Essentials of Computer Organization and Architecture, 5th Edition

4.12 EXTENDING OUR INSTRUCTION SET

Even though MARIE’s instruction set is sufficient to write any program we wish, there are a few instructions we can add to make programming much simpler. We have 4 bits allocated to the opcode, which implies that we can have 16 unique instructions, and we are using only 9 of them. Surely, we can make many programming tasks much easier by adding a few well-chosen instructions to our instruction set. Our new instructions are summarized in Table 4.6.

TABLE 4.6 MARIE’s Extended Instruction Set

Instruction Number (hex)	Instruction	Meaning
0	JnS X	Store the PC at address X and jump to X + 1.
A	Clear	Put all zeros in AC.
B	AddI X	Add indirect: Go to address X. Use the value at X        as the actual ...