1. Computer Architecture & Operations I
Instructor: Ryan Florin

2. Instructions for Making Decisions
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)

3. Conditional Operations
The University of Adelaide, School of Computer Science
21 July 2018
Conditional Operations
Branch to a labeled instruction if a condition is true
Otherwise, continue sequentially
beq rs, rt, L1
if (rs == rt) branch to instruction labeled L1;
bne rs, rt, L1
if (rs != rt) branch to instruction labeled L1;
j L1
unconditional jump to instruction labeled L1
Chapter 2 — Instructions: Language of the Computer

4. Compiling If Statements
The University of Adelaide, School of Computer Science
21 July 2018
Compiling If Statements
C code:
if (i==j) f = g+h; else f = g-h;
Chapter 2 — Instructions: Language of the Computer

5. Compiling If Statements
The University of Adelaide, School of Computer Science
21 July 2018
Compiling If Statements
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 $s3, $s4, Else add $s0, $s1, $s j Exit Else: sub $s0, $s1, $s2 Exit: …
Assembler calculates addresses
Chapter 2 — Instructions: Language of the Computer

6. Compiling Loop Statements
The University of Adelaide, School of Computer Science
21 July 2018
Compiling Loop Statements
C code:
while (save[i] == k) i = i+1;
i in $s3, k in $s5, address of save in $s6
Flowchart?
Chapter 2 — Instructions: Language of the Computer

7. Compiling Loop Statements
The University of Adelaide, School of Computer Science
21 July 2018
Compiling Loop Statements
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 $t1, $s3, add $t1, $t1, $s lw $t0, 0($t1) bne $t0, $s5, Exit addi $s3, $s3, j Loop Exit: …
Chapter 2 — Instructions: Language of the Computer

8. The University of Adelaide, School of Computer Science
21 July 2018
More Examples
C code:
for (i=1; i<=5; i++)
count++;
How to initialize a variable?
addi $t0, $zero, 0 # $t0 = 0
addi $t1, $zero, 10 # $t1 = 10
Chapter 2 — Instructions: Language of the Computer

9. The University of Adelaide, School of Computer Science
21 July 2018
More Examples
C code:
for (i=1; i<=5; i++)
sum += A[i];
What assumptions need to be made?
What is the address of array A?
Should sum be initialized?
Chapter 2 — Instructions: Language of the Computer

10. Summary Conditional Instructions Converting a C Program to MIPS beq
bne j
Converting a C Program to MIPS