Question

I want to subtract two signed 8bit integers in JavaScript. As JavaScript doesn’t have a type for ‘signed’ or ‘unsigned’ integers I need to stick to the ‘number’ type – and cannot do some ‘cast-magic’ just like it would be possible using different languages.

I want to simulate a kind of CPU register and therefore need to work on as well the signed as the unsigned values. Additionally I need to calculate the values of the Carry and the Overflow flag. The Carry Flag is set when an unsigned overflow happens (the unsigned value has left the range from 0 to 255) while the Overflow Flag is set on a signed overflow (value is not is not in range -128 – 127).

This code seems to work but is quite cumbersome and ugly. Is there any “easy” way of doing this calculation?

Thanks in advance!

//Convert a signed 8bit number to a "real" number (254 => -2)
function toSigned(number)
{
    if (number & 0x80) //sign bit is set
        number = -(0x100 - number);
    return number
}
//Inverts the sign of a signed 8bit number
function invertSign(number)
{
    return 0x100 - number;
}
//subtracts source from target
function sub(target,source)
{
    //calculate carry
    var carry = (target - source < 0);
    console.log("Carry: %b", carry);

    //Calculate result & overflow
    var result = (target + invertSign(source)) & 0xFF;
    var overflow = !(result >= -128 && result <= 127);
    console.log("Overflow: %b", overflow);
    console.log("Result: %d - %d = %d",toSigned(target),toSigned(source),result);
}
//Examples
sub(8,254); //8 - (-2), no overflow, carry set
sub(8,2); //8 - 2, no overflow, no carry
sub(0x50,0xB0); //80 - (-80), overflow, carry

Answer 1

It seems that one of best options for you is Typed Arrays (it’s not completely supported by all browsers, see Can I Use: typedarrays)

var signed8bitArray = new Int8Array(1);
var unsigned8bitArray = new Uint8Array(1);

unsigned8bitArray[0] = 254;
signed8bitArray[0] = unsigned8bitArray[0];
signed8bitArray[0] == -2 // This is true

You may want to read Int8Array, Uint8Array and general documentation on JavaScript typed arrays from MDN.