Machine Characteristics - A20 at Boot Time
Table of Contents

This is one of a number of machine-characteristics.

Boot-Time A20

When receiving control from the BIOS what state will A20 be in? This table shows for the test machines whether at boot A20 is disabled (memory is wrapped) or A20 is enabled (memory is flat). A20 must be enabled before entering Protected Mode. The K bit is from the KBC output port. The A bit is from System Control Port A.

Machine K bit A bit Boot A20 State
Dell Latitude XP 475D laptop. 80486 75MHz. Phoenix BIOS 1.10 0 0 disabled
AOpen motherboard. Pentium 3 600MHz. Award BIOS 3.60 0 0 disabled
Viglen MPC-L. Geode 399MHz. ION A503 BIOS 1 0 disabled
Asus EEE 4G netbook. Celeron M ULV 630MHz. AMI ASUS 701 BIOS 1 0 enabled
Gateway laptop. Dual-core Core 2 T5300 1.7GHz. Phoenix BIOS 0 0 enabled
Jetway NC91. Dual-core Atom 1.6GHz. Phoenix-AwardBIOS x 0 enabled


  • While capable of reflecting the state of the control the Viglen MPC-L K bit is evidently initially reported erroneously.
  • The Gateway laptop has A20 enabled but this is reflected in neither bit.
  • There is no reliable correlation between bits set and A20 state. However, the effect of the A20 enable can be determined by testing.
  • The Jetway NC91 times out when trying to read the KBC output port. Its bit is shown as x in the table.
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