diff options
Diffstat (limited to 'MdePkg/Library/BaseLib/X64/Thunk16.asm')
| -rw-r--r-- | MdePkg/Library/BaseLib/X64/Thunk16.asm | 34 |
1 files changed, 20 insertions, 14 deletions
diff --git a/MdePkg/Library/BaseLib/X64/Thunk16.asm b/MdePkg/Library/BaseLib/X64/Thunk16.asm index 642310869b..556a8b47e3 100644 --- a/MdePkg/Library/BaseLib/X64/Thunk16.asm +++ b/MdePkg/Library/BaseLib/X64/Thunk16.asm @@ -66,6 +66,14 @@ 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.
+ ;
+ ; Some instructions for manipulation of segment registers have to be written
+ ; in opcode since 64-bit MASM prevents accesses to those registers.
+ ;
DB 16h ; push ss
DB 0eh ; push cs
DB 66h
@@ -115,7 +123,7 @@ SavedCr4 DD ? ;
; rdi in the instruction below is indeed bx in 16-bit code
;
- DB 66h, 2eh
+ DB 66h, 2eh ; 2eh is "cs:" segment override
lgdt fword ptr [rdi + (SavedGdt - @Base)]
DB 66h
mov ecx, 0c0000080h
@@ -129,9 +137,8 @@ SavedCr0 DD ? @64Eip DD ?
SavedCs DW ?
@64BitCode:
- DB 48h, 0b8h ; mov rax, imm64
-SavedRip DQ ?
- jmp rax ; return to caller
+ mov rsp, r8 ; restore stack
+ ret
_BackFromUserCode ENDP
_EntryPoint DD _ToUserCode - m16Start
@@ -160,14 +167,14 @@ _ToUserCode PROC mov cr4, rbp
mov ss, esi ; set up 16-bit stack segment
mov sp, bx ; set up 16-bit stack pointer
- DB 66h
+ DB 66h ; make the following call 32-bit
call @Base ; push eip
@Base:
pop bp ; ebp <- address of @Base
push [esp + sizeof (IA32_REGS) + 2]
- lea eax, [rsi + (@RealMode - @Base)]
+ lea eax, [rsi + (@RealMode - @Base)] ; rsi is "bp" in 16-bit code
push rax
- retf
+ retf ; execution begins at next instruction
@RealMode:
DB 66h, 2eh ; CS and operand size override
lidt fword ptr [rsi + (_16Idtr - @Base)]
@@ -178,7 +185,7 @@ _ToUserCode PROC pop gs
popf ; popfd
lea sp, [esp + 4] ; skip high order 32 bits of EFlags
- DB 66h
+ DB 66h ; make the following retf 32-bit
retf ; transfer control to user code
_ToUserCode ENDP
@@ -220,8 +227,8 @@ GDT_SIZE = $ - _NullSeg ; );
;------------------------------------------------------------------------------
InternalAsmThunk16 PROC USES rbp rbx rsi rdi
- mov r10d, ds
- mov r11d, es
+ mov r10d, ds ; r9 ~ r11 are not accessible in 16-bit
+ mov r11d, es ; so use them for saving seg registers
mov r9d, ss
push fs
push gs
@@ -238,8 +245,8 @@ InternalAsmThunk16 PROC USES rbp rbx rsi rdi lea ecx, [rdx + (SavedCr4 - m16Start)]
mov eax, edx ; eax <- transition code address
and edx, 0fh
- shl eax, 12
- lea ax, [rdx + (_BackFromUserCode - m16Start)]
+ shl eax, 12 ; segment address in high order 16 bits
+ lea ax, [rdx + (_BackFromUserCode - m16Start)] ; offset address
stosd ; [edi] <- return address of user code
sgdt fword ptr [rcx + (SavedGdt - SavedCr4)]
sidt fword ptr [rsp + 38h] ; save IDT stack in argument space
@@ -257,13 +264,12 @@ InternalAsmThunk16 PROC USES rbp rbx rsi rdi pushfq
lea edx, [rdx + DATA16 - DATA32]
lea r8, @RetFromRealMode
- mov [rcx + (SavedRip - SavedCr4)], r8
+ push r8
mov r8d, cs
mov [rcx + (SavedCs - SavedCr4)], r8w
mov r8, rsp
jmp fword ptr [rcx + (_EntryPoint - SavedCr4)]
@RetFromRealMode:
- mov rsp, r8
popfq
lidt fword ptr [rsp + 38h] ; restore protected mode IDTR
lea eax, [rbp - sizeof (IA32_REGS)]
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