1. MicroComputer Engineering BranchProcCall page 1 3 + 5 + 7 Addiu$t1 $r0 3 Addiu$t1 $t1 5 Addiu$t1 $t1 7 Example

2. MicroComputer Engineering BranchProcCall page 2 La$t0 tal Lw$t1 0($t0) Lw$t2 4($t0) Lw$t3 8($t0) Addu $t4 $t1 $t2 Addu $t4 $t4 $t3 Sw$t4 12($t0) 3 5 7 tal Example 2

3. MicroComputer Engineering BranchProcCall page 3 The Branch Instruction B label “label” is an instruction address. Instruction addresses are 32 bits. But branch instructions are immediate format, ONLY 16 BITS !?

4. MicroComputer Engineering BranchProcCall page 4 How It’s Done! xBzx, y, z all have y !absolute values. ! !But the quantity z!z - y is an offset !(a relative value). If the offset z - y can be represented in 16 bits, then it will fit.

5. MicroComputer Engineering BranchProcCall page 5 But There’s One More Trick.... z and y are valid byte addresses, both word aligned! That means that: z = xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx00 y = xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx00 so z - y = xxxxxxxxxxxxxx00 Why waste space representing two bits which are always zero?

6. MicroComputer Engineering BranchProcCall page 6 But there’s one more trick.... So, shift the difference (z - y) right two bits. That way, if (z - y) fits in 18 bits we can save the offset in 16 bits!

7. MicroComputer Engineering BranchProcCall page 7 How does the structure work? xBzWhen we fetch B z, y !the program counter !contains the value x. ! z!That means y = PC + 4. ! Since offset = (z - y), then PC + 4 + offset = y + z - y, an absolute address!

8. MicroComputer Engineering BranchProcCall page 8 To calculate the branch address Program counter Instruction memory B Imm z - y shifted right twice Imm 00 Restore the two zeroes 18 bits 14 18 Sign extend to 32 bits 4 + Absolute branch address PC + 4

9. MicroComputer Engineering BranchProcCall page 9 Branch logic Sgn/Ze extend Zero ext. B label ALU A B 31 0 4 + +

10. MicroComputer Engineering BranchProcCall page 10 Branch logic Sgn/Ze extend Zero ext. B label ALU A B 31 0 4 + +

11. MicroComputer Engineering BranchProcCall page 11 Branch logic Sgn/Ze extend Zero ext. B label ALU A B 31 0 4 + +

12. MicroComputer Engineering BranchProcCall page 12 Branch logic Sgn/Ze extend Zero ext. Beq rs rt label ALU A B 31 0 4 + +

13. MicroComputer Engineering BranchProcCall page 13 Branch logic Sgn/Ze extend Zero ext. Beq rs rt label ALU A B 31 0 4 + +

14. MicroComputer Engineering BranchProcCall page 14 Branch logic Sgn/Ze extend Zero ext. Beq rs rt label ALU A B 31 0 4 + +

15. MicroComputer Engineering BranchProcCall page 15 Branch logic Sgn/Ze extend Zero ext. Beq rs rt label ALU A B 31 0 4 + + ?

16. MicroComputer Engineering BranchProcCall page 16 Procedure call Suppose we have a program that “rings the bell” at 10 different locations in the code. This sort of thing is usually complicated! many, many instructions..... That’s a waste of space can’t we write the “bell code” once and share it?

17. MicroComputer Engineering BranchProcCall page 17 Procedure call Bbell back! ! BbellProblem: The back!return address! Bell! !Branch is not !enough. Bback

18. MicroComputer Engineering BranchProcCall page 18 Return address A We need to tell the subroutine where it should branch back to, –i.e., the return address, each time we call it! That means: The return address must be changeable.

19. MicroComputer Engineering BranchProcCall page 19 Return address B We can’t use B x; X is not changeable –we can’t write to the instruction memory... But we can write to: –register file –data memory So we can write the return address there.

20. MicroComputer Engineering BranchProcCall page 20 Return address C Suppose we use $31 for example: Bell! ! Jr$31 Jump register $31: –Read the register file and stores the value into the PC

21. MicroComputer Engineering BranchProcCall page 21 Branch logic Sgn/Ze extend Zero ext. Jr $31 ALU A B 31 0 4 + +

22. MicroComputer Engineering BranchProcCall page 22 Branch logic Sgn/Ze extend Zero ext. Jr $31 ALU A B 31 0 4 + +

23. MicroComputer Engineering BranchProcCall page 23 This works (not the best way): !bell:!! La$31 x! Bbell! x:!Jr$31 ! La$31 y Bbell y:! !

24. MicroComputer Engineering BranchProcCall page 24 What’s done?? La$31 x2 instructions B bell1 instruction x:need a label We just want to get the address following the “call” into $31.

25. MicroComputer Engineering BranchProcCall page 25 save return address in reg $31 Branch logic Sgn/Ze extend Zero ext. La $31 x B bell x: ALU A B 31 0 4 + + Return address when B bell is fetched

26. MicroComputer Engineering BranchProcCall page 26 The return address The “call” is at “PC”. So, the return address must be PC + 4. But we compute PC + 4. Can we cause PC + 4 to be written to the register file ($31). Yes. Use BALbell (branch and link).

27. MicroComputer Engineering BranchProcCall page 27 Branch logic Sgn/Ze extend Zero ext. Bal label ALU A B 31 0 4 + +

28. MicroComputer Engineering BranchProcCall page 28 Branch logic Sgn/Ze extend Zero ext. Bal label ALU A B 31 0 4 + +

29. MicroComputer Engineering BranchProcCall page 29 Branch logic Sgn/Ze extend Zero ext. Bal label ALU A B 31 0 4 + +

30. MicroComputer Engineering BranchProcCall page 30 Branch logic Sgn/Ze extend Zero ext. Bal label ALU A B 31 0 4 + +

31. MicroComputer Engineering BranchProcCall page 31 Branch logic Sgn/Ze extend Zero ext. B label … next instr ALU A B 31 0 4 + +