1
CPS3340
COMPUTER ARCHITECTURE
Fall Semester, 2013
Lecture 11: MIPS-Conditional Instructions
Instructor: Ashraf Yaseen
10/15/2013
DEPARTMENT OF MATH & COMPUTER SCIENCE
CENTRAL STATE UNIVERSITY, WILBERFORCE, OH
Review
2

Last Class

Binary Integers



Representation of MIPS Instructions

R-format

I-format
This Class

Conditional Instructions



Unsigned, Signed, Signed Extension
slt
Branch Addressing
Next Class

Procedure
Instructions for Making Decisions
3

High-level programming language
C/C++:
if … else … (conditional)
goto
(unconditional)
for, while, until (loops)

Assembly Languages
MIPS:
beq (branch if equal)
bne (branch if not equal)
j (unconditional jump)
Conditional Operations
4

Branch to a labeled instruction if a condition is true
 Otherwise,

beq rs, rt, L1
 if

(rs == rt) branch to instruction labeled L1;
bne rs, rt, L1
 if

continue sequentially
(rs != rt) branch to instruction labeled L1;
j L1
 unconditional
jump to instruction labeled L1
Compiling If Statements
5

C code:
if (i==j) f = g+h;
else f = g-h;
Compiling If Statements
6

C code:
if (i==j) f = g+h;
else f = g-h;


f ($s0), g ($s1), h($s2), i($s3), j($s4)
Compiled MIPS code:
bne
add
j
Else: sub
Exit: …
$s3, $s4, Else
$s0, $s1, $s2
Exit
$s0, $s1, $s2
Assembler calculates addresses
Compiling Loop Statements
7

C code:
while (save[i] == k) i = i+1;
i

in $s3, k in $s5, address of save in $s6
Flowchart?
Compiling Loop Statements
8

C code:
while (save[i] == k) i = i+1;
i

in $s3, k in $s5, address of save in $s6
Compiled MIPS code:
Loop: sll
add
lw
bne
addi
j
Exit: …
$t1,
$t1,
$t0,
$t0,
$s3,
Loop
$s3, 2
$t1, $s6
0($t1)
$s5, Exit
$s3, 1
Basic Blocks
9

A basic block is a sequence of instructions with
 No
embedded branches (except at end)
 No branch targets (except at beginning)


A compiler identifies basic
blocks for optimization
An advanced processor
can accelerate execution
of basic blocks
More Conditional Operations
10



Less than
Greater than
Combination of logical operations
More Conditional Operations
11

Set result to 1 if a condition is true
 Otherwise,

slt rd, rs, rt
 if

(rs < rt) rd = 1; else rd = 0;
slti rt, rs, constant
 if

set to 0
(rs < constant) rt = 1; else rt = 0;
Use in combination with beq, bne
slt $t0, $s1, $s2
bne $t0, $zero, L
# if ($s1 < $s2)
#
branch to L
Exercise
12
Convert the following C++ statement into MIPS
if (i>j and i<k) {
a++;
}
Assume i in $s0, j in $s1, k in $s2, a in $s3

Exercise
13
if (i>j and i<k) {
a++;
}
Assume i in $s0, j in $s1, k in $s2, a in $s3
L:
slt $t0, $s1, $s0
slt $t1, $s0, $s2
and $t0, $t0, $t1
beq $t0, $zero, L
addi $s3, $s3, 1
…
Better Solution
14
if (i>j and i<k) {
a++;
}
Assume i in $s0, j in $s1, k in $s2, a in $s3
L:
slt $t0, $s1, $s0
beq $t0, $zero, L
slt $t0, $s0, $s2
beq $t0, $zero, L
addi $s3, $s3, 1
…
Branch Instruction Design
15


Why not blt, bge, etc?
Hardware for <, ≥, … slower than =, ≠
 Combining
with branch involves more work per
instruction, requiring a slower clock
 All instructions penalized!


beq and bne are the common case
This is a good design compromise
Signed vs. Unsigned
16



Signed comparison: slt, slti
Unsigned comparison: sltu, sltui
Example
 $s0
= 1111 1111 1111 1111 1111 1111 1111 1111
 $s1 = 0000 0000 0000 0000 0000 0000 0000 0001
 slt
$t0, $s0, $s1 # signed
 –1
< +1  $t0 = 1
 sltu
$t0, $s0, $s1
 +4,294,967,295
# unsigned
> +1  $t0 = 0
Branch Addressing
17

Branch instructions specify
 Opcode,

two registers, target address
Most branch targets are near branch
 Forward
op
rs
6 bits

or backward
rt
5 bits
constant or address
5 bits
16 bits
PC-relative addressing


Target address = PC + offset × 4
PC already incremented by 4 by this time
Jump Addressing
18

Jump (j and jal) targets could be anywhere in text
segment
 Encode
op
address
6 bits

full address in instruction
26 bits
(Pseudo)Direct jump addressing

Target address = PC31…28 : (address × 4)
Target Addressing Example
19

Loop code from earlier example
 Assume
Loop: sll
Loop at location 80000
$t1, $s3, 2
80000
0
0
19
9
2
0
add
$t1, $t1, $s6
80004
0
9
22
9
0
32
lw
$t0, 0($t1)
80008
35
9
8
0
bne
$t0, $s5, Exit 80012
5
8
21
2
19
19
1
addi $s3, $s3, 1
80016
8
j
80020
2
Exit: …
Loop
80024
20000
Branching Far Away
20


If branch target is too far to encode with 16-bit
offset, assembler rewrites the code
Example
L2:
beq $s0,$s1, L1
↓
bne $s0,$s1, L2
j L1
…
Addressing Mode Summary
21
Summary
22

Conditional Instructions
 beq
 bne
j
 slt,
slti
 sltu,
sltui
 Converting

a C Program to MIPS
Branch Addressing
What I want you to do
23


Review Chapter 2
Prepare for assignment 4