MAC
MAC is a really simple virtual machine. It's written in C and is [as of writing this] only one small C file.
Check out the improved version to see an implementation of more instructions, registers, etc...
I've also ported this to my own programming language, Ark. Check it out!
Instructions
op val usage function
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HLT 0 hlt halts program
PSH 1 psh val pushes <val> to stack
POP 2 pop pops value from stack
ADD 3 add adds top two vals on stack