Understanding Machine Instruction Characteristics

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 • 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. 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

25. Addresses • Main or Virtual memory address • Unsigned Integers

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 = 0000 0010 0100 0110 • Positive: 1100 • Negative: 1101

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)

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

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 Operation Name Description • Jump (Branch) • Jump Conditional • Jump to Subroutine • Return • Execute • Skip • Skip conditional • Halt • Wait (Hold) • No Operation • 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.

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?