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-rw-r--r--MdePkg/Library/BaseLib/X64/DisablePaging64.asm19
-rw-r--r--MdePkg/Library/BaseLib/X64/EnablePaging64.asm4
-rw-r--r--MdePkg/Library/BaseLib/X64/LongJump.asm2
-rw-r--r--MdePkg/Library/BaseLib/X64/Monitor.asm2
-rw-r--r--MdePkg/Library/BaseLib/X64/Mwait.asm2
-rw-r--r--MdePkg/Library/BaseLib/X64/ReadDr4.asm4
-rw-r--r--MdePkg/Library/BaseLib/X64/ReadDr5.asm4
-rw-r--r--MdePkg/Library/BaseLib/X64/ReadMm0.asm3
-rw-r--r--MdePkg/Library/BaseLib/X64/ReadMm1.asm3
-rw-r--r--MdePkg/Library/BaseLib/X64/ReadMm2.asm3
-rw-r--r--MdePkg/Library/BaseLib/X64/ReadMm3.asm3
-rw-r--r--MdePkg/Library/BaseLib/X64/ReadMm4.asm3
-rw-r--r--MdePkg/Library/BaseLib/X64/ReadMm5.asm3
-rw-r--r--MdePkg/Library/BaseLib/X64/ReadMm6.asm3
-rw-r--r--MdePkg/Library/BaseLib/X64/ReadMm7.asm3
-rw-r--r--MdePkg/Library/BaseLib/X64/Thunk16.asm34
-rw-r--r--MdePkg/Library/BaseLib/X64/WriteDr4.asm4
-rw-r--r--MdePkg/Library/BaseLib/X64/WriteDr5.asm4
-rw-r--r--MdePkg/Library/BaseLib/X64/WriteMm0.asm3
-rw-r--r--MdePkg/Library/BaseLib/X64/WriteMm1.asm3
-rw-r--r--MdePkg/Library/BaseLib/X64/WriteMm2.asm3
-rw-r--r--MdePkg/Library/BaseLib/X64/WriteMm3.asm3
-rw-r--r--MdePkg/Library/BaseLib/X64/WriteMm4.asm3
-rw-r--r--MdePkg/Library/BaseLib/X64/WriteMm5.asm3
-rw-r--r--MdePkg/Library/BaseLib/X64/WriteMm6.asm3
-rw-r--r--MdePkg/Library/BaseLib/X64/WriteMm7.asm3
-rw-r--r--MdePkg/Library/BaseLib/X64/WriteMsr64.asm4
27 files changed, 100 insertions, 31 deletions
diff --git a/MdePkg/Library/BaseLib/X64/DisablePaging64.asm b/MdePkg/Library/BaseLib/X64/DisablePaging64.asm
index 2609c941aa..518137ddc9 100644
--- a/MdePkg/Library/BaseLib/X64/DisablePaging64.asm
+++ b/MdePkg/Library/BaseLib/X64/DisablePaging64.asm
@@ -36,15 +36,14 @@
;------------------------------------------------------------------------------
InternalX86DisablePaging64 PROC
cli
- shl rcx, 32
+ shl rcx, 32 ; rcx[32..47] <- Cs
lea eax, @F
- mov ecx, eax
- push rcx
- mov ebx, edx
mov esi, r8d
+ or rcx, rax ; rcx[0..47] <- Cs:@F
mov edi, r9d
- mov eax, [rsp + 28h]
- retf
+ mov eax, [rsp + 28h] ; eax <- New Stack
+ push rcx
+ retf ; switch to compatibility mode
@@:
mov esp, eax ; set up new stack
mov rax, cr0
@@ -57,10 +56,10 @@ InternalX86DisablePaging64 PROC
mov rax, cr4
and al, NOT (1 SHL 5) ; clear PAE
mov cr4, rax
- push rdi
- push rsi
- call rbx
- jmp $
+ push rdi ; push Context2
+ push rsi ; push Context1
+ call rdx ; transfer control to EntryPoint
+ hlt ; no one should get here
InternalX86DisablePaging64 ENDP
END
diff --git a/MdePkg/Library/BaseLib/X64/EnablePaging64.asm b/MdePkg/Library/BaseLib/X64/EnablePaging64.asm
index 459120a886..c54843008a 100644
--- a/MdePkg/Library/BaseLib/X64/EnablePaging64.asm
+++ b/MdePkg/Library/BaseLib/X64/EnablePaging64.asm
@@ -36,7 +36,7 @@
;------------------------------------------------------------------------------
