ADDI

addi is a common mnemonic used in assembly language to denote an add-immediate instruction. In this class of instructions, a constant value encoded within the instruction (the immediate) is added to the value in a source register, and the result is written to a destination register. The addi form is a core building block for arithmetic, pointer arithmetic, and constant loading in many reduced-instruction-set computing architectures.

In the most well-known usage, for example in the MIPS architecture, the instruction has the form addi rt, rs, immediate. It adds the sign-extended 16-bit immediate to the contents of rs and places the result in rt. The immediate is sign-extended to the machine’s word length (typically 32 bits). Depending on the architecture, addi may trigger an overflow exception if the addition cannot be represented in the destination register; some related variants, such as addiu in MIPS, perform the same operation but do not trap on overflow.

Immediate range issues are a practical consideration: the immediate value is encoded within the instruction, so only a limited range is directly representable. For a 16-bit signed immediate, the allowed range is typically -32768 to 32767, though exact limits can vary by architecture and encoding.

Usage of addi streamlines code that would otherwise require moving a constant into a register before performing arithmetic, making it efficient for tasks such as loop indexing, array addressing, and constant offset calculations.

See also: add immediate instruction, MIPS instruction set, RISC architectures, overflow behavior in arithmetic.