selectors

-    mov     ds, ecx

-    mov     es, ecx

-    mov     fs, ecx

-    mov     gs, ecx

+BITS    16

+    mov     dx, ss

+    mov     ss, cx                      ; set new segment selectors

+    mov     ds, cx

+    mov     es, cx

+    mov     fs, cx

+    mov     gs, cx

     mov     cr0, eax                    ; real mode starts at next instruction

                                         ;  which (per SDM) *must* be a far JMP.

     DB      0eah

 .RealAddr: DW 0, 0

 

     mov     cr4, ebp

-    mov     ss, esi                     ; set up 16-bit stack segment

-    xchg    sp, bx                      ; set up 16-bit stack pointer

-

-;   mov     bp, [esp + sizeof(IA32_REGS)

-    DB      67h

-    mov     ebp, [esp + IA32_REGS.size] ; BackFromUserCode address from stack

-

-;   mov     cs:[bp + (_BackFromUserCode.SavedSs - _BackFromUserCode)], dx

-    mov     [cs:esi + (_BackFromUserCode.SavedSs - _BackFromUserCode)], edx

+    mov     ss, si                      ; set up 16-bit stack segment

+    xchg    esp, ebx                    ; set up 16-bit stack pointer

+    mov     bp, [esp + IA32_REGS.size]

+    mov     [cs:bp + (_BackFromUserCode.SavedSs - _BackFromUserCode)], dx

+    mov     [cs:bp + (_BackFromUserCode.SavedEsp - _BackFromUserCode)], ebx

+    lidt    [cs:bp + (_16Idtr - _BackFromUserCode)]

 

-;   mov     cs:[bp + (_BackFromUserCode.SavedEsp - _BackFromUserCode)], ebx

-    DB      2eh, 66h, 89h, 9eh

-    DW      _BackFromUserCode.SavedEsp - _BackFromUserCode

-

-;   lidt    cs:[bp + (_16Idtr - _BackFromUserCode)]

-    DB      2eh, 66h, 0fh, 01h, 9eh

-    DW      _16Idtr - _BackFromUserCode

-

-    popaw                               ; popad actually

+    popad

     pop     ds

     pop     es

     pop     fs

     pop     gs

-    popfw                                ; popfd

+    popfd

 

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

-    retf                                ; transfer control to user code

+o32 retf                                ; transfer control to user code

 

 ALIGN   16

 _NullSegDesc    DQ      0

@@ -221,6 +203,7 @@ GdtEnd:
 ;------------------------------------------------------------------------------

 global ASM_PFX(InternalAsmThunk16)

 ASM_PFX(InternalAsmThunk16):

+BITS    32

     push    ebp

     push    ebx

     push    esi