InternalX86EnablePaging64 PROC
cli
- pop rax
+ pop rax ; skip the return address
call @Base
@Base:
add dword ptr [rsp], @F - @Base ; offset for far retf, seg is the 1st arg
@@ -58,7 +58,7 @@ InternalX86EnablePaging64 PROC
mov rsp, [esp + 18h]
add rsp, -20h
call rbx
- jmp $ ; halt processor if EntryPoint() returned
+ hlt ; halt processor if EntryPoint() returned
InternalX86EnablePaging64 ENDP
END
diff --git a/MdePkg/Library/BaseLib/X64/LongJump.asm b/MdePkg/Library/BaseLib/X64/LongJump.asm
index 6ef5f964e0..38e5606081 100644
--- a/MdePkg/Library/BaseLib/X64/LongJump.asm
+++ b/MdePkg/Library/BaseLib/X64/LongJump.asm
@@ -39,7 +39,7 @@ InternalLongJump PROC
mov r13, [rcx + 30h]
mov r14, [rcx + 38h]
mov r15, [rcx + 40h]
- mov rax, rdx
+ mov rax, rdx ; set return value
jmp qword ptr [rcx + 48h]
InternalLongJump ENDP
diff --git a/MdePkg/Library/BaseLib/X64/Monitor.asm b/MdePkg/Library/BaseLib/X64/Monitor.asm
index 0140494d30..ec9c3f8d03 100644
--- a/MdePkg/Library/BaseLib/X64/Monitor.asm
+++ b/MdePkg/Library/BaseLib/X64/Monitor.asm
@@ -36,7 +36,7 @@ AsmMonitor PROC
mov eax, ecx
mov ecx, edx
mov edx, r8d
- DB 0fh, 1, 0c8h
+ DB 0fh, 1, 0c8h ; monitor
ret
AsmMonitor ENDP
diff --git a/MdePkg/Library/BaseLib/X64/Mwait.asm b/MdePkg/Library/BaseLib/X64/Mwait.asm
index 8f76d944f1..aae1d42758 100644
--- a/MdePkg/Library/BaseLib/X64/Mwait.asm
+++ b/MdePkg/Library/BaseLib/X64/Mwait.asm
@@ -34,7 +34,7 @@
AsmMwait PROC
mov eax, ecx
mov ecx, edx
- DB 0fh, 1, 0c9h
+ DB 0fh, 1, 0c9h ; mwait
ret
AsmMwait ENDP
diff --git a/MdePkg/Library/BaseLib/X64/ReadDr4.asm b/MdePkg/Library/BaseLib/X64/ReadDr4.asm
index 97740016b3..2bed12928b 100644
--- a/MdePkg/Library/BaseLib/X64/ReadDr4.asm
+++ b/MdePkg/Library/BaseLib/X64/ReadDr4.asm
@@ -31,6 +31,10 @@
; );
;------------------------------------------------------------------------------
AsmReadDr4 PROC
+ ;
+ ; There's no obvious reason to access this register, since it's aliased to
+ ; DR7 when DE=0 or an exception generated when DE=1
+ ;
DB 0fh, 21h, 0e0h
ret
AsmReadDr4 ENDP
diff --git a/MdePkg/Library/BaseLib/X64/ReadDr5.asm b/MdePkg/Library/BaseLib/X64/ReadDr5.asm
index 0582502220..3bee3a56ec 100644
--- a/MdePkg/Library/BaseLib/X64/ReadDr5.asm
+++ b/MdePkg/Library/BaseLib/X64/ReadDr5.asm
@@ -31,6 +31,10 @@
; );
;------------------------------------------------------------------------------
AsmReadDr5 PROC
+ ;
+ ; There's no obvious reason to access this register, since it's aliased to
+ ; DR7 when DE=0 or an exception generated when DE=1
+ ;
DB 0fh, 21h, 0e8h
ret
AsmReadDr5 ENDP
diff --git a/MdePkg/Library/BaseLib/X64/ReadMm0.asm b/MdePkg/Library/BaseLib/X64/ReadMm0.asm
index a81d208298..0ed5c8f8ae 100644
--- a/MdePkg/Library/BaseLib/X64/ReadMm0.asm
+++ b/MdePkg/Library/BaseLib/X64/ReadMm0.asm
@@ -31,6 +31,9 @@
; );
;------------------------------------------------------------------------------
AsmReadMm0 PROC
+ ;
