Why doesn’t Windows use 64-bit virtual address space below 0x00000000`7ffe0000?

https://devblogs.microsoft.com/oldnewthing/20221216-00/?p=107598

by signa11 • 4 years ago

206 62 4 years ago

devblogs.microsoft.com

082349872349872• 4 years ago

tl;dr several reasons, the most specific of which is "to keep linker relocs simple"

alexklarjr• 4 years ago

…on a long dead architecture.

jasoneckert• 4 years ago

I still have Windows 2000 installed on a Digital Ultimate Workstation (2x Alpha 21164). I turn it on every few years when I need a nostalgia kick ;-)

ourmandave• 4 years ago

Chen is the poster boy for backwards compat.

They used to work very hard to make sure updates didn't break popular programs that used undocumented hacks.

herpderperator• 4 years ago

What does the ` mean?

muststopmyths• 4 years ago

That’s the convention on windows for denoting 64-bit addresses, separating the two 32bit parts with the backtick. Don’t recall why now.

avianlyric• 4 years ago

I assume it just makes the address easier to read. I imagine most programs don’t deal with address spaces larger than 4GB, so being able to quickly identify the lower bytes of the address when physically looking at a memory address is probably quite handy.

darknavi• 4 years ago

I enjoy the short and sweet blog posts from Raymond Chen on "Why does Windows X?".

krylon• 4 years ago

He is great at summing up these things in a way I can understand (most of the time, not being very experienced with Windows internals) without having the feeling he is dumbing it down, just that he is explaining it very clearly.

tyleo• 4 years ago

While this article doesn’t have it, I especially like his subtitles which are essentially a 1-line TL;DR of the content. As a recent example:

Title: Instead of a C++ template parlor trick, why not just add support based on whether the header file has already been included?

Subtitle: Header file inclusion order dependencies

analog31• 4 years ago

They need to reserve the first 2147352576 bytes for the Color Graphics Adapter display buffer.

tadfisher• 4 years ago

Has anyone tried loading COMMAND.COM in there? Surely CGA isn't using all that space.

TazeTSchnitzel2• 4 years ago

Tangent: on 64-bit Apple platforms, nothing is mapped in the lowest 4GiB of address space. This ensures any pointer truncated to 32 bits and dereferenced will segfault.

CalChris4• 4 years ago

This was only 'enforced' by the linker on x86. You could handcraft a Mach-O executable to get around it. But it is now enforced enforced by OSX on ARM.

I don't see any good reason for this. My own half assed explanation is they're enforcing 32b cleanliness. It's not a very good explanation. But it's my explanation for something I don't like.

worewood• 4 years ago

When migrating 32bit apps to 64bit platforms, it's common to follow the recipe:

1. Try to recompile on 64bit without any changes 2. Realize the program does not work because the types have different sizes now 3. Cast everything on sight to types that have same size as in 32bit. 4. Now the program "works" (as in, it compiled and maybe ran but you didn't test thoroughly)

This address space restrictions ensures that if you happen to cast a pointer in this way it will not work, forcing the developer to review what he's doing.

mrpippy• 4 years ago

You didn’t need to handcraft it, the linker has a ‘pagezero_size’ argument which allows you to set it as small as one page and then the rest of the low 4GB can be used.

ridiculous_fish2• 4 years ago

Can't one just remap the zero page?

TazeTSchnitzel2• 4 years ago

Maybe they're now relying on it for safety in core system libraries that get linked into your process? If you can be sure that the offset in *(ptr + offset) is always a 32-bit unsigned type, you get null pointer checking for free, right?

bunabhucan• 4 years ago

Has anyone here ever used Windows NT on a DEC Alpha?

A company in Ireland I worked for gave a mobile phone GIS demo to the Irish national phone company and Digital lent us a beefy server with two AXP CPUs (it was gonna be the future!) running windows NT. We literally pulled it on cobbled dublin streets on a a handcart to bring it to their offices.

avidphantasm• 4 years ago

Back around 2001, I bought a secondhand Alpha ATX mobo and for a lark installed NT 4 on it. I didn’t do much with it though.

jamesfinlayson• 4 years ago

What was the overlap of both products being commercially available? I didn't think it was too long.

peteri• 4 years ago

Wikipedia says it was only commercially available for NT4, I do somewhere have the Dec Alpha Windows 2000 beta discs (It got killed very late in the beta).

gavanm• 4 years ago

In the mid-90's, I worked at a place that had one used for our intranet server running Win NT with IIS.

Mostly used a DEC Alpha as a "visible" competitor to our major server provider at the time (Compaq) to try and manage the prices they were charging (at least until Compaq bought Digital, and then eventually bought by/merged with HP).

One interesting thing was that we had a faulty RAM chip that we only discovered due to a bug in website publishing to IIS causing a big memory leak which crashed the server. Once the the chip was replaced, we still had the underlying leak - but at least the server didn't crash any more.

oakwhiz• 4 years ago

Hand delivering servers and other gear is always a part of a good story. Makes me wish for a decent cart and maybe some brakes.

