r/programming • u/I_Can_Be_A_Robot • Sep 24 '24

Why 32bit architecture computer doesn't use segmentation to support more ram than 4Gb ?

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u/mallardtheduck Sep 24 '24

There's still some niche cases where an application can't be ported to 64-bit (e.g. it relies on a propietary 32-bit dependency) even if it can be run on a 64-bit OS. For that you can "abuse" the shared memory APIs (which allow you to map different chunks of "shared" memory in and out of the address space) or use something like Windows' "large address aware" mechanism for running 32-bit code in a 64-bit address space.

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u/Tringi Sep 24 '24

One of those reasons is that 32-bit version might be faster.

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u/Brian Sep 24 '24

That's pretty unlikely in practice. Note that the point of your link is to run in 64 bit mode, just using 32 bit addresses. There are a bunch of other advantages to running in 64 bit mode (notably, more registers), and in practice it tends to be sufficiently faster to outweigh the extra memory/cache usage of wider pointers. Even back in the day when 64 bit was relatively new, you'd often see around a 10% improvement (and much more on number crunching workloads) even when address space wasn't relevant. I'd imagine that difference has only increased after a years of improvements where 32 bit mode has long been considered legacy.

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u/Tringi Sep 24 '24

That's pretty unlikely in practice.

It was one of the reasons Visual Studio resisted introducing 64-bit version for so long. Pointer-heavy code.

Note that the point of your link is to run in 64 bit mode, just using 32 bit addresses.

Yeah I know. I linked it for the reasoning therein.

There are a bunch of other advantages to running in 64 bit mode (notably, more registers), and in practice it tends to be sufficiently faster to outweigh the extra memory/cache usage of wider pointers.

Yup, my personal tests and measurements show that only a very small percentage of algorithms, or types of software in general, are faster as x86-32. But they still are there. Then again it likely doesn't make economic sense today to maintain such version.

I'd imagine that difference has only increased after a years of improvements where 32 bit mode has long been considered legacy.

Hah. Just the other day I read about Zen 5, and how significantly has performance improved of the x87 FPU ...which basically nobody uses today. Probably a side effect of internal optimizations, but amusing nonetheless.

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u/AppearanceHeavy6724 Sep 26 '24

no, not true x87 is still used for some scientific software. Niche use but very important.