1. Machine Instruction Characteristics

2. Machine Instruction Characteristics
Machine Instructions- are instructions executed by the processor.
Instruction set- a collection of instructions that the processor can execute

3. Elements of a machine instruction
Opcode- uses a binary code which specifies the operation to be preformed.
Source operand reference- the input for the operation
Result operand reference- the result produced by the operation
Next instruction reference- tells the processor where to get the next instruction

4. Source and result operands can be found in
Main or virtual memory
Processor register- contains registers that can be used by machine instructions
Immediate- the operand value is being contained when the instruction is being executed
I/O devices- instruction specifies I/O module and device but if memory mapped then just another main or virtual memory address

5. Representation of instruction
OPCODE (4 Bits)
Ref to Operand (6 Bits)
Ref to Operand 2
(6 Bits)
An instruction format is used to define the layout of the bits allocated to these elements of instructions.
The instruction format explicitly or implicitly indicates the addressing Modes used for each operand in that instruction.

6. Instruction format Instruction length: -
The longer instruction length gives:
More Opcodes
More operands
More addressing modes
Greater address range.

7. Factors considered for instruction length
Memory Size.
Memory organization.
Bus structure
Processor speed
Processor complexity.

8. Allocation of Bits for different fields in an instruction
Number of addressing modes.
Number of operands.
Register versus memory.
Number of register sets.
Address range.
Address granularity.

9. Variable length instructions
Advantages: -
Provides flexibility in addressing modes.
Provides complex addressing modes.
Reduces number of instruction fetch cycles.
Reduces amount of space taken by a program.
Disadvantages: -
Requires more complex CPU.
Decoding of instruction becomes difficult.

10. Instruction Sets Instruction Characteristics Types of Operands
Types of Operations

11. Instruction Representation
Are represented by a sequence of bits
It is common to use “symbolic representation” of machine instructions
Op codes and operands are represented by abbreviations called “mnemonics”

12. Mnemonics ADD add SUB subtract MUL multiply DIV divide
LOAD load data from memory
STOR store data to memory

13. Instruction types Data processing- arithmetic and logic instructions
Data storage- moving data into or out of register or memory locations
Data movement- I/O instructions
Control- test and branch instructions

14. Number of Addresses
A traditional way of describing processor architecture
What is the maximum number of addresses one might need in an instruction?
An instruction could be required to contain four addresses

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

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

17. Number of Addresses 1 address instructions Implicit second address
Usually a register (accumulator)
Common on early machines
E.g. ADD B

18. Number of Addresses Zero address instruction:
Special memory organization
Last in first out (Stack)
Stack is a known location
E.g. CMA

19. Utilization of Instruction Addresses (Nonbranching Instructions)
Number of Addresses
Utilization of Instruction Addresses (Nonbranching Instructions)

20. Number of Addresses 1 address instructions 2 Or 3 address instructions
One general purpose register
Multiple general purpose registers

21. Instruction Set Design
Operation Repertoire- How many and which operations to provide, and how complex the operations should be.
Data Types- The various types of data upon which operations are performed.
Instruction Format- Instruction length (in bits), number of addresses and size of fields.

22. Instruction Set Design
Registers- Number of processor registers that can be referenced by instructions, and their use
Addressing- The mode or modes by which the address of an operand is specified.

23. Types of operands

24. TYPES OF OPERANDS Addresses Numbers Characters Logical Data
Specialized Data Types
Data Structures
First 4 are the most important general categories of data
Last 2: Ex: list or string of characters

25. Addresses Main or Virtual memory address Unsigned Integers
By calculating on the operand reference we can determine the main or Virtual memory address. This can be considered an unsigned integer and therefore data.

