Re: Question about the A20 line

It tests out to see if 2 memory zones overlap. When the A20 line isn't
flipped, you only have 1Mb (20 bits) of addressable memory. Those
memory addresses wrap around when you reach the end. so address 0:0 and
address 0FFFFh:10h are actually the same! The thing is. If your A20
line was flipped correctly, these 2 memory addresses would not overlap.
If you put a byte in the first address and can't get the same value in
the second address then your A20 flip worked!

Regards
Jean-François Michaud


.

Relevant Pages

  • Re: Question about the A20 line
    ... > It tests out to see if 2 memory zones overlap. ... you only have 1Mb of addressable memory. ... > memory addresses wrap around when you reach the end. ... If your A20 ...
    (comp.lang.asm.x86)
  • Re: Accessing 2MB in real mode...
    ... > megabyte is translated to wrap-around to the beginning of memory. ... But, if A20 is disabled, then it will always be zero, thus causing the wrap you ... - Unreal mode is the standard real mode, with all of the segment limits set ...
    (comp.lang.asm.x86)
  • Re: Accessing 2MB in real mode...
    ... As it only masks off A20, A21 and up are routed to the (memory) bus as ... This means that the wrap around only occurs at odd MB addresses: ... if you don't want to fiddle with that A20 switch. ...
    (comp.lang.asm.x86)
  • Re: Limit on number of columns pulled using DBI::ODBC
    ... Please try to fix your email application (Yahoo) to wrap lines at ~76 ... I were running into memory issues, I'd rebuild my SQL query as such: ... I'll help you write a function to 'chunk' things up. ...
    (perl.beginners)
  • Re: plenty of memory. running out anyway.
    ... > Well, it's not wraped when it gets to the nntp servers, and the ... > reply if I don't wrap. ... It's definitely not that photoshop is using 1 Gb of memory. ... using the registry during log off. ...
    (microsoft.public.windowsxp.general)