1. Procedures in more detail

2. CMPE12cGabriel Hugh Elkaim 2 Why use procedures? –Code reuse –More readable code –Less code Microprocessors (and assembly languages) provide only minimal support for procedures Must build a standard form for procedures Procedures

3. CMPE12cGabriel Hugh Elkaim 3 Procedures Procedure call {.. x = larger (a, b);.. } Procedure header and parameters int larger (int x, int y) Procedure body { if (x > y) bigger = x; else bigger = y; return (bigger); }

4. CMPE12cGabriel Hugh Elkaim 4 Procedures Steps in the execution of the procedure: 1.Save return address 2.Procedure call 3.Execute procedure 4.Return What is the return address? What is the procedure call? What is the return? MAL uses registers rather than variables for return address

5. CMPE12cGabriel Hugh Elkaim 5 Modern load/store architectures (MIPS, Sparc) have a Jump and Link instruction: jal procedure _ name Places the address of the instruction following the jal call into $RA ($31) –$31 is an arbitrary (and common) choice made at architecture time –Why isn’t the register used within the instruction call? jal $31, procedure _ name #INVALID CODE Branches (jumps) to the address given by the label (procedure _ name) Procedures

6. CMPE12cGabriel Hugh Elkaim 6 Procedures The example becomes: jal proc1 # use of $ra is implied.. jal proc1.. proc1:# 1 st instruction of procedure here.. jr $ra# $ra is the alias for $31

7. CMPE12cGabriel Hugh Elkaim 7 Nested Procedures What happens if a procedure calls another procedure (nesting) using jal? jal proc1... proc1:. jal proc2. jr $ra proc2:.. jr $ra

8. CMPE12cGabriel Hugh Elkaim 8 Nested Procedures Even more exciting, what happens if a procedure calls itself (recursion)? jal proc1.... proc1:.. jal proc1.. jr $ra

9. CMPE12cGabriel Hugh Elkaim 9 Nested Procedures Must save return address as generated Unknown how many to save Needed in reverse order of saved Use a …

10. CMPE12cGabriel Hugh Elkaim 10 System Stack A stack is so frequently used for procedure call/return, that many computer systems predefine a system stack.

11. CMPE12cGabriel Hugh Elkaim 11 The system stack is for dynamic data (vs static, known before runtime) –Return addresses –Saving registers to move other data into register bank (“spilling”) –Local variables – several instances may be defined at once with multiple calls –Dynamic memory allocation The system stack is very large In a multi-user environment, how many system stacks are there? System Stack

12. CMPE12cGabriel Hugh Elkaim 12 The MIPS system stack is defined to grow towards a smaller address –very common implementation The stack pointer points to an empty location at the top of the stack The stack pointer is $sp ($29) It is defined and initialized before program execution begins If $sp ($29) is used for any other purpose, the stack is pointer is lost. –This would be bad, very bad!! System Stack

13. CMPE12cGabriel Hugh Elkaim 13 Smaller address your program here Larger address system stack here Grows towards smaller addresses System Stack Memory

14. CMPE12cGabriel Hugh Elkaim 14 System Stack push, in MAL: sw$?, ($sp) # the $? is replaced by some register sub$sp, $sp, 4 # contains the data to be pushed Or sub$sp, $sp, 4 sw$?, 4($sp) pop, in MAL: add$sp, $sp, 4 lw$?, ($sp) Or lw$?, 4($sp) add$sp, $sp, 4 Which forms are better if there is an interrupt that uses the same stack?

15. CMPE12cGabriel Hugh Elkaim 15 System Stack Often a procedure pushes/pops many things. add$sp, $sp, -4 sw$s2, 4($sp) add$sp, $sp, -4 sw$s3, 4($sp) add$sp, $sp, -4 sw$s4, 4($sp) But we do not need to change $sp each time. add$sp, $sp, -12 sw$s2, 12($sp) sw$s3, 8($sp) sw$s4, 4($sp) Can do this for the pop as well

16. CMPE12cGabriel Hugh Elkaim 16 System Stack: Saving Return Addresses jaldoit... jaldoit... doit:sub $sp, $sp, 4#push return address sw $ra, 4($sp)... jal another#overwrites $ra... lw$ra, 4($sp)#pop return address add$sp, $sp, 4 jr$ra

17. CMPE12cGabriel Hugh Elkaim 17 Note how every pop has a push. Never leave a procedure without popping everything that was pushed. Always match up your pushes and pops. System Stack: Saving Return Addresses

18. CMPE12cGabriel Hugh Elkaim 18 Parameter Passing Parameter = Argument There is even less architectural support for parameter passing Need to create a convention

19. CMPE12cGabriel Hugh Elkaim 19 Follow the convention –Follow the convention carefully –Follow the convention consistently –Never change the convention once defined Place data in a specific parameter location Both the calling program and the procedure need to know where the parameters are. Procedure may place return values there. Parameter Passing

20. CMPE12cGabriel Hugh Elkaim 20 Parameter Passing Call by value (C, C++) The parameter passed may not be modified by the procedure. This can be implemented by passing a copy of the value. The procedure can modify the value (copy) passed to it, but the value is not changed outside the scope of the procedure. Call by reference (Fortran, C/C++ &, Pascal var) The parameter passed to the subroutine can be modified. The modification is seen outside the scope of the subroutine. Similar to having access to global variable.

