segmented memory models

Jim Brain brain at
Mon Aug 4 18:55:49 CDT 2008

Patrick Finnegan wrote:
> I'll assume from that comment that you aren't familiar with PDP-11 
I understood the argument as "8086 put the 'vectors' (in quotes, since 
they are not just the addresses, but the opcode as well) at the top of 
RAM, which did not solve anything., because in the next generation of 
CPU, those addresses fell in the middle of the memory map.  So, on the 
one hand, you could constrain the memory map (ala PDP-11), which it 
appears would be considered an "elegant" design, or we lift the top of 
the address space, now exposing the 'vectors' as being in the middle of 
the memory map, and that is considered "bad".

Dunno, given the choice between fixed map with MMU to push blocks of RAM 
into view, or lifting the top of memory, I think the x86 designers 
picked a wise path.

> And, in case you're worried that hardware had to be adjusted depending 
> on what address bus used, it didn't.  The CPU asserts a I/O page line 
> on the bus whenever there's an access to the I/O page of address space, 
> So, the device only needs to check 12 address lines (memory accesses 
> are 16 bites wide) and the I/O page line, regardless of how wide the 
> address bus on the CPU is.
I noted the concern in general, but it does indeed appear the PDP-11 
designers handled it with elegance.  Still, I think we're comparing 
apples to oranges.  The Intel crowd had very little control over the 
computer design, while the DEC team controlled all aspects of the 
design, including the CPU.  If the IBM team hadworked very closely with 
the Intel designers in bringing out the 5150, I would agree with Tony's 
point.  But, we know IBM broke the rules in bringing out the unit, 
pulling off the shelf and already available parts to bring the machine 
to market.  If they *had* collaborated as described, I fear the PC would 
never have made it.  I tend to believe it arrived because the folks who 
designed it took the non-IBM approach, not in spite of that approach.

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