32 位和 64 位汇编程序之间的区别？

Question

As far as I noticed a 32bit program uses the FLAT memory model and the 64bit also. Using the 32bit program one has only 4GB to address and using 64bit (rcx for example) makes it possible to saturate the 40 to 48 address bits modern CPU provide and address even more.

So beside this and some additional control registers that a 32bit processor does not has, I ask myself if it is possible to run 32bit code in linux flawlessly.

I mean must every C code I execute be 64bit for instance?

I can understand that since C builds upon a stack frame and base pointer pushing a 32bit base pointer on stack my introduce problems where the stack pointer is 64bit and one might access the pop and push op codes in 32 bit fashion.

So what are the difference and is it possible to actually run 32bit code when running a 64bit Linux kernel?

[Update]

To state the scenario clear I am running a 64bit program and load a ELF64 file into memory map everything and call the method directly. The idea is to generate asm code dynamically.

Answer 1

The main difference between them is the different calling conventions. On 32bit there are several types: __stdcall, __fastcall, ...

On 64bit (x64) there's only one (on Windows® platforms, about others I don't know) And it has some requirements, which are very different to 32bit. More on https://future2048.blogspot.com Note that ARM and IA64 (Itanium) also are different Encodings as x64 (Intel64/AMD64)

And you have 8 more general registers r8..r15, with sub registers r8d..r15d, r8w..r15w, r8b..r15b For the SIMD-based code also 8 additional registers xmm8..xmm15 are present.

The exception handling is data-based on 64bit; on 32bit it was code-based. So on 64bit for unwinding exceptions no longer instructions are used to build the exception frame. The exceptiom handling is completely data-based so that no additional instructions are required to try/catch.

The memory limit of 2GB on 32bit apps (or with /LARGEADDRESSAWARE 3GB on an app on 32bit Win OS, or 4GB on 64bit Win OS) is now much larger More on https://msdn.microsoft.com/en-us/library/windows/desktop/aa366778(v=vs.85).aspx

And of course, the general purpose registers have 64bit width instead of 32bit. So any integer calculation can process values bigger than the 32bit limit of 0..4294967296. (signed -2147483648..+2147483647)

Also reading and storing memory with a simple MOV instruction can read and write a QWORD (64bit) at once; on 32bit that only could write a DWORD (32bit).

Some instructions have been removed: PUSHA + POPA disappeared. And one Encoding form of INC/DEC is now used as REX-Byte prefix Encoding.

Answer 2

Some 32 bit code will work in a 64 bit environment without modification. However, in general, functions won't work because the calling conventions are probably different (depends on the architecture). Depending on the program, you could write some glue to convert arguments to the calling convention you want. So you can't just link a 32-bit library into your 64-bit application.

However, if your entire application is 32-bit, you will probably be able to run it just fine. The word size of the kernel doesn't really matter. Popular operating systems all support running 32-bit code with a 64-bit kernel: Linux, OS X, and Windows support this.

In short: you can run 32-bit applications on a 64-bit system but you can't mix 32-bit and 64-bit code in the same application (barring deep wizardry).