sub method
Implementation
void sub(int opcode) {
var rxIdx = (opcode & 0x0F00) >> 8;
var ryIdx = (opcode & 0x0F0) >> 4;
registers.v[0xF] = registers.v[rxIdx] > registers.v[ryIdx] ? 1 : 0;
registers.v[rxIdx] -= registers.v[ryIdx];
registers.v[rxIdx] &= 0xFF;
}