This document discusses computer architecture and microprocessors. It covers topics such as instruction sets, instruction formats, addressing modes, and types of instructions. The key points are: - An instruction set is the complete collection of instructions understood by a CPU, usually represented in both machine code and assembly code. - Instructions contain operation codes and references to operands and results. Common addressing modes include immediate, direct, indirect, register, and displacement addressing. - Instruction types include data processing, data storage/movement, and program flow control. Design decisions include the instruction repertoire and number of operand addresses. - Common arithmetic operations are addition, subtraction, multiplication and division of signed integers and floating point numbers. Addressing modes

1. EC-252: COMPUTER ARCHITECTURE
AND MICROPROCESSORS
Vaskar Raychoudhury
Indian Institute of Technology Roorkee

2. What is an Instruction Set?
The complete collection of instructions that are
understood by a CPU
Machine Code
Binary
2
Binary
Usually represented by assembly codes

3. Elements of an Instruction
Operation code (Op code)
Do this
Source Operand reference
To this
3
To this
Result Operand reference
Put the answer here
Next Instruction Reference
When you have done that, do this...

4. Where have all the Operands Gone?
Main memory (or virtual memory or cache)
CPU register
I/O device
(Immediate)
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(Immediate)

5. Instruction Representation
In machine code each instruction has a unique bit
pattern
For human consumption (well, programmers anyway)
a symbolic representation is used
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a symbolic representation is used
e.g. ADD, SUB, LOAD
Operands can also be represented in this way
ADD A, B

6. Simple Instruction Format
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7. Instruction Types
Data processing
Data storage (main memory)
Data movement (I/O)
Program flow control
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Program flow control

8. Number of Addresses (a)
3 addresses
Operand 1, Operand 2, Result
a = b + c;
May be a fourth - next instruction (usually implicit)
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May be a fourth - next instruction (usually implicit)
Not common
Needs very long words to hold everything

9. Number of Addresses (b)
2 addresses
One address doubles as operand and result
a = a + b
Reduces length of instruction
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Reduces length of instruction
Requires some extra work
Temporary storage to hold some results

10. Number of Addresses (c)
1 address
Implicit second address
Usually a register (accumulator)
Common on early machines
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Common on early machines

11. Number of Addresses (d)
0 (zero) addresses
All addresses implicit
Uses a stack
e.g. push a
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e.g. push a
push b
add
pop c
c = a + b

12. How Many Addresses
More addresses
More complex (powerful?) instructions
More registers
Inter-register operations are quicker
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Inter-register operations are quicker
Fewer instructions per program
Fewer addresses
Less complex (powerful?) instructions
More instructions per program
Faster fetch/execution of instructions

13. Design Decisions (1)
Operation repertoire
How many ops?
What can they do?
How complex are they?
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How complex are they?
Data types
Instruction formats
Length of op code field
Number of addresses

14. Design Decisions (2)
Registers
Number of CPU registers available
Which operations can be performed on which
registers?
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Addressing modes (later…)
RISC v CISC

15. Types of Operand
Addresses
Numbers
Integer/floating point
Characters
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Characters
ASCII etc.
Logical Data
Bits or flags

16. Types of Operation
Data Transfer
Source, Destination, Amount of data
Arithmetic
Logical
Bitwise operations
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AND, OR, NOT
Conversion
Binary to Decimal
I/O
System Control
OS specific operations
Transfer of Control
Branch, Skip, Subroutine call (e.g., interrupt)

17. Arithmetic
Add, Subtract, Multiply, Divide
Signed Integer
Floating point ?
May include
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May include
Increment (a++)
Decrement (a--)
Negate (-a)

18. Addressing Modes
Immediate
Direct
Indirect
Register
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Register
Register Indirect
Displacement (Indexed)
Stack

19. Immediate Addressing
Operand is part of instruction
Operand = address field
e.g. ADD 5
Add 5 to contents of accumulator
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Add 5 to contents of accumulator
5 is operand
No memory reference to fetch data
Fast
Limited range

20. Immediate Addressing Diagram
OperandOpcode
Instruction
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21. 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
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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

22. Direct Addressing Diagram
Address AOpcode
Instruction
Memory
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Operand

23. Indirect Addressing (1)
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
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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

24. Indirect Addressing (2)
Large address space
2n where n = word length
May be nested, multilevel, cascaded
e.g. EA = (((A)))
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e.g. EA = (((A)))
Draw the diagram yourself
Multiple memory accesses to find operand
Hence slower

25. Indirect Addressing Diagram
Address AOpcode
Instruction
Memory
Pointer to operand
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Operand

26. Register Addressing (1)
Operand is held in register named in address filed
EA = R
Limited number of registers
Very small address field needed
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Very small address field needed
Shorter instructions
Faster instruction fetch

27. Register Addressing (2)
No memory access
Very fast execution
Very limited address space
Multiple registers helps performance
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Multiple registers helps performance
Requires good assembly programming or compiler
writing
N.B. C programming
register int a;
c.f. Direct addressing

28. Register Addressing Diagram
Register Address ROpcode
Instruction
Registers
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Operand

29. Register Indirect Addressing
C.f. indirect addressing
EA = (R)
Operand is in memory cell pointed to by contents of
register R
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register R
Large address space (2n)
One fewer memory access than indirect addressing

30. Register Indirect Addressing Diagram
Register Address ROpcode
Instruction
Memory
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OperandPointer to Operand
Registers

31. Displacement Addressing
EA = A + (R)
Address field hold two values
A = base value
R = register that holds displacement
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R = register that holds displacement
or vice versa

32. Displacement Addressing Diagram
Register ROpcode
Instruction
Memory
Address A
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OperandPointer to Operand
Registers
+

33. Relative Addressing
A version of displacement addressing
R = Program counter, PC
EA = A + (PC)
i.e. get operand from A cells from current location
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i.e. get operand from A cells from current location
pointed to by PC
c.f locality of reference & cache usage

34. Base-Register Addressing
A holds displacement
R holds pointer to base address
R may be explicit or implicit
e.g. segment registers in 80x86
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e.g. segment registers in 80x86

35. Indexed Addressing
A = base
R = displacement
EA = A + R
Good for accessing arrays
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Good for accessing arrays
EA = A + R
R++

36. Combinations
Postindex
EA = (A) + (R)
Preindex
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Preindex
EA = (A+(R))
(Draw the diagrams)

37. Stack Addressing
Operand is (implicitly) on top of stack
e.g.
ADD Pop top two items from stack
and add
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and add