Can a 32-bit OS machine use up all 8GB RAM + 20GB page file?
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As I understand it, this mode gains performance by sticking with 32bit pointers in user-space instead of 64bit pointers, while still allowing for taking advantage of all the new registers etc available in x86-64 ("amd64") CPUs. (Many programs doesn't really need more than 4gb virtual address space, and so using full 64bit pointers can cause quite the overhead - imagine doubly-linked lists with small data structures, for example)
But it's also yet another architecture. It's not binary compatible with either i686 or x86_64. You need your whole userspace compiled to use it. Middleware with embedded assembly (there's a surprising amount of this in glibc, and of course things like ffmpeg, libjpeg, etc...) needs to be ported. You can't run 32 bit binaries from proprietary sources.
And frankly the benefit over straight x86_64 is quite modest. I don't see x32 taking off. It's just not worth the hassle.
Some numbers mentioned on the x32abi page hint on anywhere between 4% and 40% performance gains, if that is true then I'd think the benefits would outweigh the hassle of another architecture.
(Edit: Most middleware also ship straight-C versions of the routines; whether or not an x32 C compiler can measure up to handcrafted x86 or x86_64 assembly I don't know - but I'm guessing the much higher register count would help a lot. Regarding proprietary software: There are a great many server configurations that do not need anything beyond the standard open source packages available in Debian)
Could the cpu have a mode/flag where all pointers in cpu registers were treated as 32 bits (high 32 bits ignored)?
Alignment issues might make this impossible (loading a 32 bit address in a compatible way from memory/cache...) but it would be neat if it could work. For instance, if memory layout in 32 bit chunks were A... B..., there would have to be a way to load both A and B into the low 32 bits of a register.
Of course, there are performance advantages to bigger pointers too, sometimes. For instance, it can be easier for a garbage collector to identify pointers is the ratio of memory addresses in use to total addresses is small.
Flat real mode was a hack mode on old 32bit intel systems to enable flat memory mapping with 16bit code. You'd jump into protected mode, map the full 4gig address space flat, then jump back into 16bit real mode without remapping that memory space.
Afterwards you could access all your memory in a flat address space rather than using 64K segment:offsets. You had to prefix your memory access instructions with a 0x66 byte though. Also, you had to write your own runtime library.
This is the exact opposite, Giving you all the registers, but not the memory address space.
I'm curious about what you're thinking about when you say "write your own runtime library" - wasn't the whole purpose of this "unreal mode" and staying in 16bit such that you would still have access to DOS and its API? (compared to switching to full 32bit mode, where DOS went out the window unless you implemented or embedded a "DOS-extender"?).
I rewrote all the string handling, memory management, etc functions in the Borland pascal and c++ runtimes to let me use all the memory without worrying about segments/offsets.
So, you got the DOS functions, but you also got printf, port io, memory functions, a bunch of stuff I can't remember, etc.
Basically you got the primary benefits (for the needs of the time) of pmode - flat memory, without most of the pain. Seems like the same situation here - you get all the new registers, but without the pointer overhead.
EDIT: One point to note is that your Code, Data and Heap segments all stayed 64K. This bit me in the ass once when I was presenting. Two bits of code had been tested separately when brought together went over the 64K code and heap limits. That was embarrassing.
8-bit computers running chips such as the Z80 and 6502 were limited to 16-bit addressing and a total of 64k of addressable RAM. For most computers earlier on this wasn't a problem, but in the later 80s when machines with 128k started to become more common this meant that there were lots of strange quirks to be considered.
For example, the Sinclair ZX Spectrum (Timex TS2000 in the US) originally came with 16 or 48k of RAM and 16k of ROM (64k of total addressable memory). When the +128k came out in 1985, Sinclair used a fairly interesting way of getting around the 64k limit. The memory was turned into 16k banks, which were the manipulated via a port. A bit more info on how this worked is available here[1].
Incidentally someone built a 4Mb RAM upgrade for the Spectrum![2]
[1] - http://www.worldofspectrum.org/faq/reference/128kreference.h...
Also, swapping memory in and out of the address space is a rather costly operation. And it becomes infeasible if the process's working set grows larger than the available address space [thrashing].
Nope. They can't actually disable it since the NX bit requires that PAE be enabled.
