0
$\begingroup$

So as the title suggests,

How does a programing language, such as Java or C, abstract itself away from underlying byte machines?

Im curious, because I know that static/dynamic typing, data abstraction and control abstraction play a role. But I don't understand it so I was wondering if anyone could shed a nice explanation on this question!

Thanks a lot in advance!

EDIT: I currently have this as an explanation so far..

Programming languages tend to abstract away from the underlying byte machines to provide a high-level view for the programmer. This can be for multiple reasons ranging from making code easier to write and debug, to solving platform issues.

One way languages abstract themselves away is through typing. Using java as an example, it uses predefined base types of a certain size, including but not limited to: int, float and char. It also uses object/structured types which have a dynamic size such as Object and String. Like some others, it provides a garbage-collection and automatic memory management utility. This makes it abstract even further away as you can worry less about allocating memory space to data types, compared to a language like C.

Improve this question
$\endgroup$
5
  • 3
    $\begingroup$ I'm not sure what kind of answer are you expecting. Do you want to know what the compiler does to translate, say, C to, say, x86 machine code? In that case, I think reading about compilers on Wikipedia might be a good start. And if you have a more specific question, you can ask here. Also, I have never heard the term "byte machine" and it seems google didn't either. $\endgroup$ – svick May 2 '16 at 14:27
  • $\begingroup$ C doesn't completely abstract away from the underlying byte machine. The range of an int depends on the machine. $\endgroup$ – Yuval Filmus May 2 '16 at 14:58
  • $\begingroup$ @svick I tried googling it. My lecturer put it on our slides and i can't seem to find an answer anyways. Its basically asking how languages abstract away from lower-level code such as binary/asm $\endgroup$ – madcrazydrumma May 2 '16 at 15:58
  • $\begingroup$ Your 'explanation' relates to the "why" of abstraction, not the "how"! $\endgroup$ – Johan May 2 '16 at 16:35
  • 1
    $\begingroup$ Can you be more precise about exactly what puzzles you? $\endgroup$ – user48832 May 2 '16 at 21:14
3
$\begingroup$

I'm assuming you are really asking

how is it possible that the same source code can be compiled to different architectures?

In order to achieve this feat different systems use different solutions.

Interpreter
Some languages use an interpreter. In this case there is no underlying architecture. The source code gets interpreted as written.

Virtual machine using byte code
Languages like C# and java compile to intermediary code (called byte code) for a virtual machine.
A runtime then executes this code somewhat like an interpreter would.
Frequently a just-in-time compiler further compiles (critical parts of) the byte-code to actual machine code.
So Java and C# do not care about the actual machine.

Virtual machine as intermediary target
Other compilers use the LLVM architecture. LLVM presents a virtual machine to the front end and a compiler using LLVM compiles for this virtual machine.
Specific LLVM backends tuned to a specific CPU architecture further compile this intermediary code to actual machine code.
LLVM differs from Java in that LLVM always compiles all code to machine code and it does not force any specific runtime upon the compiler.

Direct compilation
Traditional compilers like gcc (gnu C) compile the code to a specific architecture in a direct manner. Every architecture requires a different compiler which follows the language rules to create assembly code.
Maintaining separate compilers for different CPU architectures whilst ensuring all those abstractions work the same on all targets is accomplished through sheer hard work and coordination between compilers.

Hence the rise of intermediary virtual machines as compilation targets.

To what extend does a compiler abstract away the underlying CPU?

This differs per language. Older languages like C tend to less abstraction.
The trend in newer languages is towards more abstraction.
Some languages offer both abstract types (always the same irrespective of architecture) as well as implementation dependent types (different for different architectures).
An example of this is a NativeInt type that is 32 bits on X86 and 64 bits on X64.

CPU's do some of the abstraction
The other element in play is that CPU's are converging towards a single 'constellation'. More and more CPU architectures are using the X64 model of 32 bit integers and 64 bit memory.
Also many CPU's are backwards compatible with their predecessors.
As long as a compiler produces code that runs on a current or previous version of a CPU that CPU will run it.
So in that case it's the CPU that abstracts away (some of) the differences.

... Byte machine

A byte machine does not exist.
There is byte code, which is code for the virtual machine used by e.g. Java.
and there is machine code, which is what a CPU consumes.

Memory management
You talk about memory management.
This is a subject completely different from abstraction.
Every CPU architecture handles memory in essentially the same way.
All but the most esoteric CPU's have a stack and use pointer logic to address the heap.
The fact that e.g. Java and C# use a garbage collector is not technically abstraction, but relates to resource management.
How resource management gets done depends upon the runtime library in use by that language.

Dynamic types
Dynamic types are invented to solve the problem of buffer overflows (arguably the number one security issue facing computing today). If the runtime library handles string and array management then the burden on the programmer decreases.

How does a languages abstract away managed or dynamic types

By using a runtime library.
All usage of dynamic and managed types gets translated into calls to the runtime library.
E.g. when using a dynamic string, the runtime library handles the reference counting, copy-on-write and growing and shrinking the storage as needed.
None of these actions are visible in the source code.

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.