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SubjectRe: setup.S: A20 enable sequence (once again)
Date
On Mon, 06 Nov 2000 you wrote:
H. Peter Anvin wrote
> I just looked at the code, and it's worse than I first thought: if
> memory location 0x200 happens to contain 0x0001 when the kernel is
> entered, this code with loop indefinitely.

Ooops, you're right !

How about this one:


diff -ur linux-2.4.0-test10/arch/i386/boot/setup.S linux/arch/i386/boot/setup.S
--- linux-2.4.0-test10/arch/i386/boot/setup.S Mon Oct 30 23:44:29 2000
+++ linux/arch/i386/boot/setup.S Mon Nov 6 21:14:45 2000
@@ -631,6 +631,26 @@
# appropriate

# that was painless, now we enable a20
+# Try the "fast A20" version first:
+# 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
+#
+ inb $0x92, %al #
+ orb $02, %al # "fast A20" version
+ andb $0xfe, %al # be sure not to reset CPU
+ outb %al, $0x92 # some chips have only this
+
+# See if that already did the job, skip keyboard controller
+# accesses if so: some boards do not have a keyboard controller.
+# These boards (especially if running on a slowish 386 EX)
+# can spend several *minutes* in silence, polling for the
+# non-existent keyboard controller.
+
+ movw $0x100,%cx # small retry count
+ call a20_check
+ jne a20_enabled
+
call empty_8042

movb $0xD1, %al # command write
@@ -641,20 +661,20 @@
outb %al, $0x60
call empty_8042

-#
-# 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
-#
- inb $0x92, %al #
- orb $02, %al # "fast A20" version
- outb %al, $0x92 # some chips have only this
-
# 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 (0x200) is the int 0x80
# vector, which should be safe to use.

+a20_recheck:
+ movw $0xffff,%cx # large retry count
+ call a20_check
+ jne a20_enabled
+ jmp a20_recheck # loop until no longer aliased
+
+#
+# cx has retry count
+a20_check:
xorw %ax, %ax # segment 0x0000
movw %ax, %fs
decw %ax # segment 0xffff (HMA)
@@ -663,9 +683,11 @@
incw %ax # unused memory location <0xfff0
movw %ax, %fs:(0x200) # we use the "int 0x80" vector
cmpw %gs:(0x210), %ax # and its corresponding HMA addr
- je a20_wait # loop until no longer aliased
+ loopz a20_wait
+ ret

# make sure any possible coprocessor is properly reset..
+a20_enabled:
xorw %ax, %ax
outb %al, $0xf0
call delay
----------------------------------------------------------------
Robert Kaiser email: rkaiser@sysgo.de
SYSGO RTS GmbH
Am Pfaffenstein 14 phone: (49) 6136 9948-762
D-55270 Klein-Winternheim / Germany fax: (49) 6136 9948-10
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