4 Control Flow & addressing modes

1. Branch Instructions (B-Type)

• Purpose: conditional PC updates based on register comparisons
• Formats recap:

  | imm[12] imm[10:5] rs2 rs1 funct3 imm[4:1] imm[11] opcode |

• Common opcodes:
  • beq (000): branch if equal
  • bne (001): branch if not equal
  • blt (100): branch if less than (signed)
  • bge (101): branch if greater or equal (signed)
  • bltu/bgeu: unsigned variants
• Target calculation:

$${\text{PC}}_{\text{next}}=\{\begin{array}{ll}\text{PC}+\mathrm{SE}({\mathrm{imm}}_{12\mid 10:5\mid 4:1\mid 11}\Vert 0),& \text{if condition true},\\ \text{PC}+4,& \text{otherwise}.\end{array}$$

• Sign-extension: assemble imm bits then extend bit 12 across upper bits

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2. Jumps (J- and I-Type)

2.1 JAL (J-Type)

• Opcode: 1101111
• Format:

  | imm[20] imm[10:1] imm[11] imm[19:12] rd opcode |

• Operation:

  jal rd, offset

  • rd ← PC + 4
  • PC ← PC + SE(offset_20|19:12|11|10:1|0)

2.2 JALR (I-Type)

• Opcode: 1100111, funct3 = 000
• Format: I-Type
• Operation:

  jalr rd, rs1, imm

  • rd ← PC + 4
  • PC ← (rs1 + SE(imm)) & ~1
  • clears LSB for alignment

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3. Function Call/Return Conventions

• Call: jal ra, label or jal x1, label
• Return: jalr x0, ra, 0 (jump to return address register)
• Stack usage:
  • Save caller-saved registers onto stack
  • Adjust sp in prologue/epilogue
• Example prologue/epilogue:

  addi sp, sp, -16      # allocate stack frame
  sw   ra, 12(sp)
  sw   s0, 8(sp)
  addi s0, sp, 16       # set frame pointer
  ...                   # function body
  lw   ra, 12(sp)
  lw   s0, 8(sp)
  addi sp, sp, 16       # deallocate
  jalr x0, ra, 0        # return

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4. Addressing Modes

• Register-immediate (base): ld/st rs1 + imm
• PC-relative: used by auipc, jal, beq offsets
• Immediate only: U-Type for lui loads high bits
• Memory-aligned offsets: offsets in bytes, must align to data width
• Examples:
  • lw x5, 4(x10) → x5 ← M[x10 + 4]
  • sw x6, -8(x2) → M[x2 – 8] ← x6

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5. Linking Branches & Jumps

• Static labels: assembler computes offsets
• PC-relative pros: position-independent code
• Hazards: control hazards in pipelines (Lecture 8)

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