🚀 go-pugleaf

In article <1991Jun21.030948.10951@cs.mcgill.ca> storm@cs.mcgill.ca (Marc WANDSCHNEIDER) writes:


   Could somebody please tell me the difference between the 8088 and the 8086
   processors..?

   Obviously, both are 16bit processors, but is the width of the data bus the 
   only difference between them...?

   Which was developed first, and why is it that the 386 and 486 chips create 
   virtual 8086s instead of virtual 8088s...?

Internally, the 8086 and 8088 are divided into two units, the
execution unit and the bus interface unit.  Both of them use the same
execution unit and only differ in the bus interface units.  Inside,
the real CPU part (the execution units) is the same so that's my guess
as to why 386 and 486 have V86 modes.  

The differences in the bus interface units are as follows (it's been a
few years on the memory)

1.  The 8088 talks to the world in 8 bits at a time, the 8086 in 16
bits.
2.  The Prefetch queue in the 8086 is 6 bytes while the 8088 is 4
bytes.  Why? I don't remember, I do remember reading a well thought
out intel discussion about this.

3.  The other signals which come out are different.  The 8088 does
have a minimal mode where some of it's features aren't available (say
bus granting and such) but where the support chip count is reduced.
Not a big deal now, but in 1978 (or when ever it was designed) it was
a big deal.

As a result of the different bus interface units, the instruction
timing is different.  Memory fetches are much more expensive on the
8088 then on the 8086 when the 8086 is doing word aligned fetches.  A
good background reference to all of this is in Embedded Systems
Journal for (july?, the one which came to me in the mail yesterday).
Also, you could ask on comp.arch.

As I understand intel's push, the 8086 was the real processor and the
8088 was for a cheaper design.  The 8086 was first.

bruce
--
Bruce O'Neel              oneel@heasfs.gsfc.nasa.gov
NASA/GSFC/STX/Code 664