MARIE has a very simple, yet powerful, instruction set. The instruction set architecture
(ISA) of a machine specifies the instructions that the computer can perform
and the format for each instruction. The ISA is essentially an interface
between the software and the hardware. Some ISAs include hundreds of instructions.
We mentioned previously that each instruction for MARIE consists of 16
bits. The most significant 4 bits, bits 12–15, make up the opcode that specifies the
instruction to be executed (which allows for a total of 16 instructions). The least
significant 12 bits, bits 0–11, form an address, which allows for a maximum
memory size of 212–1. The instruction format for MARIE is shown in Figure 4.10.