commit 15868c4bbfff7f913487497d5165a36be1be0424
parent f0721f1be4c017d7e8d9d79b249fa04dda14a9ec
Author: rsc <rsc>
Date: Wed, 22 Aug 2007 14:21:05 +0000
better comment
Diffstat:
1 file changed, 3 insertions(+), 7 deletions(-)
diff --git a/bootasm.S b/bootasm.S
@@ -33,13 +33,9 @@ start:
movw $start,%sp # Stack Pointer
# Enable A20:
- # For fascinating historical reasons (related to the fact that
- # the earliest 8086-based PCs could only address 1MB of physical
- # memory and subsequent 80286-based PCs wanted to retain maximum
- # compatibility), physical address line 20 is tied to low when the
- # machine boots. Obviously this a bit of a drag for us, especially
- # when trying to address memory above 1MB. This code undoes this.
-
+ # For backwards compatibility with the earliest PCs, physical
+ # address line 20 is tied low, so that addresses higher than
+ # 1MB wrap around to zero by default. This code undoes this.
seta20.1:
inb $0x64,%al # Get status
testb $0x2,%al # Busy?
