bcopyxx: clear all flags before jumping to the successor code

Clear the flags register while we still have a stack to do so.
This makes the state in the target OS more predictable.  It's worth
noting that none of the instructions we use in either the PM or RM
trampolines change the flags, either (except the IF setting in the RM
trampoline, but that is intentional.)

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

diff --git a/core/bcopyxx.inc b/core/bcopyxx.inc
index 06b03b2..4d0266e 100644 (file)
--- a/core/bcopyxx.inc
+++ b/core/bcopyxx.inc
@@ -232,8 +232,10 @@ pm_shuffle:
                call pm_bcopy
                jmp .loop
 .done:
+               push ecx                ; == 0, for cleaning the flags register
                and esi,esi
                jz pm_shuffle_real_mode
+               popfd                   ; Clean the flags
                jmp edi                 ; Protected mode entry
 
                ; We have a real-mode entry point, so we need to return
@@ -250,14 +252,16 @@ pm_shuffle_real_mode:
                mov [ebx+PM_CS16+7],ah
                mov [ebx+PM_DS16+4],al
                mov [ebx+PM_DS16+7],ah
-               mov ax,PM_DS16
-               mov ds,eax
-               mov es,eax
-               mov fs,eax
-               mov gs,eax
-               mov ss,eax
                mov eax,cr0
                and al,~1
+               popfd                   ; Clean the flags
+               ; No flag-changing instructions below...
+               mov dx,PM_DS16
+               mov ds,edx
+               mov es,edx
+               mov fs,edx
+               mov gs,edx
+               mov ss,edx
                jmp PM_CS16:0
 
                align   16