A virtual machine isn’t exactly an alternative to compilers or interpreters. I think you are thinking of a JIT compiler, which is how many VMs are implemented. A virtual machine itself is exactly what the name says – it’s a machine (processor) that doesn’t actually exist. For example, most processors don’t have any intrinsic way … Read more
I assume you want a virtual machine rather than a mere interpreter. I think they are two points on a continuum. An interpreter works on something close to the original representation of the program. A VM works on more primitive (and self-contained) instructions. This means you need a compilation stage to translate the one to … Read more
This thread covers your exact question. Here is a soup-to-nuts implementation with complete documentation. I wrote a subroutine threaded Forth targeting 68K when I was in college. I defined the runtime environment and dictionary format, then wrote some C code that boot strapped a Macintosh application that loaded a default dictionary, populated some I/O vectors … Read more
There’s a definite trade-off between JIT and AOT (ahead-of-time) compilation. As you stated, JIT has access to run-time information that can aid in optimization. This includes data about the machine it’s executing on, enabling platform-specific native optimization. However, JIT also has the overhead of translating byte-code to native instructions. This overhead often becomes apparent in … Read more
When you create an application code, the apk file contains a .dex file, which contains binary Dalvik bytecode. This is the format that the platform actually understands. However, it’s not easy to read or modify binary code, so there are tools out there to convert to and from a human readable representation. The most common … Read more
In general, Just-in-time compilation refers to compiling native code at runtime and executing it instead of (or in addition to) interpreting. Some VMs, such as Google V8, don’t even have an interpreter; they JIT compile every function that gets executed (with varying degrees of optimization). On Stack Replacement (OSR) is a technique for switching between … Read more
wow, this turned out to be a lot more painful than I expected. 100% of the pain was linux protecting the program from being overwritten and/or executing data. Two solutions shown below. And a lot of googling was involved so the somewhat simple put some instruction bytes and execute them was mine, the mprotect and … Read more
There are a lot of misconceptions here. I suppose you could think of .Net as a virtual machine if you really wanted, but let’s look at how the .Net Framework really handles your code. The typical scenario looks like this You write a .Net program in C#, VB.Net, F#, or some other compatible language. That … Read more
Implemented in hardware, a register-based machine is going to be more efficient simply because there are fewer accesses to the slower RAM. In software, however, even a register based architecture will most likely have the “registers” in RAM. A stack based machine is going to be just as efficient in that case. In addition a … Read more
Constant pool is a part of .class file (and its in-memory representation) that contains constants needed to run the code of that class. These constants include literals specified by the programmer and symbolic references generated by compiler. Symbolic references are basically names of classes, methods and fields referenced from the code. These references are used … Read more