21. CMPE12cGabriel Hugh Elkaim 21 Parameter Passing Simplest method: Use registers The calling program puts the parameter(s) into specific registers, and the procedure uses them... move$s4, $s2# put parameter in register $s4 jaldecrement move$s2, $s4# copy parameter back to its # correct place.. decrement: add$s4, $s4, -1 jr$ra Why not just use $s2 within the procedure?

22. CMPE12cGabriel Hugh Elkaim 22 Another method: Use system Stack When there aren’t enough registers for all parameters, use the system stack in something called an activation record (AR). Used for all parameters in machines with few registers. eg. HC11, 6502, 8086, … Parameter Passing

23. CMPE12cGabriel Hugh Elkaim 23 sub$sp, $sp, 8# allocate space for parameters sw$s2, 8($sp)# place parameter 1 into AR of proc sw$s3, 4($sp)# place parameter 2 into AR of proc jalproc. proc: sub$sp, $sp, 12# allocate remainder of AR for proc # assume fixed size (too big) # activation record lw$t0, 20($sp)# retrieve parameter 1 lw$t1, 16($sp)# retrieve parameter 2 # use parameters in procedure calculations add$sp, $sp, 20# remove AR of proc jr$ra Parameters on system stack

24. CMPE12cGabriel Hugh Elkaim 24 Parameters on system stack Calling program Allocate space for parameters Places parameters into stack Calls procedure Procedure: Allocates AR (or remainder of AR) De-allocates AR of procedure (or at least most of it) MIPS convention The first 4 parameters are passed in register The alias for $4-$7 is $a0-$a3. The first parameter is passed in $a0. Space for all parameters passed in $a0-$a3 is allocated in the procedure’s AR.

25. CMPE12cGabriel Hugh Elkaim 25 MIPS Convention If there are nested subroutine calls, and registers $a0-$a3 are used for parameters, the values would be lost procA:#receives 3 parameters in $a0, $a1, $a2 … # set up procB’s parameters move $a0, $t0# overwrites procA’s parameter in $a0 move $a1, $t1# overwrites procA’s parameter in $a1 jal procB# the nested procedure call # procA continues after procB returns # procA’s passed parameters are needed, but have been # overwritten Solutions…

26. CMPE12cGabriel Hugh Elkaim 26 MIPS Convention Need to expand on the MIPS convention to now preserve the argument registers also Caller Place parameters in $a0 to $a3 jal procedure Procedure Allocate remainder of AR and push $ra Procedure calculations

27. CMPE12cGabriel Hugh Elkaim 27 Place current parameters ($a0-$a3) into AR Set up parameters to proc2 in $a0-$a3 Call proc2 (jal proc2) Copy any return values out of $v0-v1, $a0-$a3 Restore current parameters from AR back to $a0- $a3 MIPS Convention Procedure continued… To call proc2 More procedure calculations Get procedure’s return address from AR De-allocate AR Return (jr $ra)

28. CMPE12cGabriel Hugh Elkaim 28 Summary: Parameter Passing Styles 1.Use registers Advantages Disadvantages 2.Use some registers, and place the rest on the stack Advantages Disadvantages 3.Put all parameters on the stack (an unsophisticated compiler might do this) Advantages Disadvantages 4.Put parameters in memory set aside for them Advantages Disadvantages

29. CMPE12cGabriel Hugh Elkaim 29 Register Spilling When in a procedure may need to use registers that are already being used, what to do? Callee saved A procedure clears out some registers for its own use Register values are preserved across procedure calls MIPS calls these saved registers: $s0-$s8 (aliases for $16-$23, $30) The called procedure Saves register values in its AR, Uses the register for local variables, Restores register values before it returns.

30. CMPE12cGabriel Hugh Elkaim 30 Register Spilling Caller Saved The calling program saves the registers that it does not want a called procedure to overwrite These register values are NOT preserved across procedure calls MIPS calls these temporary registers: $t0-$t9 (aliases for $8-$15, $24-$25) Procedures use these registers for local variables The values do not need to be preserved outside the scope of the procedure. How about $v0-$v1, and $a0-$a3?

31. CMPE12cGabriel Hugh Elkaim 31 Procedure Calls From the compiler’s point of view, a procedure call looks like: call setup procedure call return cleanup. procedure: prologue. calculations. epilogue

32. CMPE12cGabriel Hugh Elkaim 32 MIPS Procedure Calls The full MIPS convention includes… Call Setup Save any callee-save registers that are currently in use Place current parameters into current stack frame Allocate space for all parameters for procedure to be called Change $sp by the total parameter size Place first 4 parameters to procedure into $a0-$a3 Place remaining parameters on stack Procedure Call jal

33. CMPE12cGabriel Hugh Elkaim 33 Prologue Allocate space for remainder of stack frame Save return address in stack frame Copy needed parameters from stack frame into registers Save any needed save registers into current stack frame Epilogue Restore (copy) return address from stack frame into $ra Restore any saved registers De-allocate stack frame (or most of it) Move $sp so the space for the procedure’s frame is gone Return Cleanup Copy needed return values and parameters from $v0-$v1, $a0-$a3, or stack frame to correct places De-allocate remainder of procedure’s stack frame Move $sp so the space for the procedure’s frame is gone Restore any saved registers from call setup MIPS Procedure Calls

34. CMPE12cGabriel Hugh Elkaim 34 Summary: Procedure Calls Minimal ASM support Need formal and consistent mechanism Why? Activation record includes Return addresses parameters Save registers Saved arguments … Caller must… Callee must…