Lecture 03 - Branches and Control Flow

Course: Computer Architectures

Topics

• Branches
• Function Calls
• Pipelining

Branches

A branch changes control flow only if its condition is satisfied.

Example:

beq x1, x2, target

If:

x1 == x2

the branch is taken and execution goes to target.

If:

x1 != x2

the branch is not taken and execution continues with the next sequential instruction.

Jumps

A jump changes the PC unconditionally.

jal

jal ra, label

jumps and saves a return address.

j

j label

jumps without keeping a useful return address.

jalr

jalr rd, offset(rs1)

uses a register-derived destination.

ret

ret

returns through ra.

Loop pattern

A loop may use a conditional branch such as:

loop:
    ...
    addi x1, x1, -1
    bne  x1, x0, loop

The branch is taken while x1 != 0, and not taken when x1 == 0.

Why branches matter to pipelines

A branch can change the PC, so the processor may not know which instruction should be fetched next until the branch condition is resolved.

This leads directly to Pipelining > 11. Control hazards.

Questions to review

• What does “taken” actually mean?
• What is the difference between branch and jump?
• Why does jal save a return address?
• Why do branches create a pipeline problem?

Related

• Branches
• Function Calls
• Pipelining > 11. Control hazards