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Computer Architecture and Organization

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

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Computer Architecture and Organization

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

  2. 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

  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)

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

  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)

  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 • 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)

  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

  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

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

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

  14. Number of Addresses Summary

  15. How Many Addresses • More 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

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

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

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

  19. 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

  20. 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

  21. Pentium Numeric Data Formats

  22. 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

  24. 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

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

  26. Shift and Rotate Operations

  27. Logical • Bitwise operations • AND, OR, NOT

  28. Conversion • E.g. Binary to Decimal

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

  30. Systems Control • Privileged instructions • CPU needs to be in specific state • Ring 0 on 80386+ • Kernel mode • For operating systems use

  31. Transfer of Control • Branch • 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

  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. 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) 12345678 can be stored in 4 x 8 bit locations as follows

  38. Byte Order (example) Address Version 1Version 2 • 0x00184 12 78 • 0x00185 34 56 • 0x00186 56 34 • 0x00187 78 12 • i.e. read top down or bottom up?

  39. 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

  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

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