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Subject[patch] Re: That horrible hack from hell called A20

Linus Torvalds wrote:
> On Tue, 5 Dec 2000, Linus Torvalds wrote:
> >
> > Right now this is my interim patch (to clean test11). The thing to
> note is
> > that I decreased the keyboard controller timeout by a factor of about
> 167,
> > while making the "delay" a bit longer.
> Oh, btw, I forgot to ask people to give this a whirl. I assume it fixes
> the APM problems for Kai.
> It definitely won't fix the silly Olivetti M4 issue (we still touch bit
> #2
> in 0x92). We'll need to fix that by testing A20 before bothering with
> the
> 0x92 stuff. Alan, that should get fixed in 2.2.x too - clearly those
> Olivetti machines can be considered buggy, but even so..
> Who else had trouble with the keyboard controller?


Here's a patch to 2.2.19-pre7 that is essentially a backport of the
2.4.0 gate-A20 code.

This speeds up booting on my fast-A20 board (Celeron 500 MHz, no KBC)
from 2 min:15 seconds to <too small to measure by my wrist watch>.

Kai, you reported that your system was OK with 2.4.0-test12-pre6.
Does that mean that it's OK with 2.4.0-final also?

Comments? Should we be merging Peter's int 0x15-first patch with this?
And test for A20-gated after each step, before going to the next
method? Get that working and then backport it to 2.2.19?
Have their been any test reports on Peter's last patch? I didn't see
any, but if that should be the goal, I'll give it a whirl.

I'd like to see this applied to 2.2.19. At least changing the long
delay so that it doesn't appear that Linux isn't going to boot...

~Randy--- linux/arch/i386/boot/ Tue Jan 9 15:59:08 2001
+++ linux/arch/i386/boot/setup.S Tue Jan 9 16:42:40 2001
@@ -578,46 +578,44 @@
lgdt gdt_48 ! load gdt with whatever appropriate

! that was painless, now we enable A20
-! if this is SC410 or a few other bits we need to do it with the 'fast' method
- in al,#0x92 ! read "System Control Port A"
- or al,#0x02 ! Set "Alternate Gate A20" - Bit
- out #0x92,al ! write "System Control Port A"
+ call empty_8042

! do it the normal way too, so as not to upset normal machines
- call empty_8042
mov al,#0xD1 ! command write
out #0x64,al
call empty_8042
mov al,#0xDF ! A20 on
out #0x60,al
call empty_8042

+! if this is SC410 or a few other bits we need to do it with the 'fast' method
+! You must preserve the other bits here. Otherwise embarrasing things
+! like laptops powering off on boot happen. Corrected version by Kira
+! Brown from Linux 2.2
+ in al,#0x92 ! read "System Control Port A"
+ or al,#0x02 ! Set "Alternate Gate A20" - Bit
+ out #0x92,al ! write "System Control Port A"
! wait until a20 really *is* enabled; it can take a fair amount of
! time on certain systems; Toshiba Tecras are known to have this
-! problem. The memory location used here is the int 0x1f vector,
-! which should be safe to use; any *unused* memory location < 0xfff0
-! should work here.
-#define TEST_ADDR 0x7c
+! problem. The memory location used here (0x200) is the int 0x80
+! vector, which should be safe to use.

- push ds
xor ax,ax ! segment 0x0000
- mov ds,ax
+ mov fs,ax
dec ax ! segment 0xffff (HMA)
mov gs,ax
- mov bx,[TEST_ADDR] ! we want to restore the value later
- inc ax
- mov [TEST_ADDR],ax
+ inc ax ! unused memory location <0xfff0
+ seg fs
+ mov [0x200],ax ! we use the "int 0x80" vector
seg gs
- cmp ax,[TEST_ADDR+0x10]
+ cmp ax,[0x210] ! and its corresponding HMA addr
je a20_wait ! loop until no longer aliased
- mov [TEST_ADDR],bx ! restore original value
- pop ds
! make sure any possible coprocessor is properly reset..

xor ax,ax
@@ -797,10 +795,17 @@
! Some machines have delusions that the keyboard buffer is always full
! with no keyboard attached...
+! If there is no keyboard controller, we will usually get 0xff
+! to all the reads. With each IO taking a microsecond and
+! a timeout of 100,000 iterations, this can take about half a
+! second ("delay" == outb to port 0x80). That should be ok,
+! and should also be plenty of time for a real keyboard controller
+! to empty.

push ecx
- mov ecx,#0xFFFFFF
+ mov ecx,#100000

dec ecx
@@ -840,7 +845,7 @@
! Delay is needed after doing I/O
- .word 0x00eb ! jmp $+2
+ out #0x80,al

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