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Today’s class

Today’s class. Instruction set architecture. Instruction Set Design. The instruction set architecture (ISA) level is between the high-level languages and the hardware When new hardware architecture comes along … Need to maintain backward compatibility

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Today’s class

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  1. Today’s class • Instruction set architecture Computer Architecture I - Class 11

  2. Instruction Set Design • The instruction set architecture (ISA) level is between the high-level languages and the hardware • When new hardware architecture comes along … • Need to maintain backward compatibility • Can add new features to exploit new hardware capabilities Computer Architecture I - Class 11

  3. ISA Level Computer Architecture I - Class 11

  4. Properties of the ISA Level • In principle, the ISA level is defined by how the machine appears to a machine language programmer • In modern times, this is refined to say that ISA-level code is what a compiler outputs • Compiler writer needs to know: • Memory model • What registers are available • What data types are available • What instructions are available Computer Architecture I - Class 11

  5. Memory Models • Consider 8-byte (64-bit) words • Do words have to be “word-aligned” (start at a word address)? Computer Architecture I - Class 11

  6. Instruction and Data Spaces • Most machines have a single linear address space at the ISA level • However, a few machines have separate address spaces for instructions and data • Effectively doubles memory! • All writes go to data space, protecting instructions from being overwritten Computer Architecture I - Class 11

  7. Registers • Special purpose • Program counter • Stack pointer • Program status word • General purpose • Hold key local variables and intermediate results of calculations • RISC machines usually have at least 32 • Trend in new CPU designs is to have even more Computer Architecture I - Class 11

  8. Instructions • LOAD and STORE instructions – move data between memory and registers • MOVE instructions – move data among the registers • Arithmetic instructions • Boolean instructions • Comparing data items and branching on the results Computer Architecture I - Class 11

  9. Pentium 4 • Three operating modes • Real mode – all features added since the 8088 are turned off (effectively behaves like a simple 8088; if a program does something wrong whole machine crashes) • Virtual 8086 mode – makes it possible to run old 8088 programs in a protected way (if a program crashes, the OS is notified instead of the machine crashing) • Protected mode – actually behaves like a Pentium 4! Computer Architecture I - Class 11

  10. Pentium 4 • Address space • 16,384 segments • Each segment has 232 addresses (4 GB) • Most operating systems, including Unix and Windows, support only one segment • Every byte has its own address • Words are 32 bits long, and low-endian Computer Architecture I - Class 11

  11. Pentium 4 • Registers • EAX is main arithmetic register • EBX holds pointers • ECX plays a role in looping • EDX used in multiplication and division • ESI – source string pointer • EDI – destination string pointer • EBP – base of the current stack frame • ESP – stack pointer • EIP – instruction pointer Computer Architecture I - Class 11

  12. Pentium 4 • Data types • Two’s complement integers • Unsigned integers • Binary coded decimal numbers • IEEE 754 floating point numbers Computer Architecture I - Class 11

  13. Instruction Formats Computer Architecture I - Class 11

  14. Design Criteria • Short instructions are better than long ones • Sufficient room in the instruction format to express all the operations desired • Number of bits in the address field Computer Architecture I - Class 11

  15. Examples • Consider an (n + k) bit instruction that has a k-bit opcode and a single n-bit address • Allows for 2k different operations • Allows for 2n addressable memory cells • Could alternatively have: • k-1 bit opcode and n+1 bit address • Half as many instructions but twice the addressable memory • k+1 bit opcode and n-1 bit address • Twice as many instructions but half the addressable memory Computer Architecture I - Class 11

  16. Expanding Opcodes • An instruction with a 4-bit opcode and three 4-bit address fields. Computer Architecture I - Class 11

  17. Expanding Opcodes • Suppose the designers need: • 15 three-address instructions • 14 two-address instructions • 31 one-address instructions • 15 no-address instructions • Use opcodes 0-14 as the three-address instructions • Interpret opcode 15 differently Computer Architecture I - Class 11

  18. Expanding Opcodes Computer Architecture I - Class 11

  19. Pentium 4 Instruction Formats • Highly complex and irregular! • Up to 6 variable-length fields, 5 of which are optional Computer Architecture I - Class 11

  20. Addressing Modes • How to specify where an operand for an instruction is located • How the bits of an address field in an instruction are interpreted Computer Architecture I - Class 11

  21. Immediate Addressing • Address part of the instruction contains the actual operand itself • An immediate instruction for loading 4 into register 1 • Does not require an extra memory reference to fetch the operand Computer Architecture I - Class 11

