Computer Architecture and
Organization
Dr. Khawar Khurshid
Spring ‘20
Instruction Set
Addressing Modes
Design Decisions
Instruction formats
Length of op code field
Number of addresses
Operations
How many ops?
Which ops?
Data types
Addressing modes
What is an Instruction Set?
The complete collection of instructions
that are understood by a CPU
Usually represented by assembly codes
Elements of an Instruction
Operation code (Op code)
Do this
Source Operand reference
To this
Result Operand reference
Put the answer here
Types of Operand
Addresses
Numbers
Logical Data
Instruction Representation
In machine code each instruction has a
unique bit pattern
For users a symbolic representation is
used
e.g. ADD, SUB, LOAD
Operands can also be represented in this
way
ADD A,B
Simple Instruction Format
Instruction Types
Data processing
Data movement
Program flow control
Number of Addresses
3 addresses
2 addresses
1 address
0 address
Number of Addresses
3 addresses
Operand 1, Operand 2, Result
c = a + b;
Not common
Needs very long instructions to hold
everything
Number of Addresses
2 addresses
One address doubles as operand and result
a=a+b
Reduces length of instruction
Requires some extra work
Temporary storage to hold some results
Number of Addresses
1 address
Implicit second address
Usually a register (accumulator)
Number of Addresses
0 (zero) addresses
All addresses implicit
Uses a stack
e.g. push a
push b
add
pop c
c=a+b
How Many Addresses
More addresses
More complex instructions
More registers
Fewer instructions per program
Fewer addresses
Less complex instructions
More instructions per program
Slower execution of program
Types of Operation
Data Transfer
Arithmetic
Logical
Transfer of Control
Types of Operations
• Data Transfer Specify
• Arithmetic Source
• Logical Destination
• Transfer of Control
Types of Operations
• Data Transfer • Add, Subtract,
• Arithmetic • Multiply, Divide
• Logical • May include
• Transfer of Control —Increment (a++)
—Decrement (a--)
—Negate (-a)
Types of Operations
• Data Transfer
• Arithmetic • Bitwise operations
• Logical • AND, OR, NOT
• Transfer of Control
• Shift left, right
Types of Operations
• Data Transfer
• Arithmetic
• Logical
• Transfer of Control
• Branch
— e.g. branch to x if
result is zero
• Skip
— e.g. increment and skip
if zero
• Subroutine call
Branch Instruction
Nested Procedure Calls
Use of Stack
Data Types
8 bit Byte
16 bit word
32 bit double word
64 bit quad word
Floating Point data
Addressing through 8 bit unit
Byte Order
What order do we read numbers that
occupy more than one byte
87654321 can be stored in 4x8bit
locations as follows
Byte Order (example)
Address Value (1) Value (2)
184 87 21
185 65 43
186 43 65
186 21 87
i.e. read top down or bottom up
Byte Order Names
The problem is called Endian
The system on the left has the most significant
byte in the lowest address
This is called big-endian
The system on the right has the least
significant byte in the lowest address
This is called little-endian
Example
Addressing Modes and
Formats
Addressing Modes
Immediate
Direct
Indirect
Register
Register Indirect
Displacement
Stack
Immediate Addressing
Operand is part of instruction
Operand = address field
e.g. ADD 5
Add 5 to contents of accumulator
5 is operand
No memory reference to fetch data
Fast
Limited range
Immediate Addressing
Instruction
Opcode Operand
Direct Addressing
Address field contains address of operand
Effective address (EA) = address field (A)
e.g. ADD A
Add contents of cell A to accumulator
Look in memory at address A for operand
Single memory reference to access data
No additional calculations to work out
effective address
Limited address space
Direct Addressing
Instruction
Opcode Address A
Memory
Operand
Indirect Addressing
Memory cell pointed to by address field
contains the address of (pointer to) the
operand
EA = (A)
Look in A, find address (A) and look there for
operand
e.g. ADD (A)
Add contents of cell pointed to by contents
of A to accumulator
Indirect Addressing
Large address space
2n where n = word length
Multiple memory accesses to find
operand
Hence slower
Indirect Addressing
Instruction
Opcode Address A
Memory
Pointer to operand
Operand
Register Addressing
Operand is held in register named in
address filed
EA = R
Limited number of registers
Very small address field needed
Shorter instructions
Faster instruction fetch
Register Addressing
No memory access
Very fast execution
Very limited address space
Multiple registers helps performance
Register Addressing
Instruction
Opcode Register Address R
Registers
Operand
Register Indirect Addressing
EA = (R)
Operand is in memory cell pointed to by
contents of register R
Large address space (2n)
One fewer memory access than indirect
addressing
Register Indirect Addressing
Instruction
Opcode Register Address R
Memory
Registers
Pointer to Operand Operand
Displacement Addressing
EA = A + (R)
Address field hold two values
R = register that holds the base address
A = displacement
Displacement Addressing
Instruction
Opcode Register R Address A
Memory
Registers
Pointer to Operand + Operand
Relative Addressing
A version of displacement addressing
R = Program counter
EA = A + (PC)
i.e. get operand from A cells from current
location pointed to by PC
Base Addressing
A version of displacement addressing
R = A dedicated Base Register
EA = A + (B)
i.e. get operand from A cells from
location pointed to by B
Stack Addressing
Operand is (implicitly) on top of stack
e.g.
ADD Pop top two items from stack
and add
Any Queries?
Thank You
