Difference between compiled, interpreted, vm and jit

ericltw commented 6 years ago

Machine code is a computer program written in machine language instructions that can be executed directly by a computer's CPU.
Low-level programming is complex, it has low productivity and often generates errors difficult to find.
The overwhelming majority of practice programs today are written in higher-level language or assembly language. The source code is then translated to executable machine code by utilities such as compiler, assemblers, and linkers.

Once the program is written in high-level language, a translator converts it into machine language. This translator is called compiler.
The set of machine instructions that are understood by different processors can be different. Programs also call different operating system routines depending on the system. These differences have forced to create a specialized compiler for each type of machine and operating system, even though they use the same high-level language.
Cobol, C and C + + are the most representative high-level programming languages.
http://www.edu4java.com/_img/concepts/compiler-c.png

An interpreted language is any programming language that isn't already in "machine code" prior to run time. Unlike compiled languages, an interpreted language's doesn't happen beforehand. Translation occurs at the same time as the program is being executed.
Many of an interpreted language's instructions can be executed directly, without compiling to machine code; however, when certain code is required, an interpreter steps in during runtime and translates it on the spot.
PHP, Javascript, Java, C#, Python are the most representative high-level programming languages.

Compiled languages	Interpreted languages
Fast	Slow
Source code private	Source code public
Not cross-platform	Cross-platform
Requires extra compiling step	No extra step
Longer to development in	Easier debugging
Take more memory	More memory efficient

This is an intermediate concept, between the compiler and the interpreter, popularized by the Java programming language. The idea is to write programs in high-level language, but instead of compiling it (translating) into machine language, it translates it into an intermediate code. This intermediate code is executed by an interpreter know as Virtual Machine.
Java and C# are the best examples of this type of languages.
http://www.edu4java.com/_img/concepts/compiler-c.png

Java has a JIT compiler as part of the Java Virtual Machine(JVM); C# has one within the .NET framework; and Android has a JIT in its Dalvik Virtual Machine(DVM).
If typical compilers take source code and completely convert it into machine code, JIT take the same source code and convert it into an intermediary "assembly language", which can then pulled from when it's needed. And that's the key. Assembly code is interpreted into machine code on call—resulting in a faster translation of only the code that you need.
JITs are key not only because they reduce the CPU's workload by not compiling everything all at once, but also because the resulting compiled code is optimized for the particular CPU. It's why language with JIT compilers are able to be so "portable" and run on any platform or OS.

ericltw commented 5 years ago

JIT Compilation是兩種傳統機械代碼翻譯的組合，是由編譯(AOT)和解釋的組合，JIT編譯混合了編譯代碼的速度和解釋的靈活性，JIT編譯的目的是增加解釋器運行效率，如解釋器每次進入循環，都需要重新的解釋代碼。

ericltw commented 5 years ago

ericltw commented 5 years ago

ericltw / notes