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The Legacy of CPU Features Since 1980s
jones_supa writes: David Albert asked the following question:
"My mental model of CPUs is stuck in the 1980s: basically boxes that do arithmetic, logic, bit twiddling and shifting, and loading and storing things in memory. I'm vaguely aware of various newer developments like vector instructions (SIMD) and the idea that newer CPUs have support for virtualization (though I have no idea what that means in practice). What cool developments have I been missing? "
An article by Dan Luu answers this question and provides a good overview of various cool tricks modern CPUs can perform. The slightly older presentation Compiler++ by Jim Radigan also gives some insight on how C++ translates to modern instruction sets.
"My mental model of CPUs is stuck in the 1980s: basically boxes that do arithmetic, logic, bit twiddling and shifting, and loading and storing things in memory. I'm vaguely aware of various newer developments like vector instructions (SIMD) and the idea that newer CPUs have support for virtualization (though I have no idea what that means in practice). What cool developments have I been missing? "
An article by Dan Luu answers this question and provides a good overview of various cool tricks modern CPUs can perform. The slightly older presentation Compiler++ by Jim Radigan also gives some insight on how C++ translates to modern instruction sets.
The first large scale availability of virtualisation was with the IBM 370 series, dating from June 30, 1970, but it had been available on some other machines in the 1960's.
So the idea that "newer machines have support for virtualisation" is a bit old.
Watch this Heartland Institute video
The latest generation of CPUs have instructions to support transactional memory.
Near future CPUs will have a SIMD instruction set taken right out of GPUs where you can conditionally execute without branching.
The IBM 360/370 line and its successors have had decimal arithmetic (in addition to binary and after the 370/158 floating point) since the 1960/70s. Others have had these also.
I remember when VMWare first came out, and there was all this amazement about all the cool things you could do with Virtual Machines. Very little mention anywhere that these were things you could do for decades already on mainframes.
Same thing with I/O offloading (compared to mainframes, x86 and UNIX I/O offload is still quite primitive and rudimentary), DB-based filesystems (MS has been trying to ship one of those for over 20 years now; IBM has been successfully selling one (the AS/400 / iSeries) for 25, built-in encryption features, and a host of other features.
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Diminishing returns on cache misses
Higher latency of larger caches
Higher latency of more layers of cache
Poor transistor scaling of fully associate caches or increased rate of false evictions for n-way caches.
Increased power usage. It's very difficult to turn off part of your cache to save power, but it's very easy to turn off a core
Not all problems scale well with more cache
I'm sure there are many other reasons.
That was during the Mega/Gigahertz war, during the Golden Age of the Desktop.
Power Usage wasn't an issue. Heating wasn't much of an issue as you can fit big honking liquid cooled heat sinks to your CPUs. We had these big upgradable towers which gave us room to fill with stuff.
So we had hotter CPU's because we wanted the faster clock speed.
What happened? Well first 3ghz kinda became the max you can go, and we moved to more parallel systems. (Multi-Core CPU's, and GPU), and we wanted more portable devices. Laptops became far more popular then smartphone and tablets. So power usages, size, and heat became a bigger issue.
If something is so important that you feel the need to post it on the internet... It probably isn't that important.