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rsc 2007-08-22 14:21:05 +00:00
parent f0721f1be4
commit 15868c4bbf
1 changed files with 3 additions and 7 deletions
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10
bootasm.S
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@ -33,13 +33,9 @@ start:
movw $start,%sp # Stack Pointer movw $start,%sp # Stack Pointer
# Enable A20: # Enable A20:
# For fascinating historical reasons (related to the fact that # For backwards compatibility with the earliest PCs, physical
# the earliest 8086-based PCs could only address 1MB of physical # address line 20 is tied low, so that addresses higher than
# memory and subsequent 80286-based PCs wanted to retain maximum # 1MB wrap around to zero by default. This code undoes this.
# 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.
seta20.1: seta20.1:
inb $0x64,%al # Get status inb $0x64,%al # Get status
testb $0x2,%al # Busy? testb $0x2,%al # Busy?