A20 gate/port 
Author Message
 A20 gate/port

I keep seeing these programs which switch from Real Mode to 286/386
Protected Mode refer to Gate A20. I know it's the address bus line,
from the keyboard.
What is it used for? Exactly what happens when you enable A20?
or Disable it? Does it make an interrupt? Is it necessary?
Is there any way around it?


Sun, 14 Aug 1994 08:03:11 GMT  
 A20 gate/port

Quote:

>I keep seeing these programs which switch from Real Mode to 286/386
>Protected Mode refer to Gate A20. I know it's the address bus line,
>from the keyboard.
>What is it used for? Exactly what happens when you enable A20?
>or Disable it? Does it make an interrupt? Is it necessary?
>Is there any way around it?

This is from the 286LX data book, which implements A20 Gate.

"With the 80C286 processor, full real mode address compatibility requires
that address 'rollover' at the 1-Mb boundary be handled the same as the
early 8088-based PC's did.  This requires system address line 20 to have
the capability of being forced to a 0 during real mode execution.  The
Am286ZX/LX integrated processor provides two different methods for
controlling the A20 system address line:  AT standard A20 control and
Fast Gate A20 support.  See Figure 4-2 for a logic diagram which
illustrates the Gate A20 control function in the Am258ZX/LX processor.
[Diagram showing A20G OR FGA20 feeding one side of AND with CPUA20,
 result being SYSTEMA20]
 AT Standard A20 Control
 The A20G input pin on the device is normally connected to an output of
the AT Keyboard controller.  A logic High on this input will force the
'pass through' of the CPU's A20 onto the internal system address bus.
A logic Low on this input will force the system address bus A20 line
low, as long as the Fast Gate A20 bit is not being utilized.
 Fast Gate A20 Support
 The Am286ZX/LX processor provides a high-performance method for
controlling the system A20 line, independent of the relatively slow
AT keyboard controller.  An industry standard control port at
I/O Address 092h contains the Fast Gate A20 bit, FGA20.
 A logic high written to this bit will force the 'pass through' of
the CPU's A20 onto the internal system address bus.  A logic low will
force the system address bus A20 line low, as long as the AT keyboard
controller A20G input is not being utilized."

Hope this helps.
--
Steven Murray

I am speaking, but  |  The important thing is not to stop questioning.
not for my employer.|                           -Albert Einstein



Mon, 15 Aug 1994 20:24:05 GMT  
 
 [ 2 post ] 

 Relevant Pages 

1. A20 gate on the 386

2. A20 gate on the 386

3. Jerry Mason method to open A20 Gate

4. A20 Gate (was: Newbie Question!)

5. Need for buffer gates in output ports

6. Bill Gates at the Pearly Gates

7. How to use synthesis tool to get gate level netlist only containing verilog basic gates

8. call gates and int gates

9. Gates-gate?

10. A20 ok ?

11. A20 setup

12. standard A20

 

 
Powered by phpBB® Forum Software