Re: 80286 protected mode

On Mon, 14 Oct 2024 19:39:41 GMT
scott@slp53.sl.home (Scott Lurndal) wrote:

mitchalsup@aol.com (MitchAlsup1) writes:

On Mon, 14 Oct 2024 15:04:28 +0000, David Brown wrote:
 

On 13/10/2024 17:45, Anton Ertl wrote: 

 

I do think it would be convenient if there were a fully standard
way to compare independent pointers (other than just for
equality).  Rarely needing something does not mean /never/ needing
it. 

>
OK, take a segmented memory model with 16-bit pointers and a 24-bit
virtual address space. How do you actually compare to segmented
pointers ?? 

 
Depends.  On the Burroughs mainframe there could be eight
active segments and the segment number was part of the pointer.
 
Pointers were 32-bits (actually 8 BCD digits)
 
  S s OOOOOO
 
Where 'S' was a sign digit (C or D), 's' was the
segment number (0-7) and OOOOOO was the six digit
offset within the segment (500kB/1000kD each).
 
A particular task (process) could have up to
one million "environments", each environment
could have up to 100 "memory areas (up to 1000kD)
of which the first eight were loaded into the
processor base/limit registers.   Index registers
were 8 digits and were loaded with a pointer as
described above.   Operands could optionally select
one of the index registers and the operand address
was treated as an offset to the index register;
there were 7 index registers.
 
Access to memory areas 8-99 use string instructions
where the pointer was 16 BCD digits:
 
   EEEEEEMM SsOOOOOO
 
Where EEEEEE was the evironment number (0-999999);
environments starting with D00000 were reserved for
the MCP (Operating System).    MM was the memory area
number and the remaining eight digits described the
data within the memory area.  A subroutine call could
call within a memory area or switch to a new environment.
 
Memory area 1 was the code region for the segment,
Memory area 0 held the stack and some global variables
and was typically shared by all environments.
Memory areas 2-7 were application dependent and could
be configured to be shared between environments at
link time.

What was the size of phiscal address space ?
I would suppose, more than 1,000,000 words?