Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!asuvax!ncar!elroy.jpl.nasa.gov!sdd.hp.com!zaphod.mps.ohio-state.edu!magnus.acs.ohio-state.edu!smsmith From: smsmith@magnus.acs.ohio-state.edu (Stephen M Smith) Newsgroups: comp.sys.ibm.pc.hardware Subject: Re: What is A20 ? Keywords: A20 Message-ID: <1991May22.202206.3923@magnus.acs.ohio-state.edu> Date: 22 May 91 20:22:06 GMT References: Sender: news@magnus.acs.ohio-state.edu Organization: The Ohio State University Lines: 32 Nntp-Posting-Host: top.magnus.acs.ohio-state.edu >I have a 386DX and when I boot I get a message saying >something like 'A20 handler active'. >I can turn it on or off in the setup, but what is it ? >-- >boquist@cs.chalmers.se This is new to me too...I just found an explanation for it in my Quarterdeck's Manifest document (p. 51): "A20 is a hardware address line which can be enabled or disabled under program control. The A20 line is important in the management of extended memory and the HMA [in this case the 64k block of memory starting at 1024k]. Programs usually enable the A20 line when using extended memory and the HMA and disable it otherwise." The way I understand this is that the 386 is able to use its built-in hardware to handle or manage extended memory (memory over 1024k), and it does this via the A20 address line controlled by your e(X)tended (M)emory (S)pecification driver. You people who have Manifest can read pp. 49-51 in the manual and view your own system setup by choosing 'Extended' and 'XMS' while using Manifest. Corrections and clarifications welcome! Stephen M. Smith \ + / ,,@ acs.ohio-state. \ + / {7%*@,..":27g)-=,#*:.#,/6&1*.4-,l@#9:-) " edu> \ + / BTW, WYSInaWYG \ + / --witty.saying.ARC