Jumps and such

• Some material is from page 113-116, part is from my memory.
• First remember (or learn) that all instructions are word aligned.
  • This means that the word at byte 0 takes up bytes 0,1,2,3
  • So the next word starts at 4 and takes up bytes 4,5,6,7
  • and 8,9, a_{16, b16}
  • and c_{16, d16 e16, f16}
  • Note all addresses 0000, 0100, 1000, 1100 end in 00
  • So we can really specify our branch address in words, not bytes
  • Thus the shift left on the picture on 263 (for example)
• We are now using 18 of the 32 bit branch address.
  • From where should the other 14 bits come?
  • The solution is a PC-relative branch
  • Ie, how far away are we likely to branch?
  • Branches are mostly used for goto, if, while.
  • We will see function calls somewhere else.
  • We will discuss the "Principle of Locality" in more detail later but for now.
  • The principle states that if we used an instruction, we are likely to use the instructions around it.
    • This is applied to data
    • Both in time and space.
    • But instructions, to us at least, are just data.
  • So we can assume that if we need to branch, we will branch "close to home".
  • Remember we are still branching within 2¹⁵-1 words (32,767 instructions each way).
  • So beq, bne say PC= PC+4+BranchAddr
  • What is the +4?
• Write this program and see if it does what you expect

  .text
  
  main:
  
          li      $t0, 0
  top:
          beq     $t0,10, out
  
          addi    $t0, $t0, 1
          b       top
  
  out:
          li      $v0, 10
          SYSCALL
  
      

• Jumps
  • There are three of these. (J, JAL, JR)
  • JAL and JR are for function calls, so we will ignore these for now.
  • But JR along with LUI and ORI could be the answer to everything.
  • The green card says PC = JumpAddr (but this is not very useful)
  • Remember, we get 2 bits free for word aligned.
  • And there are 26 bits in the jump address (J type instruction)
  • So we are short by 4 bits. Where should these come from?
  • The answer is the top for bits of the PC
  • Look at page 271.