+ ; 64-bit MASM doesn't support MMX instructions, so use opcode here
+ ;
DB 48h, 0fh, 7eh, 0c0h
ret
AsmReadMm0 ENDP
diff --git a/MdePkg/Library/BaseLib/X64/ReadMm1.asm b/MdePkg/Library/BaseLib/X64/ReadMm1.asm
index 5da8c4f9e5..e24fcb5f9d 100644
--- a/MdePkg/Library/BaseLib/X64/ReadMm1.asm
+++ b/MdePkg/Library/BaseLib/X64/ReadMm1.asm
@@ -31,6 +31,9 @@
; );
;------------------------------------------------------------------------------
AsmReadMm1 PROC
+ ;
+ ; 64-bit MASM doesn't support MMX instructions, so use opcode here
+ ;
DB 48h, 0fh, 7eh, 0c8h
ret
AsmReadMm1 ENDP
diff --git a/MdePkg/Library/BaseLib/X64/ReadMm2.asm b/MdePkg/Library/BaseLib/X64/ReadMm2.asm
index 3b2b1e3af3..860326b375 100644
--- a/MdePkg/Library/BaseLib/X64/ReadMm2.asm
+++ b/MdePkg/Library/BaseLib/X64/ReadMm2.asm
@@ -31,6 +31,9 @@
; );
;------------------------------------------------------------------------------
AsmReadMm2 PROC
+ ;
+ ; 64-bit MASM doesn't support MMX instructions, so use opcode here
+ ;
DB 48h, 0fh, 7eh, 0d0h
ret
AsmReadMm2 ENDP
diff --git a/MdePkg/Library/BaseLib/X64/ReadMm3.asm b/MdePkg/Library/BaseLib/X64/ReadMm3.asm
index 6283c30285..aca6f3d2df 100644
--- a/MdePkg/Library/BaseLib/X64/ReadMm3.asm
+++ b/MdePkg/Library/BaseLib/X64/ReadMm3.asm
@@ -31,6 +31,9 @@
; );
;------------------------------------------------------------------------------
AsmReadMm3 PROC
+ ;
+ ; 64-bit MASM doesn't support MMX instructions, so use opcode here
+ ;
DB 48h, 0fh, 7eh, 0d8h
ret
AsmReadMm3 ENDP
diff --git a/MdePkg/Library/BaseLib/X64/ReadMm4.asm b/MdePkg/Library/BaseLib/X64/ReadMm4.asm
index a65f130283..dd52f99436 100644
--- a/MdePkg/Library/BaseLib/X64/ReadMm4.asm
+++ b/MdePkg/Library/BaseLib/X64/ReadMm4.asm
@@ -31,6 +31,9 @@
; );
;------------------------------------------------------------------------------
AsmReadMm4 PROC
+ ;
+ ; 64-bit MASM doesn't support MMX instructions, so use opcode here
+ ;
DB 48h, 0fh, 7eh, 0e0h
ret
AsmReadMm4 ENDP
diff --git a/MdePkg/Library/BaseLib/X64/ReadMm5.asm b/MdePkg/Library/BaseLib/X64/ReadMm5.asm
index 5295c38ae0..1f30e4bed4 100644
--- a/MdePkg/Library/BaseLib/X64/ReadMm5.asm
+++ b/MdePkg/Library/BaseLib/X64/ReadMm5.asm
@@ -31,6 +31,9 @@
; );
;------------------------------------------------------------------------------
AsmReadMm5 PROC
+ ;
+ ; 64-bit MASM doesn't support MMX instructions, so use opcode here
+ ;
DB 48h, 0fh, 7eh, 0e8h
ret
AsmReadMm5 ENDP
diff --git a/MdePkg/Library/BaseLib/X64/ReadMm6.asm b/MdePkg/Library/BaseLib/X64/ReadMm6.asm
index acdc99b888..4e1f2a824b 100644
--- a/MdePkg/Library/BaseLib/X64/ReadMm6.asm
+++ b/MdePkg/Library/BaseLib/X64/ReadMm6.asm
@@ -31,6 +31,9 @@
; );
;------------------------------------------------------------------------------
AsmReadMm6 PROC
+ ;
+ ; 64-bit MASM doesn't support MMX instructions, so use opcode here
+ ;
DB 48h, 0fh, 7eh, 0f0h
ret
AsmReadMm6 ENDP
