1. Added comments to ASM files

[mirror_edk2.git] / MdePkg / Library / BaseLib / Ia32 / Thunk16.asm

diff --git a/MdePkg/Library/BaseLib/Ia32/Thunk16.asm b/MdePkg/Library/BaseLib/Ia32/Thunk16.asm
index 5e357991271d07efb15e95075027833d4075512a..5c9bc81a2bc3e6a1a624bddf7b65e1829a73846a 100644 (file)
--- 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.\r
 ;------------------------------------------------------------------------------\r
 _BackFromUserCode   PROC\r
+    ;\r
+    ; The order of saved registers on the stack matches the order they appears\r
+    ; in IA32_REGS structure. This facilitates wrapper function to extract them\r
+    ; into that structure.\r
+    ;\r
     push    ss\r
     push    cs\r
     DB      66h\r
@@ -104,6 +109,11 @@ _ThunkAttr  DD      ?
     mov     eax, ss\r
     DB      67h\r
     lea     bp, [esp + sizeof (IA32_REGS)]\r
+    ;\r
+    ; esi's in the following 2 instructions are indeed bp in 16-bit code. Fact\r
+    ; is "esi" in 32-bit addressing mode has the same encoding of "bp" in 16-\r
+    ; bit addressing mode.\r
+    ;\r
     mov     word ptr (IA32_REGS ptr [esi - sizeof (IA32_REGS)])._ESP, bp\r
     mov     ebx, (IA32_REGS ptr [esi - sizeof (IA32_REGS)])._EIP\r
     shl     ax, 4                       ; shl eax, 4\r
@@ -167,7 +177,7 @@ _ToUserCode PROC
     pop     fs\r
     pop     gs\r
     popf                                ; popfd\r
-    DB      66h\r
+    DB      66h                         ; Use 32-bit addressing for "retf" below\r
     retf                                ; transfer control to user code\r
 _ToUserCode ENDP\r
 \r
@@ -197,7 +207,7 @@ GdtEnd          LABEL   QWORD
 ;   );\r
 ;------------------------------------------------------------------------------\r
 InternalAsmThunk16  PROC    USES    ebp ebx esi edi ds  es  fs  gs\r
-    mov     esi, [esp + 36]             ; esi <- RegSet\r
+    mov     esi, [esp + 36]             ; esi <- RegSet, the 1st parameter\r
     movzx   edx, (IA32_REGS ptr [esi])._SS\r
     mov     edi, (IA32_REGS ptr [esi])._ESP\r
     add     edi, - (sizeof (IA32_REGS) + 4) ; reserve stack space\r
@@ -227,11 +237,11 @@ InternalAsmThunk16  PROC    USES    ebp ebx esi edi ds  es  fs  gs
     push    10h\r
     pop     ecx                         ; ecx <- selector for data segments\r
     lgdt    fword ptr [edx + (_16Gdtr - SavedCr0)]\r
-    pushfd\r
+    pushfd                              ; Save df/if indeed\r
     call    fword ptr [edx + (_EntryPoint - SavedCr0)]\r
     popfd\r
     lidt    fword ptr [esp + 36]        ; restore protected mode IDTR\r
-    lea     eax, [ebp - sizeof (IA32_REGS)]\r
+    lea     eax, [ebp - sizeof (IA32_REGS)] ; eax <- the address of IA32_REGS\r
     ret\r
 InternalAsmThunk16  ENDP\r
 \r