Memory limits in 16, 32 and 64 bit systems

Question

The theoretical memory limits in 16, 32 and 64 bit machines are as follows:

• 16 bit = 65,536 bytes (64 Kilobytes)

• 32 bit = 4,294,967,296 bytes (4 Gigabytes)

• 64 bit = 18,446,744,073,709,551,616 (16 Exabytes)

I remember from DOS / Windows 3.11 days, that 16 bit memory could be separated into segments, so that a 16 bit machine could access a greater amount of memory than 64 Kilobytes.

I have a machine with 16GB of memory, and am dual booting a 32bit operating system and a 64bit operating system. I can access all 16GB from 64bit, but only 3.21GB in 32bit.

So, my question is, if 16bit operating systems allowed greater that 64KB memory access due to memory segmenting, why do 32bit machines not follow the same pricipal?

Answer 1

They do, the system is called Physical Address Extension (PAE). Here is a list of windows OS'es and their max memory, any 32 bit system that allows for more than 4GB of RAM is using PAE to access the memory (For example Windows 2003 R2 Datacenter 32 bit allows for 128GB of ram).

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In fact Windows 8 requires a PAE capable CPU in it's minimum requirements.

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To address your "unasked" question on why your 32 bit OS can't access the ram if it exists: Licensing. They choose not to allow RAM to be above 4GB for their 32 bit OSes unless you pay for a data-center edition (that is why they sell a data-center edition, if you need that much ram, you likely can afford to spend more money on a OS).

Answer 2

Instead of explaining it myself, I'll let someone who has to maintain a kernel with PAE support speak in his charming ways, Linus Torvalds

• PAE sucks (Why didn't MS take advantage of PAE?)

Also keep in mind that the PAE support in Windows 32bit versions comes for a lot of cash. XP won't even be able to make use of full 4 GiB of RAM normally, because MS chose to not enable PAE features on it. A kernel that is closely related, Windows 2003 Server, does support PAE. However, even there your "Standard edition" will only support up to 4 GiB (but working around the BIOS memory hole), whereas the more expensive editions will then allow up to 64 GiB of RAM. The same holds for 32-bit Vista.

However, not in all cases is this limitation imposed by Windows. If it were, booting a PAE-enabled Linux kernel would still enable you to use the full 4 GiB (or more). Not so, some hardware manufacturers chose to impose this limitation at the BIOS level, although the CPU and chipset would be capable of handling PAE.

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Just a side-note: none of the current x86-based 64bit processors can even address the full range of the 64bit address space physically (for reference see this question and answers).

Answer 3

8-bit CPUs usually had a 16-bit address bus. (Motorola had a unified address bus, RAM and peripheral I/O shared the same address space, Intel chose to divide the two. In the case of Intel, the IO address limits of the 8088 and 8086 carried the limits over from the 8080 & 8085 CPUs.)

Intel's 8088 and 8086 had a 20-bit memory address bus(1MB), while Motorola's 68000 had a 24-bit address bus (16 MB). IIRC, the [80]286 jumped to a 24-bit address bus. Both later expanded to a 32-bit address bus with the [80]386 and the 68020 respectively.) With the Pentium chips, the address bus expanded to 64-bits. (I think the Motorola/IBM venture PowerPC chips also went 64-bit address bus.)

Memory available below and up to the maximum that could directly be accessed by the CPU was only limited by the supporting hardware chips (chipset) and OS. Bill Gates was famous in the past for stating that nobody needed more than 640K of RAM, thus DOS never evolved to directly access more RAM. With HiMem.sys and EMM386, DOS was extended to access more "upper" memory, with EMM386 being used to directly access all available RAM. HiMem.sys had less flexibility and could basically use the extra RAM for storage.

Memory exceeding that limit required a MMU (Memory Management Unit) to break the memory into segments and map it into the addressable memory space of the CPU. It's how the CoCo 3, Commodore 128, and other 8-bit computers could access more than 64K of RAM.

More favorable now is to use virtual memory to extend past physical memory limits, albeit with the limits imposed by the OS.

Answer 4

Because there is no practical reason to do so. Physical Address Extensions allow much the same functionality and their use is still very limited amongst users. In the Windows 3.1 days there were constraints that just aren't present today.

Answer 5

The theoretical memory limits in 16, 32 and 64 bit machines are as follows ...

The fundamental flaw here is the notion that the "bit width" of the processor, which is usually the size of the machine's general-purpose registers, is necessarily the same as the width of RAM addresses.

In x86 with paging enabled, but without PAE, the addresses that program and OS code use are called "linear addresses" by Intel - we usually call them "virtual addresses". They're 32 bits wide. This permits a 4 GiB virtual address space.

But it is more or less coincidence, merely an artifact of the format of page table entries that the size of a physical (RAM) address is also 32 bits.

With PAE the latter is 36 bits (at first... wider in later implementations). So, just because it is, for example, a "32 bit machine" does not mean that physical memory addresses are limited to 32 bits.

The industry has a long history of machines whose "bit width" did not match their maximum physical address size. For example, the VAX architecture defines a 32-bit machine, and virtual addresses (which are the addresses used by code once address translation is turned on) are indeed 32 bits wide... but the VAX's physical addresses are only 30 bits wide - and half of the physical address space is devoted to I/O device registers, so maximum RAM was only 512 MiB.

Even without address translation hardware, it is not necessarily the case that the machine's "bit width" defines the maximum RAM address. Example: The CDC "upper 3000" series were 36-bit machines. Do you think they could address 64 GiB of RAM? Not hardly! Those machines came out in the mid-60s! Heck, we couldn't even have 64 GB of disk space in those days. (The CDC 6000 series were 60-bit machines. Need I go on?)