The limitation comes from licensing and tiering, and is handled in the kernel independently from PAE.
It'll likely be your graphics card uses a portion of your main RAM for its memory. This is common.
That is not correct. Windows also supports full PAE, but adds licensing-based restrictions. The Datacenter and Enterprise editions of Windows Server 2008 will provide access to all 64GB RAM PAE can provide.
Of course, you might make sure that appropriate drivers for your laptop exist for x64. I would hope any laptop that can accommodate 8GB would have x64 drivers.
PAE is used everywhere and has been around since 1995 Pentium Pros. So the answer is yes, a 32 bit OS/CPU can use 8 gigabytes (and more) of physical memory and an infinite amount of swap space. It all depends on the CPUs physical address size, not virtual.
My "64 bit" CPU has 36 bit physical addresses and 48 bit virtual addresses.
Just to clarify for anyone that doesn't catch what you mean and thinks PAE is the devil:
x86-64 CPUs must support in PAE when operating in long mode (64 bit mode). Further the version of PAE used is 'classic PAE' with an additional translation layer added to get some more physical address space. In other words even 64bit CPUs don't have a real 64bit physical address space, they still use PAE to access large amounts of ram.
http://en.wikipedia.org/wiki/Physical_Address_Extension#Oper...
That being said, I still don't fully understand; Once I install the new RAM, will I only receive the benefits if I perform a new 64-bit install of Windows 7, or will upgrading just amplify the benefits? (I currently have 3 GB in 1 GB + 2 GB as the default factory configuration.)
Sorry if this question is dense of me, but my field is economics and the more technical aspects of HN are often lost on me
So AD1066 doesn't need a 64-bit Windows, just any 32-bit Windows newer than XP.
No. Non-server 32b Windows will never give access to more than 4GB RAM to the user unless you patch the kernel. XP 32b, Vista 32b and 7 32b — including Ultimate for the latter 2 — all restrict "available physical memory" to 4GB in all situations. So do many server editions as well, for Windows Server 2008 x86 for instance, only the Enterprise and Datacenter editions will give access to more than 4GB RAM (up to 64).
A 2008 Enterprise license is ~$1500. I doubt ad1066 has any desire to pay such a price.
That is correct, the alternative is to use a ($1000+) Server edition, some of which do give access to >4GB in 32b.
If you put 8GB RAM in a laptop running a 32b non-server Windows, you'll simply see 4GB.
You may see very small improvements nonetheless due to dual-channel activating (when pairing identical sticks of RAM, the CPU is able to "talk" to both at the same time rather than one at a time, doubling bandwidth), but I rather doubt it. The gains are usually insignificant. And you won't get any gains from actually increasing the amount of RAM in the machine.
That's how the 2GB limit on Starter, the 4GB limit on non-server and the various limits on server editions are implemented.
And there is a kernel hack/patch to lift that restriction, although it is of course fully unsupported: http://superuser.com/questions/52275/how-can-i-enable-pae-on...
>> a process can have more memory than address space
How does that work? The best I can come up with is the author is referring to mmap? In which case the address space is still limited to 4Gb although I concede that with the OS pagefile it would be possible to have more than 4Gb in core at a time via judicious use of mmap.
[1]: http://en.wikipedia.org/wiki/Address_Windowing_Extensions
I can't speak for Unix systems, but on the Windows side, it's complicated and messy:
PAE (Physical Address Extensions)
Client editions of Windows do not support PAE while server editions do. If you want to physically address more than 4GB of RAM you need PAE. Realistically, you're unlikely to be able to address even 4GB under non-PAE x86 due to reserved memory ranges from hardware taking a chunk.
Funfact 1: As for why PAE isn't supported on client editions, this isn't a licensing problem (you can buy x64 editions for the same price that have massively increased RAM limits), but rather, Microsoft playing it safe with driver compatibility. Apparently, during testing a non-trivial number of drivers were found that made assumptions that don't hold under PAE and thus broke horribly. Things like using the high bit of a 32-bit pointer for data storage. Rather than risk a tsunami of bluescreens caused by poorly written drivers (which people would blame MS for out of naivety) the decision was made to simply not support PAE on client editions. Server editions do as there's the reasonable expectation that servers should be running higher quality drivers.
