The Shape of Code

What is the difference between compiled and interpreted programs?

In the good-old-days the distinction was easy to make: compiled code is executed by hardware while interpreted code is executed by software.

These days it can be very difficult to decide whether a program will be executed by hardware or software, and in some cases both may occur. More complicated cpus implement some of their instructions in micro-code (software control of very low level hardware resources) and the virtual machines specified for software execution can be implemented in hardware (an interesting project for a group of talented students in their summer holidays wanting to learn about ASICs).

Some people make a distinction based on the abstraction level of the cpu specification, e.g., very high level abstraction means the code must be interpreted. In practice the implementation of a cpu specification in hardware or software is an economic decision (software may be slow, but its a lot cheaper to implement).

I think there is a compiler/interpreter distinction, but the difference is not about how code is executed (the hardware/software distinction is a convenient difference that is easy to explain).

The compiler/interpreter distinction is a difference of responsibility. Compilers treat programs like the Spartans treated their children, they are bundled into a file of the appropriate format and left for the Operating system to load into memory and point the cpu at the first instruction (a cpu’s one interest is executing the sequences of instructions pointed to by the program counter). Interpreters are more like dotting nannies, organizing the provision of memory and on call to provide access to the desired resources.

Sometimes a language is classified as an interpreted language. There is no such thing as an interpreted language, only languages which are much more easily implemented using an interpreter than a compiler.

The performance of the Spartan approach may be very desirable, but the cost of achieving it can be very high.