1. CSE 331 Computer Organization and Design Fall 2007 Week 4
Section 1: Mary Jane Irwin (
Section 2: Krishna Narayanan
Course material on ANGEL: cms.psu.edu
[adapted from D. Patterson slides]

2. Head’s Up Last week’s material This week’s material
MIPS control flow and logic operations
This week’s material
Supporting procedure calls and returns; addressing modes
Reading assignment - PH: , A.6, D.2
Next week’s material
Assemblers, linkers and loaders
Reading assignment - PH: 2.10, A.1-A.5
Reminders
HW 3 (another spim assignment) is due Wednesday, Sept 19th (by 11:55pm)
Quiz 2 is due Friday, Sept 21st (by 11:55pm)
Exam #1 is Tuesday, Oct 2, 6:30-7:45pm, 113 IST
Please if you have a conflict

3. The reality is that most code is not heavily performance-bound by the lack of one instruction. … So the belief that the microprocessor is but one or two instructions away from blindingly higher speed is almost always wrong.
Bob Colwell, The Pentium Chronicles

4. Review: MIPS Organization
Processor
Memory
Register File
1…1100
src1 addr
src1
data 5 32
src2 addr 32
registers
($zero - $ra) 5
dst addr
read/write
addr 5
src2
data
write data 32
230
words 32 32
32 bits
br offset
read data 32
Add PC 32 32 32 32
Add 32 4
write data
0…1100
Fetch
PC = PC+4
Decode
Exec 32
0…1000 32 4 5 6 7
0…0100 32
ALU 1 2 3
0…0000 32
word address
(binary)
32 bits 32
byte address
(big Endian)

5. Programming Styles
Procedures (subroutines, functions) allow the programmer to structure programs making them
easier to understand and debug and
allowing code to be reused
Procedures allow the programmer to concentrate on one portion of the code at a time
parameters act as barriers between the procedure and the rest of the program and data, allowing the procedure to be passed values (arguments) and to return values (results)
barriers concept is important!!

6. Six Steps in Execution of a Procedure
Main routine (caller) places parameters in a place where the procedure (callee) can access them
$a0 - $a3: four argument registers
Caller transfers control to the callee
Callee acquires the storage resources needed
Callee performs the desired task
Callee places the result value in a place where the caller can access it
$v0 - $v1: two value registers for result values
Callee returns control to the caller
$ra: one return address register to return to the point of origin

7. Instruction for Calling a Procedure
MIPS procedure call instruction: jal ProcAddress #jump and link
Saves PC+4 in register $ra as the link to the following instruction to set up the procedure return
Machine format:
Then can do procedure return with just
jr $ra #return
J format
op bit address
jr jumps to the address stored in the register – in this case $ra – which is just what we want.
????

8. Basic Procedure Flow
For a procedure that computes the GCD of two values i (in $t0) and j (in $t1)
gcd(i,j);
The caller puts the i and j (the parameters values) in $a0 and $a1 and issues a
jal gcd #jump to routine gcd
The callee computes the GCD, puts the result in $v0, and returns control to the caller using
gcd: #code to compute gcd
jr $ra #return

9. Spilling Registers
What if the callee needs to use more registers than allocated to argument and return values?
callee uses a stack – a last-in-first-out queue
One of the general registers, $sp ($29), is used to address the stack (which “grows” from high address to low address)
add data onto the stack – push
$sp = $sp – data on stack at new $sp
remove data from the stack – pop
data from stack at $sp $sp = $sp + 4
high addr
top of stack
$sp
Note that temporary registers $t0 through $t9 can also be used as by MIPS convention they are not preserved by the callee (kept intact) across subroutine boundaries
However, saved registers $s0 through $s7 must be preserved by the calle - I.e., if the callee uses one, it must first save it and then restore it to its old value before returning control to the caller
low addr

10. Compiling a C Leaf Procedure
Leaf procedures are ones that do not call other procedures. Give the MIPS assembler code for
int leaf_ex (int g, int h, int i, int j)
{ int f;
f = (g+h) – (i+j);
return f; }
where g, h, i, and j are in $a0, $a1, $a2, $a3
leaf_ex: addi $sp,$sp,-8 #make stack room
sw $t1,4($sp) #save $t1 on stack
sw $t0,0($sp) #save $t0 on stack
add $t0,$a0,$a1
add $t1,$a2,$a3
sub $v0,$t0,$t1
lw $t0,0($sp) #restore $t0
lw $t1,4($sp) #restore $t1
addi $sp,$sp,8 #adjust stack ptr
jr $ra
For lecture
When using temporary registers, must assume their old values must be saved and restored.
Show the stack contents on the board as you progress through the code

