Hardware Reference
In-Depth Information
Summary: Protection On The 32-Bit Intel Pentium Vs. The 64-Bit
AMD Opteron
Memory management in the Opteron is typical of most desktop or server computers today,
relying on page-level address translation and correct operation of the operating system to
provide safety to multiple processes sharing the computer. Although presented as alternat-
ives, Intel has followed AMD's lead and embraced the AMD64 architecture. Hence, both AMD
and Intel support the 64-bit extension of 80×86; yet, for compatibility reasons, both support the
elaborate segmented protection scheme.
If the segmented protection model looks harder to build than the AMD64 model, that's be-
cause it is. This effort must be especially frustrating for the engineers, since few customers
use the elaborate protection mechanism. In addition, the fact that the protection model is a
mismatch to the simple paging protection of UNIX-like systems means it will be used only
by someone writing an operating system especially for this computer, which hasn't happened
yet.
B.6 Fallacies and Pitfalls
Even a review of memory hierarchy has fallacies and pitfalls!
Pitfall Too Small An Address Space.
Just five years after DEC and Carnegie Mellon University collaborated to design the new
PDP-11 computer family, it was apparent that their creation had a fatal flaw. An architecture
announced by IBM six years before the PDP-11 was still thriving, with minor modifications, 25
years later. And the DEC VAX, criticized for including unnecessary functions, sold millions of
units after the PDP-11 went out of production. Why?
The fatal flaw of the PDP-11 was the size of its addresses (16 bits) as compared to the address
sizes of the IBM 360 (24 to 31 bits) and the VAX (32 bits). Address size limits the program
length, since the size of a program and the amount of data needed by the program must be
less than 2 Address size . The reason the address size is so hard to change is that it determines the
minimum width of anything that can contain an address: PC, register, memory word, and
efective-address arithmetic. If there is no plan to expand the address from the start, then the
chances of successfully changing address size are so slim that it normally means the end of
that computer family. Bell and Strecker [1976] put it like this:
There is only one mistake that can be made in computer design that is difficult to recover
from—not having enough address bits for memory addressing and memory management. The
PDP-11 followed the unbroken tradition of nearly every known computer. [p. 2]
A partial list of successful computers that eventually starved to death for lack of address bits
includes the PDP-8, PDP-10, PDP-11, Intel 8080, Intel 8086, Intel 80186, Intel 80286, Motorola
6800, AMI 6502, Zilog Z80, Cray-1, and Cray X-MP.
The venerable 80×86 line bears the distinction of having been extended twice, first to 32 bits
with the Intel 80386 in 1985 and recently to 64 bits with the AMD Opteron.
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