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 ...|