Don't close the A20 gate; leave it open

Don't close the A20 gate (restore to previous state); leave it open.
This is good for performance, but also avoids a nasty race condition
when leaving protected mode to then quickly re-enter protected mode.

diff --git a/bcopy32.inc b/bcopy32.inc
index b62d3ac..99a16d8 100644 (file)
--- a/bcopy32.inc
+++ b/bcopy32.inc
@@ -190,7 +190,9 @@ simple_pm_call:
                pop fs
                pop es
                pop ds
+%if DISABLE_A20
                call disable_a20
+%endif
 
                popfd                   ; Re-enables interrupts
                pop ebp
@@ -412,6 +414,8 @@ a20_test:
                pop es
                ret
 
+%if DISABLE_A20
+
 disable_a20:
                pushad
 ;
@@ -464,6 +468,8 @@ a20d_none:
                popad
                ret
 
+%endif
+
 ;
 ; Routine to empty the 8042 KBC controller.  If dl != 0
 ; then we will test A20 in the loop and exit if A20 is
@@ -594,8 +600,9 @@ trampoline_to_pm:
 
                align 2
 A20List                dw a20_dunno, a20_none, a20_bios, a20_kbc, a20_fast
+%if DISABLE_A20
 A20DList       dw a20d_dunno, a20d_none, a20d_bios, a20d_kbc, a20d_fast
-a20_adjust_cnt equ ($-A20List)/2
+%endif
 
 A20Type                dw A20_NONE             ; A20 type
 

diff --git a/com32.inc b/com32.inc
index c90c9ec..220c9d7 100644 (file)
--- a/com32.inc
+++ b/com32.inc
@@ -210,7 +210,9 @@ com32_enter_rm:
                jmp bx                  ; Go to whereever we need to go...
 
 com32_done:
+%if DISABLE_A20
                call disable_a20
+%endif
                sti
                jmp enter_command
 

diff --git a/config.inc b/config.inc
index d199597..d6a981e 100644 (file)
--- a/config.inc
+++ b/config.inc
@@ -28,6 +28,17 @@ MAX_FKEYS    equ 12                  ; Number of F-key help files
 %assign        DO_WBINVD 0                     ; Should we use WBINVD or not?
 
 ;
+; Set this to return the A20 gate to its previous state, instead of
+; leaving it open.  This has caused problems, because there appear
+; to be a race condition between disabling the A20 gate and trying to
+; re-enter protected mode, causing the A20 gate disable to take effect
+; after we have already done the A20 enabled check, with disastrous
+; consequences.  Plus, there seems to be little or no demand for it.
+;
+%assign DISABLE_A20    0
+
+
+;
 ; Version number definitinons
 ;
 %ifndef DEPEND                         ; Generated file