1. Lecture 6: Procedure Call (Part I)
Standard conventions for procedure call
MIPS support for procedure call
Register conventions
JR and JAL
Stack pointer register $SP and stack conventions

2. Function call conventions in C
main() { int i,j,k,m; i = add(j,k); m = add(i,i); }
/* function add */
int add (int addin1, int addin2){ int sum; sum = addin1 + addin2; } return sum; }
Calling procedure:
pass parameters to function by putting them in an agreed upon place
Called procedure: get parameters, use them, and return results by putting them in agreed upon place.

3. Function call conventions in MIPS
# One caller: j in $s1,k in $s2,I in $s JUMP ADDFUNC
----
# Another caller: j in $s7,k in $s8,I in $t JUMP ADDFUNC
# function ADDFUNC
ADDFUNC: ADD $t1,$t2,$t3
ADD $t4,$t1,$t1
JUMP ???
Calling procedure:
Where to put the parameters ???
Called procedure: where to get the parameters, what registers to use for computation, how to get back to caller??

4. Function Call Conventions in Assembly
Registers play a major role in keeping track of information for function calls.
MIPS register conventions:
Return address $ra
Arguments $a0, $a1, $a2, $a3
Return value $v0, $v1
Local variables $s0, $s1, … , $s7
The stack is also used; more later.

5. Instruction Support for Functions (1/5)
... sum(a,b);... /* a,b:$s0,$s1 */ } int sum(int x, int y) { return x+y; }
address add $a0,$s0,$zero # x = a add $a1,$s1,$zero # y = b addi $ra,$zero,1016 #$ra= j sum #jump to sum
2000 sum: add $v0,$a0,$a jr $ra # new instruction C
M I P S

6. Instruction Support for Functions (2/5)
... sum(a,b);... /* a,b:$s0,$s1 */ } int sum(int x, int y) { return x+y; }
2000 sum: add $v0,$a0,$a jr $ra # new instruction C
M I P S
Question: Why use jr instead of using j?
Answer: sum might be called by many functions, so we can’t return to a fixed location/address. The calling proc to sum must be able to specify “return address”.

7. Instruction Support for Functions (3/5)
Single instruction to jump and save return address: jump and link (jal)
Earlier approach: addi $ra,$zero,1016 #$ra= j sum #goto sum
Faster approach:
1008 jal sum # $ra=address of next
# instruction
# $ra=1012;goto sum
Why have a new instruction (jal)?
Make the common case fast, function calls are very common.
Also, you don’t need to know the memory address of individual instructions with jal.

8. Instruction Support for Functions (4/5)
Syntax for jal (jump and link) is same as for j (jump):
jal label
 jal should really be called laj for “link and jump”:
Step 1 (link): Save address of next instruction into $ra
Why next instruction? Why not current one?
Step 2 (jump): Jump to the given label

9. Instruction Support for Functions (5/5)
Syntax for jr (jump register):
jr register
Instead of providing a label to jump to, the jr instruction provides a register which contains an address to jump to.
Only useful if we know exact address to jump to.
Use this MIPS convention for function call and return:
jal stores return address in register ($ra)
jr $ra jumps back to that address

10. #########################################################
## MY FIRST SPIM PROCEDURE
## Simple procedure example: not more than 4 arguments, only
## 1 return value, no calls from within the procedure, and
## no local variables! #
# Write SPIM/MIPS code for this procedure using register
# conventions we just learned
# int foo (int ain, int bin) {
# int n= 2+ain+bin;
# return n;
# }

11. Solution: calling program
# load parameters for test call to foo(4,6)
li $a0, # set up first parameter
li $a1, # set up second parameter
# call foo
jal foo # call function
# on return from foo, the result is in $v0, save it and print it
move $a0,$v # move result into argument
li $v0, # syscall code for print integer
syscall
# end program
li $v0,10
syscall # terminate execution

12. Solution: procedure foo
# get arguments
mov $t0,$a # get first argument
mov $t1,$a # get second argument
# perform body of the procedure
addi $t0,$t0, #compute 2+first argument
add $t2,$t0,$t1 #compute 2+first arg+second arg
# set up return value in register $v0
move $v0,$t2
# return
jr $ra #return

13. Summary: Rules for Simple Procedures
Called with a jal instruction, returns with a jr $ra
Accepts up to 4 arguments in $a0, $a1, $a2 and $a3
Return value is always in $v0 (and if necessary in $v1)
Must follow register conventions (even in functions that only you will call)!

