用 C++ 编写的程序如何在多个 CPU 架构上运行？

Question

I have a C++ program that I want to compile to assembly, and then assembler will compile it to machine code.

Now, as far as I know, in order to transform assembly code to machine code the assembler needs some kind of table to map assembly instructions to the actual machine instructions.

Which table will the assembler use? Is there a chance that my C++ program won't run on all CPUs, because CPUs use different tables which means that the same machine code will do different things on different CPUs?

Answer 1

The assembler assembles for whatever architecture it has been told to/programed to assemble for. As the assembly language for each instruction set architecture (ISA) differs, you can only assemble an assembly program written for one architecture for that same architecture. It is generally not possible to accidentally or intentionally assemble the program for the wrong architecture.

When you use a compiler, the compiler invokes the correct assembler with the correct flags to assemble the assembly code it generated for the architecture you told it to compile for. The resulting program will only run on processors of the ISA your have compiled it for. If you want the program to run on processors of a different ISA, you have to compile it for that other ISA.

If your program is poorly written, it is possible that it won't compile or work when compiled for other architectures than the one(s) you developed it for. Such a program is called an unportable program. However, unless you do weird things or make assumptions about properties of the architecture you are programming for, this is unlikely to happen.

Answer 2

In general what is call assembly is roughly a human readable (text) form of machine code (binary).

As franji1 said in a comment, in general compilers emit an intermediate abstract machine code from the source. And this kind of code can easily (it is intended to) be translated to assembly/machine code.

I have a C++ program that I want to compile to assembly, and then assembler will compile it to machine code.

This is what a compiler is designed to. Compiler is somehow misleading. Compiler can be the "compiler phase" or "compiler toolchain". compiler phase is the one that translate your source code to the intermediate abstract form, that then needs to be translated to target assembly/machine code by the assembler . Compilation is commonly what denotes the whole process from source code to executable machine code.

Now, as far as I know, in order to transform assembly code to machine code the assembler needs some kind of table to map assembly instructions to the actual machine instructions.

Roughly yes. This is what a document like Instruction Set Reference Manual is for: describing how textual form must be translated to byte form.

Which table will the assembler use?

See document...

Is there a chance that my C++ program won't run on all CPUs, because CPUs use different tables which means that the same machine code will do different things on different CPUs?

You have to generate a byte form of your program for each platform (machine/os). A compiler is designed to generate a machine code for a given platform that realizes exactly what your source code specifies. This is why compilers exist, to free you from writing program in assembly (that is very hard to do).