Jumps and Branches

• All Branches are performed with slt, beq, bne
  • How?
  • == and != are easy
  • How about <
  • Just slt, beq
  • How about <=
  • How about >, >=
• SLT
  • How could we implement this operation?
  • Where would we implement this operation?
• BEQ, BNE
  • How could we test for equality?
  • Where would we implement this?
• Branches are pc relative
  • Look at the instruction, what is the possible addressable space?
    • Remember, instructions must be on word aligned boundaries.
    • So a 16 bit address becomes a 18 bit address padding with 00
  • Where do the other bits for the address come from?
    • Look at the picture on 263
    • Why the shift left 2
    • Why add to the pc?
  • When is the value of PCSrc set?
    • How do we decided if we are taking a branch?
    • Look at the picture on the next page
    • It is really easy to decide if an operation has produced all zeros
    • Just or the 32 bits of the result.
• Jump
  • Look at the picture on page 271
  • How is jump implemented? (It is not where you think.)
  • What are the limitations due to jumping this way?
  • When would the JUMP control line be lit?
• Just for completeness
  • Using the image on 264 (or any other)
  • What is the value of the control lines for lw, sw, add, addi