A20 and A21 
Author Message
 A20 and A21

Hello !!!
If we haven't enable A20 state , the A20 bit will alway be zero but the A21 to A32 can be one , right?
If no , why Reobert Collin say this "Therefore, without enabling the input to the A20 gate, the CPU can address every even megabyte of memory as follows: 0-1M, 2-3M, 4-5M, etc. In fact, duplicates of these memory blocks appear at 1-2M, 3-4M, 5-6M, etc. as a result of holding A20 low on the address bus."

If i entered protected mode , it doesn't not mean the A20 can be 1 unless i enable it by pic ,right?

thank you very much!!!!



Sun, 03 Nov 2002 03:00:00 GMT  
 A20 and A21

Quote:
> Hello !!!
> If we haven't enable A20 state , the A20 bit will alway be zero but

the A21 to A32 can be one , right?

   Right. A2..A19 and A21..A31 (there isn't A32 up to Pentium Pro) pass
to the system bus as they are set by CPU.

Quote:
> If i entered protected mode , it doesn't not mean the A20 can be 1

unless i enable it by pic ,right?

   Aside from the weird RAM addressing there are no problems working
with A20 disabled. However you probably want to enable it and have
access to all the RAM

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And life's only made of paper --

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Sun, 03 Nov 2002 03:00:00 GMT  
 A20 and A21
so you mean i don't need to enable A20 before enter pmode ?
thanks a lot

Quote:

> > Hello !!!
> > If we haven't enable A20 state , the A20 bit will alway be zero but
> the A21 to A32 can be one , right?

>    Right. A2..A19 and A21..A31 (there isn't A32 up to Pentium Pro) pass
> to the system bus as they are set by CPU.

> > If i entered protected mode , it doesn't not mean the A20 can be 1
> unless i enable it by pic ,right?

>    Aside from the weird RAM addressing there are no problems working
> with A20 disabled. However you probably want to enable it and have
> access to all the RAM

> -- http://bphantom.hypermart.net --

> -- Too many flames with too much to burn
> And life's only made of paper --

> Sent via Deja.com http://www.deja.com/
> Before you buy.



Mon, 04 Nov 2002 03:00:00 GMT  
 A20 and A21

Gave forth this really invaluable piece of information:

Quote:
>so you mean i don't need to enable A20 before enter pmode ?
>thanks a lot

You don't _need_ to, but if you don't then you can only ever access
half of the memory in your system.


Sat, 09 Nov 2002 03:00:00 GMT  
 
 [ 4 post ] 

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