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Procedures in more detail

Procedures in more detail. Procedures. 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. Procedure call { . . x = larger (a, b);

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Procedures in more detail

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  1. Proceduresin more detail

  2. Procedures • 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 2

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

  4. Procedures • Steps in the execution of the procedure: • Save return address • Procedure call • Execute procedure • Return • What is the return address? • What is the procedure call? • What is the return? • MAL uses registers rather than variables for return address 4

  5. Procedures • 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) 5

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

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

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

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

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

  11. System Stack • 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? 11

  12. System Stack • 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!! 12

  13. System Stack Memory Smaller address yourprogramhere Grows towards smaller addresses systemstack here Largeraddress 13

  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, 4lw $?, ($sp) • Or • lw $?, 4($sp) • add $sp, $sp, 4 • Which forms are better if there is an interrupt that usesthe same stack? 14

  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 15

  16. System Stack: Saving Return Addresses jal doit ... jal doit ... 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 16

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

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

  19. Parameter Passing • 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. 19

  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. 20

  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 • jal decrement • 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? 21

  22. Parameter Passing 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, … 22

  23. Parameters on system stack • 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 • jal proc • . • 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 23

  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. 24

  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… 25

  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 26

  27. MIPS Convention Procedure continued… • To call proc2 • 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 • More procedure calculations • Get procedure’s return address from AR • De-allocate AR • Return (jr $ra) 27

  28. Summary: Parameter Passing Styles • Use registers • Advantages • Disadvantages • Use some registers, and place the rest on the stack • Advantages • Disadvantages • Put all parameters on the stack (an unsophisticatedcompiler might do this) • Advantages • Disadvantages • Put parameters in memory set aside for them • Advantages • Disadvantages 28

  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. 29

  30. Register Spilling • Caller Saved • The calling program saves the registers that it doesnot 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 thescope of the procedure. • How about $v0-$v1, and $a0-$a3? 30

  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 31

  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 becalled • Change $sp by the total parameter size • Place first 4 parameters to procedure into $a0-$a3 • Place remaining parameters on stack • Procedure Call • jal 32

  33. MIPS Procedure Calls • 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 33

  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… 34

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