14. Nested Procedures (1/2)
int sumSquare(int x, int y) { return mult(x,x)+ y; }
A procedure called sumSquare, now sumSquare is calling mult.
Problem: there’s a return address in $ra that sumSquare wants to jump but it will be overwritten by the call to mult.
Need to save sumSquare return address before call to mult.

15. In general, may need to save some other info in addition to $ra.
Nested Procedures (2/2)
In general, may need to save some other info in addition to $ra.
When a C program is run, there are 3 important memory areas allocated:
Static: Variables declared once per program, cease to exist only after execution completes. e.g., C globals
Heap: Variables declared dynamically
Stack: Space to be used by procedure during execution; this is where we can save register values

16. Space for saved procedure information
C Memory Allocation
Address
7fff fff
Stack
Space for saved procedure information
$sp
stack
pointer
Heap
Explicitly created space, e.g., malloc(); C pointers
Static
Variables declared
once per program
Code
Program

17. C stack grows downward in memory
Using the Stack (1/2)
C stack grows downward in memory
register $sp always points to the last used space in the stack.
To use stack, we decrement this pointer by the amount of space we need and then fill it with info.
So, how do we compile this?
int sumSquare(int x, int y) { return mult(x,x)+ y; }

18. Using the Stack (2/2) Hand-compile
sumSquare: addi $sp,$sp,-8 # space on stack sw $ra, 4($sp) # save ret addr sw $a1, 0($sp) # save y
int sumSquare(int x, int y) { return mult(x,x)+ y; }
“push”
add $a1,$a0,$zero # mult(x,x) jal mult # call mult
lw $a1, 0($sp) # restore y add $v0,$v0,$a1 # mult()+y lw $ra, 4($sp) # get ret addr addi $sp,$sp,8 # update sp jr $ra mult: ...
“pop”

19. Register Conventions (1/4)
Caller: the calling function
Callee: the function being called
When callee returns from executing, the caller needs to know:
which registers may have changed, and
which registers are guaranteed to be unchanged.
Register Conventions: A set of generally accepted rules as to which registers will be unchanged after a procedure call (jal) and which registers may be changed.

20. Register Conventions (2/4) - saved
$0: No Change. Always 0.
$s0-$s7: Save and restore if you change. Very important, that’s why they’re called saved registers. If the callee changes these in any way, it must restore the original values before returning.
$sp: Save and restore if you change. The stack pointer must point to the same place before and after the jal call, or else the caller won’t be able to restore values from the stack.
HINT -- All saved registers start with S!

21. Register Conventions (3/4) - volatile
$ra: Can Change. The jal call itself will change this register. Caller needs to save on stack if nested call.
$v0-$v1: Can Change. These will contain the new returned values.
$a0-$a3: Can change. These are volatile argument registers. Caller needs to save if they’ll need them after the call.
$t0-$t9: Can change. That’s why they’re called temporary: any procedure may change them at any time. Caller needs to save if they’ll need them afterwards.

22. Register Conventions (4/4)
What do these conventions mean?
If function R calls function E, then function R/caller must save any t (temporary) registers that it may be using onto the stack before making a jal call.
Function E/callee must save any S (saved) registers it intends to use before garbling up their values
Remember: Caller/callee need to save only temporary/saved registers they are using, not all registers.

23. Structure of a Procedure
Beta:
Save Saved Registers
Restore Saved Registers
Return Results …
Send Parameters
Receive Results
Call Procedure
Save Unsaved Registers
$t’s, $v’s, $a’s, $ra
Restore Unsaved Regs.
Return
Retrieve Parameters
Alpha:
Structure of a Procedure

24. Use names for registers -- code is clearer!
MIPS Registers
The constant 0 $0 $zero Reserved for Assembler $1 $at Return Values $2-$3 $v0-$v1 Arguments $4-$7 $a0-$a3 Temporary $8-$15 $t0-$t7 Saved $16-$23 $s0-$s7 More Temporary $24-$25 $t8-$t9 Used by Kernel $26-27 $k0-$k1 Global Pointer $28 $gp Stack Pointer $29 $sp Frame Pointer $30 $fp Return Address $31 $ra
Use names for registers -- code is clearer!