diff --git a/MdePkg/Library/BaseLib/X64/ReadMm7.asm b/MdePkg/Library/BaseLib/X64/ReadMm7.asm
index 7a1b4e6111..d871a9f8dc 100644
--- a/MdePkg/Library/BaseLib/X64/ReadMm7.asm
+++ b/MdePkg/Library/BaseLib/X64/ReadMm7.asm
@@ -31,6 +31,9 @@
; );
;------------------------------------------------------------------------------
AsmReadMm7 PROC
+ ;
+ ; 64-bit MASM doesn't support MMX instructions, so use opcode here
+ ;
DB 48h, 0fh, 7eh, 0f8h
ret
AsmReadMm7 ENDP
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)]
diff --git a/MdePkg/Library/BaseLib/X64/WriteDr4.asm b/MdePkg/Library/BaseLib/X64/WriteDr4.asm
index ad618f1089..188a9d4ddc 100644
--- a/MdePkg/Library/BaseLib/X64/WriteDr4.asm
+++ b/MdePkg/Library/BaseLib/X64/WriteDr4.asm
@@ -31,6 +31,10 @@
; );
;------------------------------------------------------------------------------
AsmWriteDr4 PROC
+ ;
+ ; There's no obvious reason to access this register, since it's aliased to
+ ; DR6 when DE=0 or an exception generated when DE=1
+ ;
DB 0fh, 23h, 0e1h
mov rax, rcx
ret
diff --git a/MdePkg/Library/BaseLib/X64/WriteDr5.asm b/MdePkg/Library/BaseLib/X64/WriteDr5.asm
index 91eeef20e3..86009c3287 100644
--- a/MdePkg/Library/BaseLib/X64/WriteDr5.asm
+++ b/MdePkg/Library/BaseLib/X64/WriteDr5.asm
@@ -31,6 +31,10 @@
; );
;------------------------------------------------------------------------------
AsmWriteDr5 PROC
+ ;
+ ; There's no obvious reason to access this register, since it's aliased to
+ ; DR7 when DE=0 or an exception generated when DE=1
+ ;
DB 0fh, 23h, 0e9h
mov rax, rcx
ret
diff --git a/MdePkg/Library/BaseLib/X64/WriteMm0.asm b/MdePkg/Library/BaseLib/X64/WriteMm0.asm
index 0ad65e366d..7496362d86 100644
--- a/MdePkg/Library/BaseLib/X64/WriteMm0.asm
+++ b/MdePkg/Library/BaseLib/X64/WriteMm0.asm
@@ -31,6 +31,9 @@
; );
;------------------------------------------------------------------------------
AsmWriteMm0 PROC
+ ;
+ ; 64-bit MASM doesn't support MMX instructions, so use opcode here
+ ;
DB 48h, 0fh, 6eh, 0c1h
ret
AsmWriteMm0 ENDP
diff --git a/MdePkg/Library/BaseLib/X64/WriteMm1.asm b/MdePkg/Library/BaseLib/X64/WriteMm1.asm
index 66425a43c2..a00f551f8a 100644
--- a/MdePkg/Library/BaseLib/X64/WriteMm1.asm
+++ b/MdePkg/Library/BaseLib/X64/WriteMm1.asm
@@ -31,6 +31,9 @@
; );
;------------------------------------------------------------------------------
AsmWriteMm1 PROC
+ ;
+ ; 64-bit MASM doesn't support MMX instructions, so use opcode here
+ ;
DB 48h, 0fh, 6eh, 0c9h
ret
AsmWriteMm1 ENDP
diff --git a/MdePkg/Library/BaseLib/X64/WriteMm2.asm b/MdePkg/Library/BaseLib/X64/WriteMm2.asm
index bd0f0a0e1a..01db79007d 100644
--- a/MdePkg/Library/BaseLib/X64/WriteMm2.asm
+++ b/MdePkg/Library/BaseLib/X64/WriteMm2.asm
@@ -31,6 +31,9 @@
; );
;------------------------------------------------------------------------------
AsmWriteMm2 PROC
+ ;
+ ; 64-bit MASM doesn't support MMX instructions, so use opcode here
+ ;
