Re: 80286 protected mode

On Fri, 18 Oct 2024 14:06:17 GMT
scott@slp53.sl.home (Scott Lurndal) wrote:

Michael S <already5chosen@yahoo.com> writes:

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? 

 
It varied based on the generation.  In the
1960s, a half megabyte (10^6 digits)
was the limit.
 
In the 1970s, the architecture supported
10^8 digits, the largest B4800 systems
were shipped with 2 million digits (1MB).
In 1979, the B4900 was introduced supporting
up to 10MB (20 MD), later increased to
20MB/40MD.
 
In the 1980s, the largest systems (V500)
supported up to 10^9 digits.  It
was that generation of machine where the
environment scheme was introduced.
 
Binaries compiled in 1966 ran on all
generations without recompilation.
 
There was room in the segmentation structures
for up to 10^18 digit physical addresses
(where the segments were aligned on 10^3
digit boundaries).

So, can it be said that ar least some of B6500-compatible models
suffered from the same problem as 80286 - the segment of maximal size
didn't cover all linear (or physical) address space?
Or their index register width was increased to accomodate 1e9 digits in
the single segment?

 
Unisys discontinued that line of systems in 1992.

I thought it lasted longer. My impresion was that there were still
hardware implemntation (alongside with emulation on Xeons) sold up
until 15 years ago.