Lots of good responses regarding why 128-bit isn’t a thing, but I’d like to talk about something else.

Extrapolating from two data points is a folly. It simply can’t work. You can’t take two events, calculate the time between them, and then assume that another event will happen after the same amount of time.

Besides, your points are wrong. (Edit: That also has been mentioned in another response.)

x86 (8086) came out in 1978 as a 16-bit CPU. 32-bit came with the 386 in 1985. x64, although described in 1999, was released in 2003.

So now you have three data points: 1978 for 16-bit, 1985 for 32-bit and 2003 for 64-bit. Differences are 7 years and 18 years.

Not that extrapolating from 3 points is good practice, but at least it’s more meaningful. You could, for example, conclude that it took about 2.5 times more to move from 32-bit to 64-bit than it did from 16-bit to 32-bit. Multiply 18 years by 2.5 and you get 45 years. So the move from 64-bit to 128-bit would be expected in 2003+45 = 2048.

This is nonsense, of course, but at least it’s a calculation backed by some data (which is still rather meaningless data).

What are you gonna do with 128 that you can’t do with 64

This is a bit pedantic, but x64 refers to Alpha, which existed long before 1999. 64 bit x86 (x86-64, or amd64) wasn’t purchasable until 2003, although it was announced in 2000.

There were several additional shifts between 1978 and 2003:

• 8088 / 8086 has what’s essentially bank switched 16 bit addressing which gives 1 MB, or 2^20 bytes
• 80286 has physical support for 16 megs, or 2^24 bytes
• 80386 has physical support 4 gigs, or 2^32 bytes
• Pentium Pro has PAE support for 64 gigs, or 2^36 bytes
• AMD Opteron from 2003 has support for 1024 gigs, or 1 terabyte, or 2^40 bytes
• Current AMD and Intel CPUs physically support anywhere between 2^48 and 2^57 bytes of physical hardware (256 terabytes to 128 petabytes)

But let’s just use three points of data: 8086 / 8088, 80386, and let’s say the first 64 bit AMD Opteron supports 64 bits:

• 8086 / 8088, 1978, 20 bits
• 80386, 1985, 32 bits
• AMD Opteron, 2003, 64 bits

1978 to 1985 is 7 years, with a change in addressing of 12 bits, or about .6 bits per year.

1985 to 2003 is 18 years, with a change in addressing of 32 bits, or about .56 bits per year. So far, pretty consistent.

How long would it take to go from 64 bits to 128 bits? At around .56 bits per year, that’d be about 114 years, and we’ve had twenty so far.

Check back in 94 years.

I think what you need to know, in layman terms, is that 128bit is not the double of 64bit. 65bit is double the amount of 64bit.

128bit is an absurd huge amount. And 64 is so much that even I as a radar engineer do not have to worry about it for a second.

Because there is no need from an address space or compute standpoint.

to understand how large 128bit memory space really is; you’d need a memory size larger than all the number of atoms in the solar system

In the rare cases where you need to deal with a 128bit integer or floating, you can do it in software with not that much overhead by concatenating registers/ops. There hasn’t been enough pressure in terms of use cases that need 128bit int/fp precision for manufacturers to invest the resources in die area to add direct HW support for it.

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FWIW there have been 64bit computers since the 60s/70s.

There have been a number of 128bit systems over the years.
As it is, 64bit should be good for the life of x86

Requirements aside you’d need an insane page cache to deal with the page table hierarchy. In most architectures you need a new level of page tables per 9 bits of address space.

What is this? The console wars of the 90s all over again?

No need. 32 bit became a hindrance because a 32 bit address bus can only address up to 4GB of RAM.

A 64 bit address bus can address up to 16 EB of RAM. Exabytes. To get there we would need to pass Gigabytes, Terabytes, and Petabytes of RAM capacity before we get to Exabytes.

That is a limit that we simply will never hit, at least not with silicon based computers.

The 32 bit limit was a real constraint, 64 bit is not. Also, modern architectures do actually compute 128 bit data in parallel (say 4x32 bit), so it’d just be a matter of representing that data on the screen in a 128 bit way. Any actual need for 128 bit can just be emulated, and it’s likely you don’t need to process such data at the limit of a 2023 tier processor anyway. In fact if anything for machine learning the direction seems to be going in the other direction, preferring faster hardware at half-precision (https://en.wikipedia.org/wiki/Half-precision_floating-point_format)

Modern CPUs have 512bit registers, and don’t need bigger memory addresses. Not sure what the issue is.

If it took 21 years to go from 32-bit to 64-bit, imagine it will take about 21^2 = 441 years to go from 64-bit to 128-bit. This is because 2^128 / 2^64 is the square of 2^64 / 2^32.

Would a third leg help you walk faster? Not the way we currently walk.

Is address space the only reason we moved away from 32-bit for high-performance computers though? Does 64-bit have any performance advantages over 32-bit apart from that? What about SIMD performance?

The world’s biggest super computer, Frontier, has 9,2 PB of RAM. It’s not available to one CPU, so no need to address everything in one address space, but let’s say it is. That still leaves room to build around 1 000 times more RAM into that theoretical CPU. I’m not sure we would be able to build such a computer today. One that needs more than ~10 000 PB RAM to address, which is what 128 bits means.

Sure, RAM isn’t the only reason for bigger address space, but there are also other ways to handle data beyond one address space. For the consumer, we are far from there.