25. Other Registers
$at: may be used by the assembler at any time; unsafe to use
$k0-$k1: may be used by the OS at any time; unsafe to use
$gp, $fp: don’t worry about them
Note: Feel free to read up on $gp and $fp in Appendix A, but you can write perfectly good MIPS code without them.

26. Quickie Quiz (True or False?)
int fact(int n){
if(n == 0) return 1; else return(n*fact(n-1));}
When translating this to MIPS…
We COULD copy $a0 to $a1 (not store $a0 or $a1 on the stack) to preserve n across recursive calls.
We MUST save $a0 on the stack since it gets changed.
We MUST save $ra on the stack since we need to know where to return to… F
 Tail Recursion
Answer:
A: False! That won’t work because it’ll be clobbered on the next recursive call
B: False! Not really because we won’t need it anymore
C: False! Not really because we could rewrite this iteratively in MIPS

27. Quickie Quiz Volatile! -- need to push Saved
r: # R/W $s0,$v0,$t0,$a0,$sp,$ra,mem ### PUSH REGISTER(S) TO STACK? jal e # Call e # R/W $s0,$v0,$t0,$a0,$sp,$ra,mem jr $ra # Return to caller of r e: # R/W $s0,$v0,$t0,$a0,$sp,$ra,mem jr $ra # Return to r
What does r have to push on the stack before “jal e”?
1: Nothing 2: 1 of ($s0,$sp,$v0,$t0,$a0,$ra) 3: 2 of ($s0,$sp,$v0,$t0,$a0,$ra) 4: 3 of ($s0,$sp,$v0,$t0,$a0,$ra) 5: 4 of ($s0,$sp,$v0,$t0,$a0,$ra) 6: 5 of ($s0,$sp,$v0,$t0,$a0,$ra)
Volatile! -- need to push
Saved
e can’t return changed, no need to push
e can return changed

28. #------------- Function fibo ---------------------------------
## int fibo(int n) {
## if (n <= 2)
## return 1;
## else return(fib(n-2) + fib(n-1));
## }
## fibo is a recursive procedure to find a given Fibbonacci number
fibo: sub $sp, $sp, # push stack frame
sw $ra, 0($sp) # save return address
sw $s0, 4($sp) # and s registers used here
sw $s1, 8($sp)
bgt $a0, 2, f # check for input greater than 1
li $v0, 1 # if not, just return a 1
b fret

29. # input OK, set up recursive calls
f0: move $s0, $a # copy a0 to s0 so it's saved across calls
sub $a0, $s0, # set param to n-2
jal fibo # and make recursive call
move $s1, $v # save fib(n-2)
sub $a0, $s0, # param = n-1
jal fibo # compute fib(n-1)
add $v0, $v0, $s # add fib(n-2)
# return value from this call is already in $v0 as needed
fret: lw $ra, 0($sp) # restore return address,
lw $s0, 4($sp) # s registers,
lw $s1, 8($sp)
add $sp, $sp, # pop stack frame,
jr $ra # and return....

30. # RECURSIVE PROCEDURE TO COMPUTE FACTORIALS
#save the registers that FACT uses
sub $sp,$sp, #modify stack top
sw $s0,0($sp) #save registers
sw $ra,4($sp)
move $s0,$a #get parameter out of a0
#For n=1, return 1
bne $s0,1,MORE
li $v0, #return a 1
j RET

31. #For n>1, return n*FACT(n-1)
MORE:
## move arguments in for call
sub $a0,$s0, #compute n-1
## call FACT on n-1
jal FACT #recursive call
## use returned value and set up return value for this call
mul $v0,$v0,$s0
RET:
lw $s0,0($sp) #restore registers
lw $ra,4($sp)
add $sp,$sp, #reposition stack top
jr $ra #return

32. “And in Conclusion…” Functions called with jal, return with jr $ra.
Use stack to save anything you need. Just be sure to leave it the way you found it.
Instructions we know so far
Arithmetic: add, addi, sub, addu, addiu, subu
Memory: lw, sw, lh, sh, lb, sb
Decision: beq, bne, (blt, ble, bgt, bge)
slt, slti, sltu, sltiu
Unconditional Branches (Jumps): j, jal, jr
Registers we know so far
All of them!