Talk:A20 Line

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I think this topic needs some practical information about the A20 line. (Added stub tag). - Combuster 16:12, 17 December 2006 (CST)

There is some information about it on OS FAQ Wiki... I will adapt it when I have the time... JJeronimo 14:41, 23 December 2006 (CST)
The pre-pentium remark seems to be invalid. It has nothing to do with the processor, only with the chipset used. - Combuster 08:33, 24 December 2006 (CST)
Yes, yes... on my old 486 DX it worked... JJeronimo 11:29, 24 December 2006 (CST)

"Don't forget to restore the original bootsector identifier." Why is it important? Adek336 21:44, 1 August 2008 (UTC)

Since we are talking about the OS-specific bootloader here, it's not. The OS would know (and probably doesn't care) that the memory image of its own bootloader was slightly self-modified. If the OS happens to boot from a generic bootloader, then the OS had best not assume anything about the bootloader except what is in the bootloader's specs.
However, this article definitely needs an additional section about INT15h AX=2400h and 2401h.
Maybe there should also be a mention that the Compaq method, the HP Vectra method, and the Port 0xEE method also exist -- and pointing to external links for descriptions. Bewing 20:53, 2 August 2008 (UTC)

Interesting Info on A20 Line

Just some facts that should probably be thrown into the mix here.

  • The Intel64 Architecture Manual states, "The functionality of A20M# is used primarily by older operating systems and not used by modern operating systems. On newer Intel 64 processors, A20M# may be absent."
  • The PC 2001 System Design Guide states, "If A20M# generation logic is still present in the system, this logic must be terminated such that software writes to I/O port 92, bit 1, do not result in A20M# being asserted to the processor."
  • The AMD64 Chipset Manuals allows the USB A20 function to be merged with PS2 A20gate through I/O port 64, bit 30.
  • The AMD64 Chipset Manuals guarantee I/O port 92, bit 1 to be available for use (if set to 1 A20M# function is disabled.)
  • The AMD64 BKDG states, that after entering SMM mode, "A20M# is disabled. A20M# assertion or deassertion have no effect on SMM mode."

The above facts could probably be used to safely assume that all modern computer systems, 2001 and later, support port 92, bit 1 to control the A20 Line. Furthermore, any modern computer system that does not support port 92, bit 1 will already have the A20 Line disabled. 54616e6e6572 08:06, 12 August 2010 (UTC)

Just wanted to write again that some systems don't have Gate A20. BTW, I don't know a SINGLE program which requires Gate A20 off. Alliumnsk 06:13, 16 August 2017 (CDT)
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