1. Computer Architecture and Organization
Instruction Sets:
Characteristics and Functions

2. The complete collection of instructions that are a CPU can perform
Instruction set
The complete collection of instructions that are a CPU can perform
A program is a “machine code” of consecutive machine instructions
Represented in binary form
Usually described by assembly codes 2

3. Elements of an Instruction
Operation code (Op code)
Do this
Source Operand reference
To this
Result Operand reference
Put the answer here
Next Instruction Reference
When you have done that, do this...
Implicit in case of sequential execution (PC stores it) 3

4. Operand source and destination
Main memory
Virtual memory
Cache
CPU register
I/O device 4

5. Instruction Representation
In machine code each instruction has a unique bit pattern
For human user (programmer) a symbolic representation is used
e.g. ADD, SUB, LOAD
Operands can also be represented in this way
ADD R, A
Means add the contents of memory location A to Register R
And store the result back in Register R
Note: operation performed on the contents of A (not on the address itself) 5

6. Simple Instruction Format
Characteristics:
2 operand instruction
Could be 0, 1, 2, 3 or more …
4 bit opcode
Instruction set can have at most 16 instructions
6 bit operands
limit on memory + number representation)

7. Instruction Cycle State Diagram
Note: it is NOT guaranteed that a fetched instruction is executed!
There are traps/exceptions

8. Example of Program Execution
Fetch
Execute

9. Instruction Types Data processing Data storage Data movement
Arithmetic and logic instruction
Data storage
Memory instructions:
Load from memory, Store to memory
Data movement
I/O operations
Read from I/O device, write to I/O device
Program flow control
Test instructions (used e.g. in conditionals)
Branch instructions (used in conditionals, loops) 6

10. Number of Addresses 3 addresses Operand 1, Operand 2, Result
ADD A, B, C
A B + C
May be a forth - next instruction (usually implicit)
Not common
Needs very long words to hold everything 7

11. Number of Addresses 2 addresses
One address doubles as operand and result
ADD A, B
A A + B
Reduces length of instruction
Requires some extra work
Temporary storage to hold some results 8

12. Number of Addresses 1 address Implicit second address
Usually a register (accumulator)
ADD A
AC AC + A
Common on early machines 9

13. Number of Addresses 0 (zero) address All addresses implicit
Uses a stack
Compute C = A + B
PUSH A
PUSH B
ADD
POP C 10

14. Number of Addresses Summary

15. How Many Addresses More addresses Fewer addresses
More complex (powerful?) instructions
More registers
Inter-register operations are quicker
Fewer instructions per program
Fewer addresses
Less complex (less powerful) instructions
More instructions per program
Faster fetch/execution of instructions 11

16. Design Decisions Operation repertoire Data types Instruction formats
How many ops?
What can they do?
How complex are they?
Data types
Instruction formats
Length of op code field
Number of addresses 12

17. Addressing modes (later…) RISC v CISC
Design Decisions
Registers
Number of CPU registers available
Which operations can be performed on which registers?
Addressing modes (later…)
RISC v CISC 13

18. Types of Operand Addresses Numbers Characters Logical Data
Fixed point = Integer
Floating point
Characters
Encoded
IRA (in USA known as ASCII), EDCDIC
Logical Data
Bits or flags 14

19. Addressing is by 8 bit unit
Pentium Data Types
8 bit byte
16 bit word
32 bit double word
64 bit quad word
Addressing is by 8 bit unit
A 32 bit double word is read at addresses divisible by 4 15

20. General - arbitrary binary contents Integer - single binary value
Specific Data Types
General - arbitrary binary contents
Integer - single binary value
Ordinal - unsigned integer
Unpacked BCD - One digit per byte
Packed BCD - 2 BCD digits per byte
Near Pointer - 32 bit offset within segment
Bit field
Byte String
Floating Point 16

21. Pentium Numeric Data Formats17

22. Some instructions need operand aligned on 32 bit boundary
PowerPC Data Types
8 (byte), 16 (halfword), 32 (word) and 64 (doubleword) length data types
Some instructions need operand aligned on 32 bit boundary
Can be big- or little-endian
Fixed point processor recognises:
Unsigned byte, unsigned halfword, signed halfword, unsigned word, signed word, unsigned doubleword, byte string (<128 bytes)
Floating point
IEEE 754
Single or double precision

23. Types of Operation Data Transfer Arithmetic Logical Conversion I/O
System Control
Transfer of Control 18

24. May be different instructions for different movements
Data Transfer
Specify
Source
Destination
Amount of data
May be different instructions for different movements
e.g. IBM 370
Or one instruction and different addresses
e.g. VAX 19

25. Add, Subtract, Multiply, Divide Signed Integer Floating point ?
Arithmetic
Add, Subtract, Multiply, Divide
Signed Integer
Floating point ?
May include
Increment (a++)
Decrement (a--)
Negate (-a) 20

26. Shift and Rotate Operations

27. Logical
Bitwise operations
AND, OR, NOT 21

28. Conversion
E.g. Binary to Decimal 22

29. May be specific instructions
Input/Output
May be specific instructions
May be done using data movement instructions (memory mapped)
May be done by a separate controller (DMA) 23

30. Privileged instructions CPU needs to be in specific state
Systems Control
Privileged instructions
CPU needs to be in specific state
Ring 0 on
Kernel mode
For operating systems use 24

31. Transfer of Control Branch Skip Subroutine call
e.g. branch to x if result is zero
Skip
e.g. increment and skip if zero
ISZ Register1
Branch xxxx
ADD A
Subroutine call
E.g. interrupt call 25

32. x86 Branch instructions Compare: cmp sets flags
Conditional jump: jne = jump on not equal, checks flags.
Conditional jumps: there is a lot of them. Some examples:
jpo = jump if parity odd
jpe = jump if parity even
jo = jump if overflow
jno = jump if no overflow
jz = jump if zero
jnz = jump if not zero
je = jump if equal
jl = jump if less than
jle = jump if less than or equal
jg = jump if greater than
jge = jump if greater than or equal

33. Branch Instruction

34. Nested Procedure Calls

35. Use of Stack

36. Stack Frame Growth Using Sample Procedures P and Q

37. What order do we read numbers that occupy more than one byte
Byte Order
What order do we read numbers that occupy more than one byte
e.g. (numbers in hex to make it easy to read)
can be stored in 4 x 8 bit locations as follows 27

38. Address Version 1 Version 2 0x00184 12 78 0x00185 34 56 0x00186 56 34
Byte Order (example)
Address Version 1 Version 2 0x 0x 0x 0x
i.e. read top down or bottom up? 28

39. The problem is called Endian
Byte Order Names
The problem is called Endian
The system on the right has the least significant byte in the lowest address
This is called little-endian
The system on the left has the least significant byte in the highest address
This is called big-endian 29

40. Example of C Data Structure

41. Alternative View of Memory Map

42. Endianess is not standardized
Pentium (80x86), VAX are little-endian
IBM 370, Motorola 680x0 (Mac), and most RISC are big-endian
Internet is big-endian
Makes writing Internet programs on PC more awkward!
WinSock provides htoi and itoh (Host to Internet & Internet to Host) functions to convert 30