🚀 go-pugleaf

In article <MYX2SzW00XoF42d1lf@andrew.cmu.edu>, Hugh Brinkley Sprunt writes:
> 
> The 386 and 486 architectures claim a virtual address space size of
> 2^46 bytes.  The virtual address is formed from a 14 bit selector
> and a 32 bit offset.  
> 	b) This scheme gives us 2^14 independent address spaces of 2^32 
>      bytes each.  In other words, the virtual memory scheme 
>      consists of as many as 2^14 segments of 2^32 bytes each.
> 
> If (b) is the answer, how does the processor communicate the 14 bits of 
> selector information to the memory mapping hardware/software?

b) is the correct answer.  Page 2-2 of the 80386 Programmer's Ref Manual
says this: "Applications programmers view the logical address space of
the 80386 as a collection of 16,383 one-dimensional subspaces, each
with a specified length [ranging] from one byte upto a maximum
of 2^32 bytes".

The 14 bit selector information is held in a segment register.  For code
segments, the segment register is CS.  For data segments DS, ES, FS and GS
may contain the segment selector bits.  The stack segment selector is SS.

During code execution, the instructions are fetched from the code segment
selected by CS.  The data may come from any data segment selected by the
four data segment registers.  More than four data segments can be accessed
by appropriately loading the data segment registers with selector
information.

The default segment selection is:
   CS for code
   SS for stack
   DS for local data
   ES for string instruction destinations

Special instruction prefix elements may be used to override the default
segment selection.

So, to answer your question, "the processor [communicates] the
14 bits of selector information to the memory mapping hardware"
by looking up the corresponding selector register.


Sundar Iyengar                      Microprocessor Design

UUCP:  intelca!mipos3!mipos2!sundar Intel, SC4-59
ARPA:  sundar@mipos2.intel.com      2625, Walsh Avenue
CSNET: sundar@mipos2.intel.com      Santa Clara, CA 95051
AT&T:  O: (408) 765-5206