Extend the witdh of base address in 386 segment selector to excess the 4GB RAM limit in 32-bit OS?

Question

As the memory requirement grows fast, today more and more system requires 64-bit machines to access even larger RAM.

FWIK in 386 protected mode, a memory pointer consists of two part: the base address (32-bit) specified by a segment selector, and the offset address (32-bit) added to the base address.

To re-compile all programs in 64-bit there's a lot of work to do, for example for C/C++ programs, the machine-dependent `int' type (which is 32-bit in 32-bit machine, and 64-bit in 64-bit machine) will cause problems if it's not used correctly. Even it's being rebuilt with no-problem, as the memory requirement continuous grow, for example someday we'll use 128-bit machines, and do we need to rebuild all the programs again to conform the new word size?

If we just extend the base address to 64-bit, thus make a segment like a 4GB window on the entire RAM, we don't even need a 64-bit OS at all, isn't it? Most of applications/processes won't have to access 4G+ memory, at server side, for example if a file server utilizes 20GB RAM for caching purpose, it may be split into 10 processes with each access 2GB, thus a 32-bit pointer is enough. And put each in different segment to cover 20GB memory.

Extend the segment limit is transparent to upper layer programs, what should be done is only about CPU and the OS, if we can let Linux to support to allocate memory on different 64-bit segments (though currently the segment base address is 32-bit yet), we can easily utilize 1TB RAM on 32-bit machine, isn't it?

Am I right?

Answer 1

The memory access is done on CPU, using assembly instructions. If the CPU have 32 bit for addressing a memory segment, it can address up to 4 GB, but no more. To extend this behavior, the CPU needs a 64 bit register.

A 32 bit OS has the same limitation. A 64 bit OS can execute 32 bit programs and make them access a base address higher than 4 GB, but needs a 64 bit processor.

As conclusion, the limit of the memory window accessible by the OS (and indirectly by the process running on that OS) are limited by the processor register width, in bits.

So, you are not right.

Propably the PAE fits your needs, but you need the hardware and the operative system support, which is very common as far as I know.

Answer 2

You can get exactly this effect today by running 32 bit processes on a 64 bit kernel. Each 32 bit process only has a 4GB virtual address space, but those addresses can be mapped anywhere in the physical memory accessible to the kernel. It's not done using segmentation, though; it's just done through paging.