

The prospects for 128 bit processors (1995) - luu
http://yarchive.net/comp/128bit.html

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shameless_1
Scaling to a 128 bit system is pretty hard on the layout level. On current
designs, something like 95% of the surface is metal interconnects. Where
there's metal you're limited in freedom to place transistors. So with 128-bit
systems you're looking at 128-bit registers, address buses and data buses.
Those take up a lot of real estate on the die and you can only go so far in
narrowing them by using layers. 128-bit is a really really wide highway on a
CPU.

That doesn't mean that 128-bit systems can't be built. What it does mean is
that upgrading a current 64-bit CPU to a 128-bit CPU involves sacrifices.
Current-gen 64-bit systems contain a lot of optimizations in silico (branch
predictors, cache predictors, ...) and control structures and that's where the
problem lies: each optimization (literally) taps into that wide highway and
creates dead zones on the die that contain nothing more than metal. And
everywhere you tap you create zones where layering can only be applied with
reduced freedom.

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zw123456
There was once something call a cellarc CPU that promised a 1024 bit
architecture and teraflop speeds. Hard to find any info about it now.

