But if you really need more than 4GB of memory, then sure, go ahead and use it.
On x86-64, the start of the linear memory is typically put into one of the two remaining segment registers: GS or FS. Then the code can simply use an address mode such as "GS:[RAX + RCX]" without any additional instructions for addition or bounds-checking.
[1] https://devblogs.microsoft.com/oldnewthing/20070801-00/?p=25...
https://gcc.gnu.org/onlinedocs/gcc/Named-Address-Spaces.html
Unfortunately the obvious `__attribute__((mode(...)))` errors out if anything but the standard pointer-size mode (usually SI or DI) is passed.
Or you may be able to do it based on x32, since your far pointers are likely rare enough that you can do them manually. Especially in C++. I'm pretty sure you can just call "foreign" syscalls if you do it carefully.
Especially how you could increase the segment value by one or the offset by 16 and you would address the same memory location. Think of the possibilities!
And if you wanted more than 1MB you could just switch memory banks[1] to get access to a different part of memory. Later there was a newfangled alternative[2] where you called some interrupt to swap things around but it wasn't as cool. Though it did allow access to more memory so there was that.
Then virtual mode came along and it's all been downhill from there.
[1]: https://en.wikipedia.org/wiki/Expanded_memory
[2]: https://hackaday.com/2025/05/15/remembering-more-memory-xms-...
Schulman’s Unauthorized Windows 95 describes a particularly unhinged one: in the hypervisor of Windows/386 (and subsequently 386 Enhanced Mode in Windows 3.0 and 3.1, as well as the only available mode in 3.11, 95, 98, and Me), a driver could dynamically register upcalls for real-mode guests (within reason), all without either exerting control over the guest’s memory map or forcing the guest to do anything except a simple CALL to access it. The secret was that all the far addresses returned by the registration API referred to the exact same byte in memory, a protected-mode-only instruction whose attempted execution would trap into the hypervisor, and the trap handler would determine which upcall was meant by which of the redundant encodings was used.
And if that’s not unhinged enough for you: the boot code tried to locate the chosen instruction inside the firmware ROM, because that will have to be mapped into the guest memory map anyway. It did have a fallback if that did not work out, but it usually succeeded. This time, the secret (the knowledge of which will not make you happier, this is your final warning) is that the instruction chosen was ARPL, and the encoding of ARPL r/m16, AX starts with 63 hex, also known as the ASCII code of the lowercase letter C. The absolute madmen put the upcall entry point inside the BIOS copyright string.
(Incidentally, the ARPL instruction, “adjust requested privilege level”, is very specific to the 286’s weird don’t-call-it-capability-based segmented architecture... But it’s has a certain cunning to it, like CPU-enforced __user tagging of unprivileged addresses at runtime.)
Isn’t that an arbitrary string, though? Presumably AMI and Insyde have different copyright messages, so then what?
If the search doesn’t succeed or if you’ve set SystemROMBreakPoint=off in the [386Enh] section of SYSTEM.INI[1] or run WIN /D:S, then the trap instruction will instead be placed in a hypervisor-provided area of RAM that’s shared among all guests, accepting the risk that a misbehaving guest will stomp over it and break everything (don’t know where it fits in the memory map).
As to the chances of failing, well, I suspect the original target was the c in “(c)”, but for example Schulman shows his system having the trap address point at “chnologies Ltd.”, presumably preceded by “Phoenix Te”. AMI and Award were both “Inc.”, so that would also work. Insyde wasn’t a thing yet; don’t know what happened on Compaq or IBM machines. One way or another, looks like a c could be found somewhere often enough that the Microsoft programmers were satisfied with the approach.
At least most people design non-overlaping segments. And I'm not sure wasm would gain anything from it, being a virtual machine instead of real.
This multi-memory setup reminds me of my array juggling I had to do back then. While intellectually challenging it was not fun at all.
With 64-bit pointers, you can't really reserve all the possible space a pointer might refer to. So you end up doing manual bounds checks.
Can't bounds checks be avoided in the vast majority of cases?
See my reply to nagisa above (https://news.ycombinator.com/item?id=45283102). It feels like by using trailing unmapped barrier/guard regions, one should be able to elide almost all bounds checks that occur in the program with a bit of compiler cleverness, and convert them into trap handlers instead.
Yeah, certainly compiler smarts can remove many bounds checks (in particular for small deltas, as you mention), hoist them, and so forth. Maybe even most of them in theory?
Still, there are common patterns like pointer-chasing in linked list traversal where you just keep getting an unknown i64 pointer, that you just need to bounds check...
With 64-bit addresses, and the requirements for how invalid memory accesses should work, this is no longer possible. AND-masking does not really allow for producing the necessary traps for invalid accesses. So every one now needs some conditional before to validate that this access is in-bounds. The addresses cannot be trivially offset either as they can wrap-around (and/or accidentally hit some other mapping.)
The biggest contributor to pointer arithmetic is offset reads into pointers: what gets generated for struct field accesses.
The other class of cases are when you're actually doing more general pointer arithmetic - usually scanning across a buffer. These are cases that typically get loop unrolled to some degree by the compiler to improve pipeline efficiency on the CPU.
In the first case, you can avoid the masking entirely by using an unmapped barrier region after the mapped region. So you can guarantee that if pointer `P` is valid, then `P + d` for small d is either valid, or falls into the barrier region.
In the second case, the barrier region approach lets you lift the mask check to the top of the unrolled segment. There's still a cost, but it's spread out over multiple iterations of a loop.
As a last step: if you can prove that you're stepping monotonically through some address space using small increments, then you can guarantee that even if theoretically the "end" of the iteration might step into invalid space, that the incremental stepping is guaranteed to hit the unmapped barrier region before that occurs.
It's a bit more engineering effort on the compiler side.. and you will see some small delta of perf loss, but it would really be only in the extreme cases of hot paths where it should come into play in a meaningful way.
Why does it need to trap? Can't they just make it UB?
Specifying that invalid accesses always trap is going to degrade performance, that's not a 64-bit problem, that's a spec problem. Even if you define it in WASM, it's still UB in the compiler so you aren't saving anyone from UB they didn't already have. Just make the trapping guarantee a debug option only.
Seems like they got overly attached to the guaranteed trapping they got on 32-bit and wanted to keep it even though it's totally not worth the cost of bounds checking every pointer access. Save the trapping for debug mode only.
Maybe. Bugs that come from spooky behavior at a distance are notoriously hard to debug, especially in production, and it's worthwile to pay for it to avoid that.
To operate on any other size, you need to insert extra instructions to mask addresses to the desired size before they are used.