J-Type

1Learning Outcomes¶

• TODO

• TODO

More coming soon!

2Conditionally Branching Far Away¶

Conditional Branches are typically used for if-else statements, for/while loops. In general, because these control structure are usually pretty small (<50 lines of C code), we can use B-Type instructions. However, as we saw earlier, B-Type instructions have limited range: $\pm 2^{10}$ instructions from the current instruction (PC).

To jump even further, we can use J-Type unconditional jumps in combination with B-Type instructions.

Admittedly, J-Type instructions also have a limited range: $\pm 2^{18}$ instructions from the currrent insturction (PC).

If we wanted to jump to any address, RISC-V opts for jumping with absolute addressing and jalr. As discussed in a previous section, jalr is an I-Type instruction that sets PC to PC = R[rs1] + imm, where imm specifies a 12-bit immediate imm.

• To call functions with absolute addressing, instead of jal ra Label^[1]:

  lui  ra <hi20bits*>
  jalr ra ra <lo12bits>

• To break out of loops using absolute addressing, instead of j Label (e.g., jal x0 Label):

  auipc ra <hi20bits*>
  jalr  x0 ra <lo12bits>

See the discussion of U-Type instructions lui and auipc. Just like with resolving li pseudoinstructions, jalr sign-extends immediates. So if lo12bits has sign bit set, increment hi20bits by 1.