Funfact 2: Technically, PAE is supported on client editions, as it's required for hardware DEP support, so it's more a hard limit on addressable physical memory than it is a lack of support for PAE itself.
AWE (Address Windowing Extensions)
This is a Windows-specific capability (unlike PAE, which is a hardware capability provided by modern CPUs) which allows a process to address more memory than it has virtual address space. Further, it works hand-in-hand with PAE as you by definition need PAE support in order for the OS to address memory higher than 4GB in the first place. However, in order to use AWE, the application has to be explicitly written to utilise it as the usage of AWE requires the application itself to take a far more active role in managing its memory. AWE support is indicated via a flag in the PE header of the application binary. Needless to say, a tiny minority of applications have this support. The only one I could name right now would be Microsoft's SQL Server.
4GT (4-Gigabyte Tuning)
This is one other Windows-specific capability that isn't really related to a discussion on 4GB limitations but inevitably will be raised regardless. While PAE provides for the OS to address more than 4GB of physical memory, and AWE provides for an application to address more memory than in its virtual address space, neither of these features increase the amount of addressable memory at any given time in an application (ie. AWE allows you to change your view of memory, but not address more than 2GB of memory simultaneously). A Windows process in 32-bit has a 2GB virtual address space, with the remaining 2GB mapped to the kernel address space. 4GT, in contrast to PAE and AWE does allow you to modify the address space available to a user-mode application. By providing the appropriate bootloader parameter you can modify the usermode/kernelmode split to skew it up to a maximum of 3GB/1GB user/kernel-mode respectively. This used to be done quite a lot on large Terminal Servers for example, or to optimise database servers, however, the ramifications of doing so need to be carefully understood. You are reducing the amount of memory available to the kernel, and so, memory hungry drivers may now fail to load, or worse, outright crash the system if they haven't been coded to properly handle low memory conditions.
EDIT: I forgot to mention, that like AWE, an application won't even use the increased address space 4GT provides unless it is marked as capable in its PE Header. Merely enabling the setting won't get you anything unless your application supports this capability. Firefox, as an example, is large address aware and being 32-bit, can thus address up to 3GB instead of 2GB. While on 32-bit systems this capability is not likely used (how many 32-bit Windows users have modified their bootloader parameters to support 4GT?) it is fully used on 64-bit Windows and can use the full 3GB address space out of the box. And god knows Firefox needs it...
In summary, in the age of x64, I tend to view all of the above as far more trouble than it's worth. The virtual address space of a Windows 64-bit process is currently 8TB. Windows 7 Professional and up give you 192GB of physical memory, while Windows Server 2008 R2 Enterprise and up give you up to 2TB. So if at all possible, avoid 32-bit, 64-bit is the solution; PAE, AWE & 4GT were only ever workarounds.
If any of the above is incorrect, please correct me, as it's been a while since I studied much of the above, and if anyone can shed more light on the Unix situation, I'd be interested (in a very nerdy way).
That is not correct, client editions fully support PAE.
> As for why PAE isn't supported on client editions
It is supported (and enabled), the limits are implemented separately and independently.
> this isn't a licensing problem (you can buy x64 editions for the same price that have massively increased RAM limits)
Windows 7 Home Basic x64 will only make 8GB RAM available, Premium will restrict to 16GB, professional and up will allow up to 192GB. Meanwhile Server 2008 R2 Enterprise and Datacenter allow 2TB. These are very much licensing restrictions, as is Windows 7 Starter (x86)'s 2GB limit.
> but rather, Microsoft playing it safe with driver compatibility.
That is the excuse they give for it, yes. It just happens to make no sense when applied to the x64 limits I quoted above, to Starter's 2GB limit (and Vista Starter's even lower 1GB) or to e.g. Windows Server 2008 R2 Foundation's 8GB limit (2008 R2 only runs on x64 and Itanium so 32b isn't even remotely a factor)
> so it's more a hard limit on addressable physical memory
It's not "hard", since it's merely based on the licensing mode of the kernel. It can even be hacked out.
Why not go x64? x32 can be faster, and MS does not allow OS upgrades from 32 to 64 bit.
PAE isn't a workaround, and is mandatory for x86-64 when operating in long (64bit) mode as it is still how the CPU addresses physical memory.