EvanAnderson• 4 years ago

I had a Customer in 2000 using SQL Server 7.0 on Windows NT 4.0 on an Alphaserver 1200. The performance, as compared to contemporary x86 machines, was very good. Was it worth the cost? I don’t know. I never did a dollars to performance benchmark. I suspect it wasn’t really worth the DEC tax.

icedchai• 4 years ago

In 1998-99, I worked at a company where we had an Alpha running Windows NT Server. The company was a large DEC customer and we had tons of Alpha servers sitting around.

olvy0• 4 years ago

Me, as a QA in Microsoft. I assume other groups in Microsoft at the time were running it too. I wrote a bit about that here:

https://news.ycombinator.com/item?id=27639461

13of40• 4 years ago

I had one for running very early builds of Win64 at Microsoft. It was a very aesthetically pleasing piece of hardware. The builds of Windows I ran on it were truly at the stage of "hey it built" so I remember getting a dialog one time that said something like "Invalid Error Code" with half the warning icon rendered and the other half random garbage. We moved on to IA64 and AMD64 shortly afterwards.

p_l5• 4 years ago

I recall my father mentioning having an Alpha with Windows NT4 available at work, circa ~1998. This was probably explicitly to run computational packages like ANSYS. It wasn't main work machine, which AFAIK was something more like random Pentium, maybe Pentium Pro (doubtful - I think there was a dual Pentium Pro which was similarly a shared machine but not daily driver)

ggm• 4 years ago

I worked briefly on a real-estate web system which ran on it. It wasn't bad, as these things go (I was coming off OSF/1)

nikau• 4 years ago

Only once for around a week, we purchased a alphaserver and it didn't come with the correct OSF/1 installation media, so one of the guys installed NT4 just to see what its like - we were all impressed as it ran multiple copies of Doom without issue.

acdha• 4 years ago

Yes – back in the 90s, my first tech job was doing QA for a COBOL vendor and we had that on our support matrix. The Alpha processors were really fast but did you ever know there was A VERY SERIOUS COMPUTER in that room with the industrial fans and a case like a filing cabinet which probably weighed 50 pounds empty. We even for a time had an Alpha on Linux build for a single customer.

titzer2• 4 years ago

No. The virtual address space starts at the 64KB boundary. You can confirm this by calling GetSystemInfo and checking the lpMinimumApplicationAddress. It will be 0x00000000`00010000.

Aw, man. I recently found out there is also a similar restriction on Linux. Bummer, because I have a good use for putting code into the lower 64KB of memory (not 4KB, still want to use that for catching NPEs). My use case is a fast interpreter for bytecode. Since every bytecode handler needs an entry in the dispatch table, and I have 7--yes 7!--dispatch tables, I'd like to make their entries 2 bytes. So instead of taking 1KB each, they would take only 512B each. But alas.

Another great use case is Wasm. Since the memory is a sandboxed 4GB range and only indexed into by 32-bit offsets, placing it at virtual address 0 would save an add instruction (or use of segment register) on every access.

p_l5• 4 years ago

On linux it depends on a sysctl parameter, and on 4kB platforms some runtimes used the fact that it was set to "one page" as a method to have very short pointers starting at address 4097 for things very often references (and which might need guard page, like NULL value).

Made for some borked code with Clozure Common Lisp when people started running 64kB pagesize on PPC64, and that meant that even with minimal zero page it still was too big to map the "low memory" where CCL stuck it's NULL definition.

messe• 4 years ago

Since every bytecode handler needs an entry in the dispatch table, and I have 7--yes 7!--dispatch tables, I'd like to make their entries 2 bytes. So instead of taking 1KB each, they would take only 512B each. But alas.

Hmm. If the upper half of the handler addresses are the same, then why not store just the lower 2-bytes of each handler address in the dispatch table. If you're on x86, there's no add needed if the upper bytes of the register you're loading into already contain the upper half of the address, for example:

   mov eax, HANDLER_BASE
   mov ax, [edi + ebx]

titzer2• 4 years ago

Interesting idea. I think it means using another register to permanently hold the HANDLER_BASE though? The dispatch sequence is only 4 instructions. I tried it as a delta encoding, requiring another add instruction on every dispatch, but that is an approx. 5% penalty. So I was hoping that putting it at absolute address < 64KB I could do the sequence without another add.

cihangir• 4 years ago

That is a special page of memory that is mapped read-only into user mode from kernel mode, and it contains things like the current time, so that applications can get this information quickly without having to take a kernel transition. This page is at a fixed location for performance reasons.

I didn't know there was a vdso-like mechanism in Windows as in Linux.

monocasa• 4 years ago

Yeah, it's called KUSER_SHARED_DATA, and contains a lot of interesting items.

https://learn.microsoft.com/en-us/windows-hardware/drivers/d...

Why doesn’t Windows use 64-bit virtual address space below 0x00000000`7ffe0000?

Related Stories