CH>Hi..
 > Lesson 1:
 > In Real mode, the 80x86 can access up yo 1,024KB of memory using
 > segments and offsets. An offset is a 16-bit logical address used to
 > reference bytes within a segment. The first byte in a segment is at
 > offset 0000, the second is at 0001 and so on (The last is at FFFF).
CH>Correction to lesson one: in real mode, the 286+ can access 1088kb-16
CH>bytes of memory. This is achieved by segmentreg=FFFFh offset reg=0 to
CH>FFFFh. This is how the 286+ ITSELF operates.
It can't access 1088kb. A 16-bit pointer can only access up to 2^16 bytes, or 
64kb.
"When in real mode, the 80x86 processor is has limited access to only 1,024 
kb using a segment:offset resolution."
"Extending DOS, Ray Gwin".
CH>Now, hardware incorporated in the **specific design** of the IBM-PC
CH>effectively
CH>disables this feature whilst in real mode. This specifc hardware is
CH>called the
CH>A20 Gate. This feature was included so that the design would remain 100%
CH>software compatable with the older, 8086 based design. The fact that you
CH>only
CH>program on a PC whose hardware design limits you to 1024kb whilst in
CH>real mode
CH>does not subtract from the fact that the 286+ it's self does NOT have
CH>this
CH>limitation.
I doubt it does, there is no mention of it in the DPMI specification!
CH>Finally, you can switch on the A20 gate yourelf if you like and restore
CH>the
CH>full addresability - you just need to know how to program the hardware,
CH>or, in this day and age, ask Himem.sys to do it for you.
Well, perhaps you could give me your "reference" so I could go over it, it 
seems doubtful that they would include that feature.
 > The 286 (technically referred to as the 80286)
 > processor was the first in the Intel family to introduce separate
 > operating modes. The architectural features of Protected Mode
CH>included
 > larger physical memory, better memory management, and hardware
CH>support
 > for multi-tasking and task-switching operations.
CH>AFAIK the 80386 was the first processor to introduce hardware support
CH>for multi-tasking and task-switching. Care to say how the '286
CH>implements this so-called 'hardware support'?
I'll reply to this some other time -- I'm nearly out of time.
CH>    Craig
... And God said: "I'll buy a vowel."
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