GameBoy Emulation in JavaScript: The CPU (2010)(imrannazar.com)

    CPr_b: function() {
        var i = Z80._r.a;                          // Temp copy of A
        i -= Z80._r.b;                             // Subtract B
        Z80._r.f |= 0x40;                          // Set subtraction flag
        if(!(i & 255)) Z80._r.f |= 0x80;           // Check for zero
        if(i < 0) Z80._r.f |= 0x10;                // Check for underflow
        Z80._r.m = 1; Z80._r.t = 4;                // 1 M-time taken
    }