11. What happens to return addresses with nested procedures?
int rt_1 (int i) {
if (i == 0) return 0;
else return rt_2(i-1); }
caller: jal rt_1 next: . . .
rt_1: bne $a0, $zero, to_2
add $v0, $zero, $zero
jr $ra
to_2: addi $a0, $a0, -1
jal rt_2
rt_2: . . .
So caller must save $ra on the stack and callee must save any other registers that it uses on the stack

12. Nested Procedures Outcome
caller: jal rt_1 next: . . .
rt_1: bne $a0, $zero, to_2
add $v0, $zero, $zero
jr $ra
to_2: addi $a0, $a0, -1
jal rt_2
rt_2: . . .
So caller must save $ra on the stack and callee must save any other registers that it uses on the stack
On the call to rt_1, the return address (next in the caller routine) gets stored in $ra. What happens to the value in $ra (when i != 0) when rt_1 makes a call to rt_2?

13. Saving the Return Address, Part 1
Nested procedures (i passed in $a0, return value in $v0)
rt_1: bne $a0, $zero, to_2
add $v0, $zero, $zero
jr $ra
to_2: addi $sp, $sp, -8
sw $ra, 4($sp)
sw $a0, 0($sp)
addi $a0, $a0, -1
jal rt_2
bk_2: lw $a0, 0($sp)
lw $ra, 4($sp)
addi $sp, $sp, 8
Save the return address (and arguments) on the stack
high addr
old TOS
 $sp
caller rt addr
old $a0
$sp
low addr
For lecture
Note that the original values of $ra and I ($a0) are restored on return
NEXT TIME - Add tracking of the PC to the animation!!
caller rt addr
bk_2
$ra

14. Saving the Return Address, Part 2
Nested procedures (i passed in $a0, return value in $v0)
rt_1: bne $a0, $zero, to_2
add $v0, $zero, $zero
jr $ra
to_2: addi $sp, $sp, -8
sw $ra, 4($sp)
sw $a0, 0($sp)
addi $a0, $a0, -1
jal rt_2
bk_2: lw $a0, 0($sp)
lw $ra, 4($sp)
addi $sp, $sp, 8
Save the return address (and arguments) on the stack
high addr
old TOS
$sp
caller rt addr
old $a0
 $sp
low addr
For lecture
caller rt addr
bk_2
$ra

15. MIPS Register Convention
Name
Register Number
Usage
Preserve on call?
$zero
the constant 0
n.a.
$v0 - $v1
2-3
returned values no
$a0 - $a3
4-7
arguments
yes
$t0 - $t7
8-15
temporaries
$s0 - $s7
16-23
saved values
$t8 - $t9
24-25
$gp 28
global pointer
$sp 29
stack pointer
$fp 30
frame pointer
$ra 31
return address
$a0-$a3 – used to pass parameters, loaded by caller, read by callee and preserved by callee
$v0-$v1 – used to pass results back from callee to caller (note, can also use to pass parameters from caller to callee by is NOT preserved)

16. Bob Colwell, The Pentium Chronicles
We all go through stages of computer maturity when learning computer science and programming. At first, the computer strikes us as really obtuse and stupidly designed, and then positively malevolent, as though it is actually sentient and working very hard to make our lives miserable. Eventually, the student outgrows this feeling and no longer takes it personally when an attempted compile generates a long list of error messages. In effect, the student learns that the computer is simply following its internal rules and has no opinion about that student whatsoever, so its is clearly a waste of indignation to react to it emotionally.
Bob Colwell, The Pentium Chronicles

17. Compiling a Recursive Procedure
A procedure for calculating factorial
int fact (int n) {
if (n < 1) return 1;
else return (n * fact (n-1)); }
A recursive procedure (one that calls itself!)
fact (0) = 1
fact (1) = 1 * 1 = 1
fact (2) = 2 * 1 * 1 = 2
fact (3) = 3 * 2 * 1 * 1 = 6
fact (4) = 4 * 3 * 2 * 1 * 1 = 24
. . .
Assume n is passed in $a0; result returned in $v0

