addi means add immediate, and the key word is immediate. So far, adding has meant combining two registers. But often one of the numbers you want to add is a fixed constant known when you write the program — like adding 1 to a counter. An immediate is exactly that: a constant value written directly into the instruction itself, instead of being read from a register. addi adds such a constant to a register.

The form is addi rd, rs1, imm: take the value in source register rs1, add the constant imm, and store the result in destination register rd. So addi t0, t0, 1 reads t0, adds one, and writes it back — the assembly way of saying t0 = t0 + 1.

There is a size limit on the constant. The immediate must fit in 12 bits and is signed (it can be negative), giving a range from -2048 to 2047. The assembler refuses anything larger; for bigger constants you first load them into a register with li. Because the immediate can be negative, addi also handles subtraction of a constant — addi t0, t0, -1 counts down — which is why RISC-V never needed a separate subtract-immediate instruction.

addi is one of the most-used instructions in any program: bumping counters, stepping a pointer to the next item, and reserving scratch space on the stack are all addi. The example shows two directions at once — adding 1 to advance a counter, and adding -16 to set aside 16 bytes of working memory, the standard opening move of a function that needs room to work.