26. Numbers All machine languages include numeric types
Non-numeric processing: counters, field widths, etc
Numbers in a machine are limited by magnitude and precision
Consequences: rounding, overflow, underflow

27. Numerical Data Binary integer or Binary fixed point
Binary floating point
Decimal

28. Decimal Majority of users deal with decimals
Necessity to convert from decimal to binary on input and binary to decimal on output
Packed decimal

29. Packed Decimal
Used when there is a great deal of I/O and simple computation
Each decimal digit is represented by a 4-bit code. Ex: 0 = 0000, 1 = 0001
Numbers are formed by a string of 4-bit codes usually in multiples of 8-bit
246 =
Positive: 1100
Negative: 1101
Clearly less compact than a straight binary representation
Negative numbers can be represented by putting a 4-bit sign digit at either left or right end of a string

30. Characters Text or character strings Represented by a sequence of bits
Morse Code
International Reference Alphabet (IRA)
American Standard Code for Information Interchange (ASCII)
Extended Binary Coded Decimal Interchange Code (EBCDIC)
Data processing and Communication Systems
Each character is represented in a 7-bit pattern. Thus, 128 characters can be represented. Some of these are control characters maybe for printing and whatnot
Transmitted using 8-bit per characters. 8th bit may be a 0 or used as parity error check
EBCDIC: IBM mainframes, 8-bit code, compatible with packed decimals as well

31. Logical Data
Consider n-bit unit as consisting of n 1-bit items of data, each having a value of 0 or 1
Advantages:
Store array of Boolean or Binary data items
Manipulating bits of data items
Ex: Convert from IRA to packed decimals (only need the first four bits), Floating point Operations (you might want to round)

32. Types of Operations Data Transfer Arithmetic Logical Conversion
Input/output
System Control
Transfer of Control

33. Data Transfer Operation Name Description Move(Transfer) Store
Load (Fetch)
Exchange
Clear ( Reset)
Set
Push
Pop
Transfer word from source to destination
From processor to memory
From memory to processor
Swap contents of source and destination
Transfer 0s to destination
Transfer 1s to destination
Word from source to top of stack
From top of stack to destination

34. Arithmetic Operation Name Description Add Subtract Multiply Divide
Absolute
Negate
Increment
Decrement
Compute sum of two operands
Difference of two operands
Product of two operands
Quotient of two operands
Replace by its absolute value
Change sign
Add 1 to operand
Subtract 1 to operand

35. Logical Operation Name Description AND; OR; NOT; OR Test Compare
Set Control Variables
Shift
Variables
Perform the specified logical operation bitwise
Test specified condition; set flags based on outcome
Make logical and arithmetic comparison of two operands
Class of instructions to set controls for protection purposes, interrupt handling, timer control.
Left/right operand introducing constants at end.
Left/right with wraparound end

36. Conversion Operation Name Description Translate Convert
Translate values in a section of memory based on a table of correspondences
Convert the contents of a word from one form to another. E.g(packed decimal to binary

37. I/O Operation Name Description Input (Read) Output ( Write) Start I/O
End I/O
Transfer data from specified I/O port or device to destination
From specified source to I/O port or device
Transfer instructions to I/O processor to initiate I/O operation
Transfer status info from I/O system to specified destination

38. System Of Control
Can be executed only while the processor is in a certain privileged state or is executing a program in a special privileged area of memory.
These instructions, are reserved for the use of the operating system.
Examples:
A system control instruction may read or alter a control register
An instruction to read or modify a storage protection key
Access to process control blocks in multiprogramming system

39. Transfer Of Control Description Operation Name Jump (Branch)
Unconditional Transfer: load PC with specified address
Test condition: either load PC with specified address or do nothing.
Place current program control info in known location; jump to specified address
Replace contents of PC and other register from known location
Fetch operand from location and execute as instructed
Increment PC to skip next instruction
Either skip or do nothing based on condition
Stop program execution
Test specified repeatedly and resume condition
No operation is performed; but program execution is continued.
Jump (Branch)
Jump Conditional
Jump to Subroutine
Return
Execute
Skip
Skip conditional
Halt
Wait (Hold)
No Operation

40. QUESTIONS How many different address designs are there?
Name two types of operations.
What are the most general categories of data?
What type of endian mode does x86 use?
What are the most used Character Codes nowadays?
What are the elements of machine instruction?
What are the advantages of Logical Data?