  22. Direct Addressing • Instruction specifies full address of the operand • Instruction will always access the same memory location • Useful for accessing global variables Computer Architecture I - Class 11

  23. Register Addressing • Specifies a register for the operand • Most common addressing mode on computers Computer Architecture I - Class 11

  24. Register Indirect Addressing • Register contains the address in memory of the operand (the register acts as a pointer) • The following example computes the sum of the elements of an array Computer Architecture I - Class 11

  25. Indexed Addressing • Contents of a register plus a constant offset form the address of the operand • Consider the following program for computing the OR of corresponding products in two arrays Computer Architecture I - Class 11

  26. Reverse Polish Notation • Infix form: a + b • Postfix form: a b + • Any expression can be expressed without parentheses: • (a + b) * c = ab + c * • Convenient for evaluating formulas using stacks on a computer Computer Architecture I - Class 11

  27. Reverse Polish Notation • Some examples of infix expressions and their reverse Polish notation equivalents. Computer Architecture I - Class 11

  28. Evaluation of RPN Formulas • Evaluation of (8 + 2 x 5) / (1 + 3 x 2 - 4) • RPN form is 8 2 5 x + 1 3 2 x + 4 - / Computer Architecture I - Class 11

  29. Pentium 4 Addressing Modes • Highly irregular • Depends on which operating mode you are in (16-bit or 32-bit) • Not all modes apply to all instructions • Not all registers can be used in all modes • Compiler writer’s nightmare! Computer Architecture I - Class 11

  30. Pentium 4 Addressing Modes Computer Architecture I - Class 11

  31. Data Movement Instructions • Copying data is the most fundamental of all operations • Four possibilities: • Register to register • Register to memory • Memory to register • Memory to memory • May have one general instruction to cover all cases, or separate instructions for each case • Need to indicate how much data to copy Computer Architecture I - Class 11

  32. Dyadic Operations • Combine two operands to produce a result • Arithmetic instructions • Boolean instructions Computer Architecture I - Class 11

  33. Monadic Operations • Have one operand and produce one result • Shift and rotate instructions • CLR, INC, NEG Computer Architecture I - Class 11

  34. Comparisons and Conditional Branches • Common operation is a comparison to 0 • Use bits in the PSW, such as C, N, Z Computer Architecture I - Class 11

  35. Other Instructions • Procedure calls • Loop control Computer Architecture I - Class 11

  36. Programmed Input/Output • Single input instruction and single output instruction • Selects an I/O device and a single character is transferred • CPU must execute an explicit sequence of instructions for each and every character read or written Computer Architecture I - Class 11

  37. Programmed Input/Output • A simple terminal might have four 1-byte registers • Registers might be part of computer’s memory address space Computer Architecture I - Class 11

  38. Direct Memory Access • DMA gets rid of most of the interrupts of programmed I/O Computer Architecture I - Class 11

  39. Pentium 4 Instructions Computer Architecture I - Class 11

  40. Pentium 4 Instructions Computer Architecture I - Class 11

  41. Pentium 4 Instructions Computer Architecture I - Class 11

  42. Pentium 4 Instructions Computer Architecture I - Class 11

  43. Pentium 4 Instructions Computer Architecture I - Class 11

  44. Flow of Control • Most statements execute sequentially • Branches alter statement flow • Procedures also alter statement flow, but unlike branches, control is returned to the statement after the procedure call once the procedure is finished • Recursive procedures are procedures that call themselves Computer Architecture I - Class 11

  45. The Towers of Hanoi • An ancient problem with a simple recursive solution • In a certain monastery in Hanoi there are 3 gold pegs • Around the first peg are 64 gold disks, each disk slightly smaller in diameter than the disk below it • The monks have to move the disks to the third peg, one disk at a time, but at no time may a larger disk sit on top of a smaller one • It is said that when they finish the world will come to an end Computer Architecture I - Class 11

  46. 5-Disk Initial Configuration Computer Architecture I - Class 11

  47. Solution Outline • First move n-1 disks from peg 1 to peg 2 • Now move 1 disk from peg 1 to peg 3 • Then move the n-1 disks from peg 2 to peg 3 Computer Architecture I - Class 11

  48. Solution • This procedure moves n disks from peg I to peg j. Computer Architecture I - Class 11

  49. The Stack • Need a stack to store the parameters and local variables for each invocation of a recursive procedure • Do not want to confuse values of these variables in different invocations Computer Architecture I - Class 11

  50. The Stack Computer Architecture I - Class 11

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