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From: josh@v1.UUCP (Josh Knight)
Newsgroups: net.arch,net.micro.16k,net.micro.68k
Subject: Re: 24 bit vs. 32 bit address space
Message-ID: <133@v1.UUCP>
Date: Wed, 27-Feb-85 00:39:04 EST
Article-I.D.: v1.133
Posted: Wed Feb 27 00:39:04 1985
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Organization: IBM Research, Yorktown Heights, N.Y.
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> 
> >   From: chuqui@nsc.UUCP (The Phantom)
> >   ...there really isn't much that a 32bit address gives you that a 24bit
> >   address doesn't also give you in a manufactured product EXCEPT a marketing
> >   tool. 
  From: ...!seismo!cmu-cs-spice!skef
> If your operating system and VM hardware let you have sparse virtual address
> spaces, there's a world of difference between 24 and 32 address bits.

IBM ships processors with 128 Meg of main memory.  I know of applications in IBM
that are running out of VIRTUAL address space with 24bit addressing.  32 (or 31
if you're worried about a 370/XA, or if you're speaking about user space on a Vax) 
bit addressing isn't just for "sparse" use of the address space.  Processors
already need to address more than 24 bits worth of main memory and programs 
will use more than 24 bits worth or virtual address space WITHOUT spreading
the data thinly over it.

"The biggest (and most common) mistake that can be made in a computer design
is that of not providing enough address bits for memory addressing and managment"

Bell and Mudge, "The Evolution of the PDP-11" in _Computer_Engineering,_A_
_DEC_View_of_Hardware_Systems_Design, Digital Press, 1978.

Any opinions (stated or implied) or errors (obvious or subtle) are mine,
not my employers.

		Josh Knight, IBM T.J. Watson Research
    josh at YKTVMX on BITNET, josh.yktvmx.ibm on CSnet,
    ...!philabs!v1!josh