18. Compiling a Recursive Procedure
fact: addi $sp, $sp, -8 #adjust stack pointer
sw $ra, 4($sp) #save return address
sw $a0, 0($sp) #save argument n
slti $t0, $a0, 1 #test for n < 1
beq $t0, $zero, L1 #if n >=1, go to L1
addi $v0, $zero, 1 #else return 1 in $v0
addi $sp, $sp, 8 #adjust stack pointer
jr $ra #return to caller (1st)
L1: addi $a0, $a0, -1 #n >=1, so decrement n
jal fact #call fact with (n-1)
#this is where fact returns
bk_f: lw $a0, 0($sp) #restore argument n
lw $ra, 4($sp) #restore return address
mul $v0, $a0, $v0 #$v0 = n * fact(n-1)
jr $ra #return to caller (2nd)
Note, no need to load $ra, and $a0, since they have not changed values (expect when n is >= to 1 as on next page)
We assume a multiply instruction is available, even though we haven’t covered it yet!

19. A Look at the Stack for $a0 = 2, Part 1
Stack state after execution of the first encounter with jal (second call to fact routine with $a0 now holding 1)
saved return address to caller routine (i.e., location in the main routine where first call to fact is made) on the stack
saved original value of $a0 on the stack
old TOS
 $sp
caller rt addr
$a0 = 2
$sp
For lecture
NEXT TIME - Add tracking of the PC to the animation!!
caller rt addr
bk_f
$ra 2 1
$a0
$v0

20. A Look at the Stack for $a0 = 2, Part 2
Stack state after execution of the second encounter with jal (third call to fact routine with $a0 now holding 0)
saved return address of instruction in caller routine (instruction after jal) on the stack
saved previous value of $a0 on the stack
old TOS
caller rt addr
$a0 = 2
 $sp
bk_f
$a0 = 1
$sp
For lecture
bk_f
bk_f
$ra 1
$a0
$v0

21. A Look at the Stack for $a0 = 2, Part 3
Stack state after execution of the first encounter with the first jr ($v0 initialized to 1)
stack pointer updated to point to third call to fact
old TOS
caller rt addr
$a0 = 2
bk_f
$a0 = 1
$sp
 $sp
bk_f
$a0 = 0
 $sp
For lecture
bk_f
$ra
$a0 1
$v0

22. A Look at the Stack for $a0 = 2, Part 4
Stack state after execution of the first encounter with the second jr (return from fact routine after updating $v0 to 1 * 1)
return address to caller routine (bk_f in fact routine) restored to $ra from the stack
previous value of $a0 restored from the stack
stack pointer updated to point to second call to fact
old TOS
caller rt addr
$a0 = 2
$sp
bk_f
$a0 = 1
 $sp
bk_f
$a0 = 0
For lecture
bk_f
$ra 1
$a0
1 * 1 1
$v0

23. A Look at the Stack for $a0 = 2, Part 5
Stack state after execution of the second encounter with the second jr (return from fact routine after updating $v0 to 2 * 1 * 1)
return address to caller routine (main routine) restored to $ra from the stack
original value of $a0 restored from the stack
stack pointer updated to point to first call to fact
old TOS
$sp
caller rt addr
$a0 = 2
 $sp
bk_f
$a0 = 1
bk_f
$a0 = 0
For lecture
caller rt addr
bk_f
$ra 2 1
$a0
2 * 1 * 1
1 * 1
$v0

24. Allocating Space on the Stack
The segment of the stack containing a procedure’s saved registers and local variables is its procedure frame (aka activation record)
The frame pointer ($fp) points to the first word of the frame of a procedure – providing a stable “base” register for the procedure
$fp is initialized using $sp on a call and $sp is restored using $fp on a return
high addr
$fp
Saved argument regs (if any)
Saved return addr
Saved local regs (if any)
Local arrays & structures (if any)
$sp
On the other hand, the stack pointer (sp) might change during the procedure (not the case for all of our examples) so references to a local variable in memory might have different offsets wrt the sp depending on where they are in the procedure.
low addr

