Rs1

rs1 is a term used in the RISC-V instruction set architecture to denote the first source register field in an instruction encoding. It is a 5-bit field that selects one of the 32 general-purpose registers, commonly written as x0 through x31 in the machine code, with the usual ABI aliases such as zero, ra, sp, gp, tp, t0–t6, s0–s11, and a0–a7 in assembly.

In the encoding of RISC-V instructions, rs1 appears in several formats, notably R-type and I-type. For R-type instructions like add, sub, and, or, rs1 provides the first operand, while rs2 provides the second operand and rd designates the destination register. In I-type instructions such as addi, lw, and jalr, rs1 supplies the base or source register used together with an immediate value or offset. The rs1 field is typically located in bits 15–19 of the 32-bit instruction word.

In assembly language, rs1 is the label for the field, but programmers typically refer to the registers by their concrete names (for example, x1 or the ABI alias ra). An example is “add x5, x1, x2,” where x1 is the rs1 source register and x2 is the rs2 source register.

rs1 is a fundamental part of the instruction encoding and is distinct from the destination field rd and the second source field rs2. It is used across arithmetic, logical, memory-access, and control-flow instructions to specify the first operand.