haha, i did the same way. But now i see there are lots of better solutions...

why does my solution always return 0?
registers are declared as global variables. Should I implement readReg / writeReg methods in my solution?

var reg = {'a':0,'b':0,'c':0,'d':0};
var stack = [];

function Machine(cpu) {

function exec(inst) {
console.log(inst);
const getArg = (arg) => /[0-9]+/.test(arg)? +arg : reg[arg];

inst = inst.match(/[0-9a-z]+/g);
console.log(inst);
switch(inst[0]) {
	case 'push':	stack.push(getArg(inst[1])); break;
	case 'pop':	if (inst[1]) reg[inst[1]] = stack.pop(); 	else stack.pop(); break;
	case 'pushr': stack.push(reg['a'],reg['b'],reg['c'],reg['d']); break;
	case 'pushrr': stack.push(reg['d'],reg['c'],reg['b'],reg['a']); break;
	case 'popr': [reg['d'],reg['c'],reg['b'],reg['a']] = stack.splice(-4,4); break;
	case 'poprr': [reg['a'],reg['b'],reg['c'],reg['d']] = stack.splice(-4,4); break;
	case 'add':	reg['a'] = Array.from({length:getArg(inst[1])},()=>stack.pop()).reduce((a,c)=>a+c); break;
}

}
return { exec }
}

Very good Kata!
...
Nice job, mr.MindWanderer!

You've been modifying inputs, haven't you?

Naughty naughty!

I think this Kata has some mistakes(

• = Thou shalt not clobber thy input! = - Expected: [[8.88, 7.96], [7.48, 1.91]], instead got: [[7.48, 1.91], [8.88, 7.96]]
  8.88 * 7.96 is less then 7.48 * 1.91 ?