Machine Characteristics - A20 Support in the BIOS
Table of Contents

This is one of a number of machine-characteristics.

A20 BIOS Support

What support will the BIOS give for A20 management? The table below lists four subfunctions of BIOS interrupt 0x15 ah=0x24. All, some or none of these may be available on a particular machine. The table shows where these subfunctions are supported or not and, where they are supported, what they report at boot time. Note, in particular, the A20 Controls column. It shows where such a BIOS call is supported and that, where supported, different machines provide different control mechanisms.
Machine 00 (Disable A20) 01 (Enable A20) 02 (Report A20 State) 03 (Report A20 Controls)
Dell Latitude XP 475D laptop. 80486 75MHz. Phoenix BIOS 1.10 carry=1 carry=1 carry=1 carry=1
AOpen motherboard. Pentium 3 600MHz. Award BIOS 3.60 carry=1 carry=1 carry=1 carry=0 0000_0000_0000_0001
Viglen MPC-L. Geode 399MHz. ION A503 BIOS carry=0 carry=0 carry=0 carry=0 0000_0000_0000_0010
Asus EEE 4G netbook. Celeron M ULV 630MHz. AMI ASUS 701 BIOS carry=1 carry=1 carry=1 carry=0 0000_0000_0000_0010
Gateway laptop. Dual-core Core 2 T5300 1.7GHz. Phoenix BIOS carry=0 but the BIOS failed to change the A20 state carry=0
carry=0 carry=0 0001_0100_0000_0011 (NB. Two other bits also set)
Jetway NC91. Dual-core Atom 1.6GHz. Phoenix-AwardBIOS carry=1 carry=1 carry=1 carry=0 0000_0000_0000_0001


  • Reading horizontally shows that of the BIOS calls a number of machines support ONLY int 0x15-2403 while NOT supporting other calls in the same family. Thus even though they don't provide controls for A20 the BIOS writers seem to have made an effort to tell the OS writer which mechanisms are available. However, compare the reports with mc-a20-controls which shows controls which have been found to work.
  • Where int 0x15-2403 is supported its returned BX (A20 controls available) is presented in binary.
  • For int 0x15-2403 the Gateway laptop returns bits set which are not documented in RBIL.
  • When the Phoenix BIOS on the Gatway laptop is told to disable A20 it fails even though it returns carry clear.
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