summaryrefslogtreecommitdiff
diff options
context:
space:
mode:
authorrsc <rsc>2007-08-22 14:21:05 +0000
committerrsc <rsc>2007-08-22 14:21:05 +0000
commit15868c4bbfff7f913487497d5165a36be1be0424 (patch)
tree62c3736ae134b39a2266aa56aab57eca1f8f9c8e
parentf0721f1be4c017d7e8d9d79b249fa04dda14a9ec (diff)
downloadxv6-labs-15868c4bbfff7f913487497d5165a36be1be0424.tar.gz
xv6-labs-15868c4bbfff7f913487497d5165a36be1be0424.tar.bz2
xv6-labs-15868c4bbfff7f913487497d5165a36be1be0424.zip
better comment
-rw-r--r--bootasm.S10
1 files changed, 3 insertions, 7 deletions
diff --git a/bootasm.S b/bootasm.S
index c9982d2..65026f0 100644
--- 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?