Alpha CPUs, was Re: [rescue] i860 Success

Nathan Raymond nate at
Mon May 3 19:09:44 CDT 2004

On Mon, 3 May 2004, Phil Stracchino wrote:

> (However, I don't personally know whether the 68000 supported virtual
> memory.)

"The 68000 was also unable to correctly return from an exception on a
failing memory access, a crucial feature to enable true virtual memory. To
simulate unlimited RAM, one wants to interrupt when a memory access fails,
and then the interrupt routine will allocate a block of real RAM and read
a piece of data on disk into it. Several companies did succeed in making
68000 based Unix workstations with virtual memory that worked, by using
two 68000 chips running in parallel on different phased clocks. When the
'leading' 68000 encountered a bad memory access, extra hardware would
interrupt the 'main' 68000 to prevent it from also encountering the bad
memory access. This interrupt routine would handle the virtual memory
functions and restart the 'leading' 68000 in the correct state to continue
properly synchronized operation when the 'main' 68000 returned from the
interrupt. Obviously this is an expensive, tricky, and very inconvenient
technique, and they upgraded to the 68010 as quickly as possible.

"A more subtle problem was that the 68000 could not easily run a virtual
image of itself without simulating a large number of instructions. This
problem persists in many modern versions of the architecture, which is
rarely used in these applications. This lack caused the later versions of
the Intel 80386 to win designs in avionic control, where software
reliability was achieved by executing software virtual machines.

"In the next major revision, the 68010, most of these problems were

Nathan Raymond

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