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In short, WebAssembly is a binary instruction format for a
stack-based VM. To understand it better, we will talk a little bit more
about what a Binary Instruction Format means and what a
Stack-based VM means.
It was design for improving performance of web application and allow it to do computational expensive tasks that was previously not possible with JS. It is portable format, which can work in enviroment outside of browsers such as desktop/mobile apps, client/server application, IoTs, and even embedded devices.
Before WASM, asm.js existed that compiled statically-typed manual memory management language such as C to optimized version of JS. It used source-to-source compiler that targeted specific subset of JS. This subsets consisted of static types and virtually no garbage collection, which improved performanc by magnitudes. It also optimized size by employing techniques like dead-code elimination, and also remove unnecessary whitespace, newlines, etc and shortening long identifier to few chars identifiers.
Basically garbage means allocated memory that isn’t used anymore, and
collection means de-allocating this not-used-anymore memory.
In manual memory management languages, you allocate and deallocate
memory by yourself. But in Garbage Collected language, the language do
it for you. How GC does it and when depends upon implemented technique,
you wouldn’t want GC to do it work at inconvient time in inconvient
ways.
Just like humans use languages to understand and communicate with
each other, computer need something similar. Human languages consist of
atomic symbols such as alphabets, just as machine language consist of 0s
and 1s. These symbols randomly stringed together, themselves doesn’t
means anything. Just like kdjhsk doesn’t means anything,
random 0s and 1s 01010101 doesn’t means anything. Language
defines rules for these symbols and strings to give a meaning, like
hello means greeting in English languge. Zeros
and Ones can be given meaning with language designed particularly for a
machine.
For sake of understanding, we can design our own machine language.
Let say, our language is based on bit string of length 8-bit (called as
byte), which is analogous to a word in english language (although a word
can be of arbitarary length). To understand a sentence, we say that
string of bytes, start with a verb and than, if any, we have noun. For
example, a byte string "10100000 00100101",
10100000 is a verb and 00100101 is a noun.
Similarly, "10010011 00100100 00010000 10000001" is verb,
noun, verb and noun respectively. Then we can go an add rules for what a
verb and what a noun is. For example,
| Byte | Meaning |
|---|---|
| 00______ | Walk |
| 01______ | Run |
| 10______ | Sprint |
| 11______ | Stop |
Then we can define next 2-bits for vocal cords, and then next-next 2-bits and so on and on. For verb, it could means human names somehow mapped to a byte. This will effectively give us a way to command any (actually upto 2^8 = 255 here) humans in our machine languge.
There already exist machine language for computer, these language are
sometimes called Instruction Set Architecture (ISA), such
as x86_64, RISC-V, ARM64, etc.
WebAssembly is another one of these language, but for a made-up
hardware.
Virtual Machine (VM) as the name implies, a machine that is virtual and not a physical hardware. VMs are software that emulates a specific machine, typically they emulates ISAs. These works for decoding the binary files (executables), then it try to make sense of the structure, and then execute appropriate similar instruction that are available natively.
Stack-based means that it uses stack as storage for immediate temporary values instead of registers like ISAs. For example, you want to execute following line of code:
printf(1 + 2);The order of instructions and it execution look something like this:
| Instruction | Operation | Stack |
|---|---|---|
| 1 | push 1 | 1 |
| 2 | push 2 | 1, 2 |
| 3 | add | 3 |
| 4 | call printf | null |
So, WebAssembly is just a portable specification developed for a machine that uses stack, which main goal is to be used in Web. But since it is just a format, it can be used anywhere as long as VM is compilant and provides interface for the enviroment (just like how ISAs interface with OS enviroment through function calls and other stuffs).