1. COMPUTER ARCHITECTURE & OPERATIONS I
Instructor: Ryan Florin

2. Representing Instructions
The University of Adelaide, School of Computer Science
11 June 2018
Representing Instructions
Instructions are encoded in binary
Called machine code
MIPS instructions
Encoded as 32-bit instruction words
Small number of formats encoding operation code (opcode), register numbers, …
Regularity!
Register numbers
$t0 – $t7 are reg’s 8 – 15
$t8 – $t9 are reg’s 24 – 25
$s0 – $s7 are reg’s 16 – 23
§2.5 Representing Instructions in the Computer
Chapter 2 — Instructions: Language of the Computer

3. R-Format and I-Format

4. MIPS R-format Instructions
The University of Adelaide, School of Computer Science
11 June 2018
MIPS R-format Instructions op rs rt rd
shamt
funct
6 bits
5 bits
Instruction fields
op: operation code (opcode)
rs: first source register number
rt: second source register number
rd: destination register number
shamt: shift amount (00000 for now)
funct: function code (extends opcode)
Chapter 2 — Instructions: Language of the Computer

5. The University of Adelaide, School of Computer Science
11 June 2018
R-format Example op rs rt rd
shamt
funct
6 bits
5 bits
add $t0, $s1, $s2
R-format
$s1
$s2
$t0
add 17 18 8 32
000000
10001
10010
01000
00000
100000 =
Chapter 2 — Instructions: Language of the Computer

6. The University of Adelaide, School of Computer Science
11 June 2018
Example
What is:
012a 8020 ?
add $s0, $t1, $t2
Chapter 2 — Instructions: Language of the Computer

7. MIPS I-format Instructions
The University of Adelaide, School of Computer Science
11 June 2018
MIPS I-format Instructions op rs rt
constant or address
6 bits
5 bits
16 bits
Immediate arithmetic and load/store instructions
rt: destination register number
Constant: –215 to +215 – 1
Address: offset added to base address in rs
*** This slide is VERY important ***
** Use this slide to figure which is rs and which is rt **
Chapter 2 — Instructions: Language of the Computer

8. MIPS I-format Instructions
The University of Adelaide, School of Computer Science
11 June 2018
MIPS I-format Instructions
Which is rs and which is rt?
addi $t5, $s4, 5
lw $t0, 24 ($s0)
sw $t1, 16 ($t8)
addi $t0, $t1, -191
Chapter 2 — Instructions: Language of the Computer

9. In Class Exercises
Convert the following MIPS instructions into Machine Instructions
add $t1, $t2, $t3
addi $t5, $s3, 5
lw $t4, 200($s3)

10. In Class Exercises (Answers)
Convert the following MIPS instructions into Machine Instructions
add $t1, $t2, $t3
R-format
$t2
$t3
$t1
add 10 11 9 32
000000
01010
01011
01001
00000
100000

11. In Class Exercises (Answers)
Convert the following MIPS instructions into Machine Instructions
addi $t5, $s3, 5
I-format
$s3
$t5 5 8 19 13 5
001000
10011
01101

12. In Class Exercises (Answers)
Convert the following MIPS instructions into Machine Instructions
lw $t4, 200($s3)
I-format
$s3
$t4
200 35 19 12
200
100011
10011
01100

13. Design Principle 4: Good design demands good compromises
Different formats complicate decoding, but allow 32-bit instructions uniformly
Keep formats as similar as possible

14. Stored Program Computers
The University of Adelaide, School of Computer Science
11 June 2018
Stored Program Computers
Instructions represented in binary, just like data
Instructions and data stored in memory
Programs can operate on programs
e.g., compilers, linkers, …
Binary compatibility allows compiled programs to work on different computers
Standardized ISAs
The BIG Picture
Chapter 2 — Instructions: Language of the Computer

15. The University of Adelaide, School of Computer Science
11 June 2018
Logical Operations
Instructions for bitwise manipulation
§2.6 Logical Operations
Operation C
Java
MIPS
Shift left
<<
sll
Shift right
>>
>>>
srl
Bitwise AND
&
and, andi
Bitwise OR |
or, ori
Bitwise NOT ~
nor
Useful for extracting and inserting groups of bits in a word
Chapter 2 — Instructions: Language of the Computer

16. The University of Adelaide, School of Computer Science
11 June 2018
Shift Operations op rs rt rd
shamt
funct
6 bits
5 bits
shamt: how many positions to shift
Shift left logical
Shift left and fill with 0 bits
sll by i bits multiplies by 2i
Shift right logical
Shift right and fill with 0 bits
srl by i bits divides by 2i (unsigned only)
Chapter 2 — Instructions: Language of the Computer

17. The University of Adelaide, School of Computer Science
11 June 2018
AND Operations
Useful to mask bits in a word
Select some bits, clear others to 0
and $t0, $t1, $t2
$t2
$t1
$t0
Chapter 2 — Instructions: Language of the Computer

18. The University of Adelaide, School of Computer Science
11 June 2018
OR Operations
Useful to include bits in a word
Set some bits to 1, leave others unchanged
or $t0, $t1, $t2
$t2
$t1
$t0
Chapter 2 — Instructions: Language of the Computer

19. The University of Adelaide, School of Computer Science
11 June 2018
NOT Operations
Useful to invert bits in a word
Change 0 to 1, and 1 to 0
MIPS has NOR 3-operand instruction
a NOR b == NOT ( a OR b )
nor $t0, $t1, $zero
Register 0: always read as zero
$t1
$t0
Chapter 2 — Instructions: Language of the Computer

20. What I want you to do
Review Chapter 2