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| author | H. Peter Anvin <hpa@linux.intel.com> | 2009-04-21 16:08:07 -0700 |
|---|---|---|
| committer | H. Peter Anvin <hpa@linux.intel.com> | 2009-04-21 16:08:07 -0700 |
| commit | 8e1b32e8f5f28e49eff7a0cf39e7d2d31e379b1c (patch) | |
| tree | 16733882e862aa806c94952e6b8f9223b6e823bc | |
| parent | 31937531087fb47aa4a3a218e8114b32176f8683 (diff) | |
| download | syslinux-8e1b32e8f5f28e49eff7a0cf39e7d2d31e379b1c.tar.gz | |
a20: try to avoid io_delay if A20 is already enabled
We have at least the possibility to avoid io_delay if A20 is already
enabled. Thus, give it a try. Furthermore, when calling enable_a20,
always try the zero-work case first, since we may have been enabled by
a previous call. This should improve performance of the rm/pm
ping-pong.
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
| -rw-r--r-- | core/bcopy32.inc | 44 |
1 files changed, 19 insertions, 25 deletions
diff --git a/core/bcopy32.inc b/core/bcopy32.inc index c4edc715..f2cc6cab 100644 --- a/core/bcopy32.inc +++ b/core/bcopy32.inc @@ -193,13 +193,6 @@ simple_pm_call: %define IO_DELAY_PORT 80h ; Invalid port (we hope!) %define disable_wait 32 ; How long to wait for a disable -; Note the skip of 2 here -%define A20_DUNNO 0 ; A20 type unknown -%define A20_NONE 2 ; A20 always on? -%define A20_BIOS 4 ; A20 BIOS enable -%define A20_KBC 6 ; A20 through KBC -%define A20_FAST 8 ; A20 through port 92h - slow_out: out dx, al ; Fall through _io_delay: out IO_DELAY_PORT,al @@ -228,22 +221,19 @@ try_enable_a20: %endif ; -; If the A20 type is known, jump straight to type -; - jmp word [cs:A20Ptr] - -; -; First, see if we are on a system with no A20 gate +; First, see if we are on a system with no A20 gate, or the A20 gate +; is already enabled for us... ; -a20_dunno: a20_none: - mov word [cs:A20Ptr], a20_none call a20_test jnz a20_done + ; Otherwise, see if we had something memorized... + jmp word [cs:A20Ptr] ; ; Next, try the BIOS (INT 15h AX=2401h) ; +a20_dunno: a20_bios: mov word [cs:A20Ptr], a20_bios mov ax,2401h @@ -337,20 +327,24 @@ a20_done: popad ; This routine tests if A20 is enabled (ZF = 0). This routine ; must not destroy any register contents. ; +; The no-write early out avoids the io_delay in the (presumably common) +; case of A20 already enabled (e.g. from a previous call.) +; a20_test: push es push cx - push ax - mov cx,0FFFFh ; HMA = segment 0FFFFh + push eax + mov cx,0FFFFh ; HMA = segment 0FFFFh mov es,cx - mov cx,32 ; Loop count - mov ax,[cs:A20Test] -.a20_wait: inc ax - mov [cs:A20Test],ax - io_delay ; Serialize, and fix delay - cmp ax,[es:A20Test+10h] - loopz .a20_wait -.a20_done: pop ax + mov eax,[cs:A20Test] + mov cx,32 ; Loop count + jmp .test ; First iteration = early out +.wait: inc eax + mov [cs:A20Test],eax + io_delay ; Serialize, and fix delay +.test: cmp eax,[es:A20Test+10h] + loopz .wait +.done: pop eax pop cx pop es ret |