LC-3 - Instruction Set

Instruction Set

The LC-3 instruction set implements fifteen types of instructions, with a sixteenth opcode reserved for later use. The architecture is a load-store architecture; values in memory must be brought into the register file before they can be operated upon.

Arithmetic instructions available include addition, bitwise AND, and bitwise NOT, with the first two of these able to use both registers and sign-extended immediate values as operands. These operations are sufficient to implement a number of basic arithmetic operations, including subtraction (by negating values) and bitwise left shift (by using the addition instruction to multiply values by two). The LC-3 can also implement any bitwise logical function, because NOT and AND together are logically complete.

Memory accesses can be performed by computing addresses based on the current value of the program counter (PC) or a register in the register file; additionally, the LC-3 provides indirect loads and stores, which use a piece of data in memory as an address to load data from or store data to. Values in memory must be brought into the register file before they can be used as part of an arithmetic or logical operation.

The LC-3 provides both conditional and unconditional control flow instructions. Conditional branches are based on the arithmetic sign (negative, zero, or positive) of the last piece of data written into the register file. Unconditional branches may move execution to a location given by a register value or a PC-relative offset. Three instructions (JSR, JSRR, and TRAP) support the notion of subroutine calls by storing the address of the code calling the subroutine into a register before changing the value of the program counter. The LC-3 does not support the direct arithmetic comparison of two values; comparing two register values arithmetically requires subtracting one from the other and evaluating the result.