Branches & Jumps

Control flow instructions:

Branches (B-type, conditional, PC-relative):

• BEQ rs1, rs2, offset: branch if equal.
• BNE rs1, rs2, offset: branch if not equal.
• BLT rs1, rs2, offset: branch if less (signed).
• BGE rs1, rs2, offset: branch if greater-or-equal (signed).
• BLTU / BGEU: unsigned variants.

If condition true: pc = pc + sign_extend(offset). Else: pc += 4 (next instruction).

Branch offsets are 13-bit signed (range ±4 KiB), multiples of 2.

JAL (J-type, jump and link):

• jal rd, offset
• rd = pc + 4 (return address).
• pc += sign_extend(offset).

Used for function calls: jal ra, function.

JALR (I-type, jump and link register):

• jalr rd, offset(rs1)
• rd = pc + 4.
• pc = (rs1 + sign_extend(offset)) & ~1.

Used for indirect calls and returns: jalr x0, 0(ra) (= ret).

Implementation:

After every instruction, advance pc. Branches may override.

Pseudoinstructions:

• j label → jal x0, label (no return address saved).
• jr rs → jalr x0, 0(rs).
• ret → jalr x0, 0(ra).
• call label → auipc + jalr (for far calls).
• bgt rs1, rs2, lbl → blt rs2, rs1, lbl (operands swapped).