RISC-V Registers

RISC-V has the following 32-bit registers:
  • 32 general-purpose registers
  • A program counter (PC)
A 32 general-purpose registers have the following assigned functions:
  • x0 is hard-wired to 0 and can be used as a target register for any instructions where the result must be discarded.
  • x0 can also be used as a source of zero (0) if needed.
  • x1-x31 are general-purpose registers. The 32-bit integers they hold are interpreted, depending on the instruction.
A PC has the following assigned functions and characteristics:
  • PC points to the next instruction to be executed.
  • The PC cannot be written or read using load/store instructions.
The following figure shows the 32 general-purpose registers in a RISC-V ISA1 CPU.
Figure 1. RISC-V Base Unprivileged Integer Register State

Calling Convention for RISC-V Registers

The symbolic name in the table is the name used by the RISC-V register when applying the inline assembly in the design.

Table 1. Symbolic Names in RISC-V General Purpose Registers
Register Name Symbolic Name Description
x0 Zero Hardwired zero register, always read as zero (0), and writes are ignored.
x1 Ra Return address register, used to store the return address.
x2 Sp Stack pointer register, used to point to the top of the call stack.
x3 Gp Global pointer register, used to addressing global data.
x4 Tp Thread pointer, used for addressing thread-local data.
x5 t0 Temporary register/alternate link register, used for general temporary storage.
x6-x7 t1-t2 Temporary registers, used for general temporary storage.
x8 s0/fp Saved register/frame pointer, often used to establish and maintain stack frames.
x9 s1 Saved register, used for saving and restoring values across function calls.
x10-x11 a0-a1 Function argument registers/return value register.
x12-x17 a2-a7 Function argument registers.
x18-x27 s2-s11 Saved registers, used for saving and restoring values across function calls.
x28-x31 t3-t6 Temporary registers, often used for general temporary storage.
Note: Ensure correct registers are used when designing your program to avoid any data corruption.
1 ISA: Instruction Set Architecture