The mechanism provided to switch between opcode planes is a simple mode-altering instruction in the Bplane. The esc instruction serves as an escape prefix for the instructions that follows it, making a local selection from among the available opcode planes. In the most simple case, esc specifies the opcode plane for the next instruction only. However, it can simultaneously specify the opcode planes that apply to each of the three instructions that follow it. It does so by retaining the now familiar 3-operand instruction format. Each 4-bit operand designates a selected opcode plane, instead of a general register. The opcode plane escape is only in eect for the specific operations, in sequence. Furthermore, any such escape is cancelled if an explicit branch instruction is taken.