DB 48h, 0fh, 6eh, 0d1h
ret
AsmWriteMm2 ENDP
diff --git a/MdePkg/Library/BaseLib/X64/WriteMm3.asm b/MdePkg/Library/BaseLib/X64/WriteMm3.asm
index fa36ea64bb..59464641eb 100644
--- a/MdePkg/Library/BaseLib/X64/WriteMm3.asm
+++ b/MdePkg/Library/BaseLib/X64/WriteMm3.asm
@@ -31,6 +31,9 @@
; );
;------------------------------------------------------------------------------
AsmWriteMm3 PROC
+ ;
+ ; 64-bit MASM doesn't support MMX instructions, so use opcode here
+ ;
DB 48h, 0fh, 6eh, 0d9h
ret
AsmWriteMm3 ENDP
diff --git a/MdePkg/Library/BaseLib/X64/WriteMm4.asm b/MdePkg/Library/BaseLib/X64/WriteMm4.asm
index d9bc46e16c..6848dbf7ec 100644
--- a/MdePkg/Library/BaseLib/X64/WriteMm4.asm
+++ b/MdePkg/Library/BaseLib/X64/WriteMm4.asm
@@ -31,6 +31,9 @@
; );
;------------------------------------------------------------------------------
AsmWriteMm4 PROC
+ ;
+ ; 64-bit MASM doesn't support MMX instructions, so use opcode here
+ ;
DB 48h, 0fh, 6eh, 0e1h
ret
AsmWriteMm4 ENDP
diff --git a/MdePkg/Library/BaseLib/X64/WriteMm5.asm b/MdePkg/Library/BaseLib/X64/WriteMm5.asm
index f268bea8ec..fbe44ae078 100644
--- a/MdePkg/Library/BaseLib/X64/WriteMm5.asm
+++ b/MdePkg/Library/BaseLib/X64/WriteMm5.asm
@@ -31,6 +31,9 @@
; );
;------------------------------------------------------------------------------
AsmWriteMm5 PROC
+ ;
+ ; 64-bit MASM doesn't support MMX instructions, so use opcode here
+ ;
DB 48h, 0fh, 6eh, 0e9h
ret
AsmWriteMm5 ENDP
diff --git a/MdePkg/Library/BaseLib/X64/WriteMm6.asm b/MdePkg/Library/BaseLib/X64/WriteMm6.asm
index cb32ea6dc3..cd5938828d 100644
--- a/MdePkg/Library/BaseLib/X64/WriteMm6.asm
+++ b/MdePkg/Library/BaseLib/X64/WriteMm6.asm
@@ -31,6 +31,9 @@
; );
;------------------------------------------------------------------------------
AsmWriteMm6 PROC
+ ;
+ ; 64-bit MASM doesn't support MMX instructions, so use opcode here
+ ;
DB 48h, 0fh, 6eh, 0f1h
ret
AsmWriteMm6 ENDP
diff --git a/MdePkg/Library/BaseLib/X64/WriteMm7.asm b/MdePkg/Library/BaseLib/X64/WriteMm7.asm
index f5713823bb..9636a4cd8d 100644
--- a/MdePkg/Library/BaseLib/X64/WriteMm7.asm
+++ b/MdePkg/Library/BaseLib/X64/WriteMm7.asm
@@ -31,6 +31,9 @@
; );
;------------------------------------------------------------------------------
AsmWriteMm7 PROC
+ ;
+ ; 64-bit MASM doesn't support MMX instructions, so use opcode here
+ ;
DB 48h, 0fh, 6eh, 0f9h
ret
AsmWriteMm7 ENDP
diff --git a/MdePkg/Library/BaseLib/X64/WriteMsr64.asm b/MdePkg/Library/BaseLib/X64/WriteMsr64.asm
index 6b8066f922..b7704b8185 100644
--- a/MdePkg/Library/BaseLib/X64/WriteMsr64.asm
+++ b/MdePkg/Library/BaseLib/X64/WriteMsr64.asm
@@ -32,8 +32,8 @@
; );
;------------------------------------------------------------------------------
AsmWriteMsr64 PROC
- mov rax, rdx
- shr rdx, 20h
+ mov rax, rdx ; meanwhile, rax <- return value
+ shr rdx, 20h ; edx:eax contains the value to write
wrmsr
ret
AsmWriteMsr64 ENDP