Architecture: Operands & Registers

Operand

Operand Location

Physical location in computer

• Registers

• Memory

• Constants (also called immediates)

Registers

It is faster than memory and RISC-V has 32 32-bit registers. Can use either name (i.e., ra, zero) or x0, x1, etc. Using name is preferred. Registers used for specific purposes:

• zero always holds the constant value 0.

• the saved registers, s0-s11, used to hold variables

• the temporary registers, t0-t6, used to hold intermediate values during a larger computation

Why 12-bit Constant (Immediate)?

1. Instruction size constraint

• RISC-V uses fixed 32-bit instruction encoding (for the base ISA).

• The I-type format (used by addi) allocates:

• 7 bits → opcode

• 5 bits → destination register

• 5 bits → source register

• 3 bits → funct3

• 12 bits → immediate

• 2. Balance between compactness and usefulness • 12 bits signed immediate is enough for: • small constants • stack pointer/frame pointer adjustments • offset addressing for loads/stores • For larger constants, RISC-V provides lui (load upper immediate) and auipc (add upper immediate to PC). Combining these with addi lets you construct 32-bit constants efficiently

Why 12-bit Constant (Immediate)?

1. Instruction size constraint: RISC-V uses fixed 32-bit instruction encoding (for the base ISA).

2. Balance between compactness and usefulness: 12 bits signed immediate is enough for small constants, stack pointer/frame pointer adjustments and offset addressing for loads/stores.

   For larger constants, RISC-V provides lui (load upper immediate) and auipc (add upper immediate to PC). Combining these with addi lets you construct 32-bit constants efficiently

zero

Register Number: x0

Usage: Hardwired to Constant value 0

ra

Register Number: x1

Usage: Return address

sp

Register Number: x2

Usage: Stack Pointer

gp

Register Number: x3

Usage: Global Pointer

tp

Register Number: x4

Usage: Thread Pointer

t0-2

Register Number: x5-7

Usage: Temporaries

s0/fp

Register Number: x8

Usage: Saved register / Frame pointer

s1

Register Number: x9

Usage: Stack Pointer

a0-1

Register Number: x10-11

Usage: Function arguments / return values

a2-7

Register Number: x12-17

Usage: Function arguments

s2-11

Register Number: x18-27

Usage: Saved registers (to hold variables)

t3-6

Register Number: x28-31

Usage: Temporaries