Introduction 2

Overview

• CPU design
• Address spaces, interruts
• OS structures/implementation

CPU design

• A CPU basic cycle consist of fetch[取], decode, execute
• Every CPU has his own instruction set
• CPU has a set of registers
• Registers are used to store data and for special functions (e.g. program counter, program status word – mode bit)
• The compiler/programmer decides what to keep in the registers
• Context switching[上下文切换] must save and restore the CPU’s internal state, including its registers

Memory Management Unit (MMU)

• Memory adsresses from 0 to MAX
• Variables are mnemonic[帮助记忆的] names for memory addresses

• You don’t know where the process will run in physical memory at compile time

  • Multiple processes run on modern machines
  • The compiler assumes that it will start running at 0 (logical address space)
  • An offset[补偿] is added at runtime by the MMU (physical address space)

    physical address = logical address + offset

• Modern computer use a logical and physical memory addresses:

  • Every process has a logical address space – [0,MAX64] (theoretically理论上)
  • The machine has a physical address space – [0, MAX ] (MAX determined by the amount of physical memory)

• Address translation takes place in MMU

  physical = f (logical )

• A context switch between processes invalidates[使无效] the MMU (as well as registers, cache, … )

Timer interrupts

• Interrupts temporarily pause a process’s normal operation
• Different types of interrupts:
  • Timer interrupts by CPU clock
  • I/O interrupts for I/O completion or error codes
  • Software generated, e.g. errors and exceptions
    

    1. Timer generates an interrupt
    2. CPU finishes current instruction and tests for interrupt
    3. Transfer to interrupt service routine

    • Hardware saves current process state (PSW, program counter)
    • Set program counter to interrupt service routine
    • Save registers and other state information

    1. Carry out[执行] interrupt service routine (scheduler)
    2. Restore next process to run

Moore’s “law”

Moore’s “law”: “The number of transistors on an integrated circuit (chip) doubles roughly every two years”

• Closely linked, but not necessarily related to performance
• The “power wall” slows performance improvements of single core/single processor systems
  • A few cores for multiple “programs” is easy to justify
  • How to use massively[大规模的] parallel computers/CPUs/many core machines
  • Can we extract parallelism automatically, can we implement parallelism at the lowest level (similar to multiprogramming)

Multi-core, hyperthreaded processors

• Modern CPUs contain multiple cores and are often hyper-threaded
• Evolution in hardware has implications on operating system design
  • XP did not support multi processor architectures
  • Process scheduling needs to account for load balancing and CPU affinity[亲和性]
  • Cache coherency[缓存一致性] becomes important

Memory

• Memory hierarchies[层级] used to balance cost and performance
  • Fast and expensive memory is used for caching
  • Slow and inexpensive memory is used for long term storage
  • Memory includes, registers, L1/L2 cache, main/core memory, disk, etc.
  • L2 Cache can be shared or dedicated[专注的] to individual cores
  • Cache management is mainly done by hardware
  • The CPU can only access main memory directly (i.e. files have to be brought into memory first)

I/O Devices

• Device driver interacts[交互] with the controller, controller interacts with the device (e.g., disk controller)
• The operating system/device driver typically communicates with the controller through registers
• I/O can take place through:
  • Busy waiting
  • Interrupt based
  • Direct memory access (using DMA chip)

Operating System Structure

• Systems contain a lot of functionality
• Operating Systems are structured by Micro kernels[微内核] and Monolithic[单内核]

Micro Kernels

• All non-essential functionality is extracted[取出] from the kernel
  • Communication, memory management and CPU scheduling are likely to be included in the kernel
  • The file system, GUI, device drivers are likely to be user processes
    除了保留基本功能，其他功能移出到user mode
• Micro kernels are more easy to extend, more portable[便携], and usually more reliable
• Frequent system calls and kernel traps[陷阱] cause significant overhead[开销] (mode switches)
• Some Unix version, Mac OS X, Minix, and early versions of Windows (NT4.0) were (partially) micro kernels

Monolithic Systems

• All procedures are linked together into one single executable running in kernel mode
• Monolithic kernels are difficult to maintain
• Current versions of Windows, Linux are implemented as monolithic kernels
  操作系统高度紧密，移植性不佳。但是若设计完善，效率高

Summary

• Operating Systems are closely linked to computer architecture
• Address translation and interrupts
• OS structures

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谢谢投喂 (～o￣3￣)～

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