25. Allocating Space on the Heap
Static data segment for constants and other static variables (e.g., arrays)
Dynamic data segment (aka heap) for structures that grow and shrink (e.g., linked lists)
Allocate space on the heap with malloc() and free it with free()
Memory
$sp
0x 7f f f f f f c
Stack
Dynamic data
(heap)
$gp 0x
Static data 0x
Text
(Your code)
Stack and heap grow towards each other.
Malloc() allocates space on the heap and returns a pointer to it. Memory allocation is controlled by programs (in C) and is the source of many common and difficult bugs. Forgetting to free space leads to “memory leak” and the need for “garbage collection” and freeing space to early to “dangling pointers”. PC 0x
Reserved 0x

26. MIPS Addressing Modes
Register addressing – operand is in a register
Base (displacement) addressing – operand is at the memory location whose address is the sum of a register and a 16-bit constant contained within the instruction
Immediate addressing – operand is a 16-bit constant contained within the instruction
PC-relative addressing –instruction address is the sum of the PC and a 16-bit constant contained within the instruction
Pseudo-direct addressing – instruction address is the 26-bit constant contained within the instruction concatenated with the upper 4 bits of the PC

27. Addressing Modes Illustrated
1. Register addressing
op rs rt rd funct
Register
word operand
op rs rt offset
2. Base addressing
base register
Memory
word or byte operand
3. Immediate addressing
op rs rt operand
4. PC-relative addressing
op rs rt offset
Program Counter (PC)
Memory
branch destination instruction
5. Pseudo-direct addressing
op jump address
Program Counter (PC)
Memory
jump destination instruction ||

28. Review: MIPS Instructions, so far
Category
Instr
OpC
Example
Meaning
Arithmetic
(R & I format)
add
0 & 20
add $s1, $s2, $s3
$s1 = $s2 + $s3
subtract
0 & 22
sub $s1, $s2, $s3
$s1 = $s2 - $s3
add immediate 8
addi $s1, $s2, 4
$s1 = $s2 + 4
shift left logical
0 & 00
sll $s1, $s2, 4
$s1 = $s2 << 4
shift right logical
0 & 02
srl $s1, $s2, 4
$s1 = $s2 >> 4 (fill with zeros)
shift right arithmetic
0 & 03
sra $s1, $s2, 4
$s1 = $s2 >> 4 (fill with sign bit)
and
0 & 24
and $s1, $s2, $s3
$s1 = $s2 & $s3 or
0 & 25
or $s1, $s2, $s3
$s1 = $s2 | $s3
nor
0 & 27
nor $s1, $s2, $s3
$s1 = not ($s2 | $s3)
and immediate c
and $s1, $s2, ff00
$s1 = $s2 & 0xff00
or immediate d
or $s1, $s2, ff00
$s1 = $s2 | 0xff00
load upper immediate f
lui $s1, 0xffff
$s1 = 0xffff0000

29. Review: MIPS Instructions, so far
Category
Instr
OpC
Example
Meaning
Data
transfer
(I format)
load word 23
lw $s1, 100($s2)
$s1 = Memory($s2+100)
store word 2b
sw $s1, 100($s2)
Memory($s2+100) = $s1
load byte 20
lb $s1, 101($s2)
$s1 = Memory($s2+101)
store byte 28
sb $s1, 101($s2)
Memory($s2+101) = $s1
load half 21
lh $s1, 101($s2)
$s1 = Memory($s2+102)
store half 29
sh $s1, 101($s2)
Memory($s2+102) = $s1
Cond. branch
(I & R format)
br on equal 4
beq $s1, $s2, L
if ($s1==$s2) go to L
br on not equal 5
bne $s1, $s2, L
if ($s1 !=$s2) go to L
set on less than immediate a
slti $s1, $s2, 100
if ($s2<100) $s1=1; else $s1=0
set on less than
0 & 2a
slt $s1, $s2, $s3
if ($s2<$s3) $s1=1; else $s1=0
Uncond. jump
jump 2 j
go to 10000
jump register
0 & 08
jr $t1
go to $t1
jump and link 3
jal
go to 10000; $ra=PC+4

30. Instruction Categories
Review: MIPS R3000 ISA
Instruction Categories
Load/Store
Computational
Jump and Branch
Floating Point
coprocessor
Memory Management
Special
3 Instruction Formats: all 32 bits wide
Registers
R0 - R31 PC HI LO
6 bits
5 bits
5 bits
5 bits
5 bits
6 bits
R format OP rs rt rd
shamt
funct
I format OP rs rt
16 bit number
J format OP
26 bit jump target