Introduction: Hardware and Operating System

Introduction

Operating System

• A program that acts as an intermediary between a user of a computer and the computer hardware

Operating system goals

• Execute user programs and make solving user problems easier
• Use the computer hardware in an efficient manner

Role of Operating System

• A resource mannager and allocator
  • Decide between conflicting requests for hardware access
  • Attempt to be efficient and fair
• A control program
  • Control execution of user programs

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Two common OS families

POSIX

• Unix-ish
• e.g. Linux, Mac, Android, iOS

Windows

• Shipped by Microsoft

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Computer Component

CPU: Central Processing Unit

• IBM PCs are Intel 80386 compatible
• Intel, AMD
• Today's dominant ISA(Instruction Set Architecture): x86-64 (developed by AMD)

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RAM: Random Access Memory

• Periodically charge to store in a capacitor
  $\rightarrow$ High power consumption

Traditional RAM

• Execute data transfer at falling edges

DDR RAM: Double data rate SDRAM

• Execute data transfer on both rising and falling edges

North Bridge

• Coordinate access to main memory
• CPU - RAM

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I/O device

South Bridge

• Facilitate I/O between devices, CPU and main memory
• CPU - I/O

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Architecture Review

CPU

RAM

• 16-bit CPU can address 64KB of memory(= $2^{16}$ bytes)
• 1 byte is stored at each address
• Not all memory is free

Bit of CPU determines address space

Commuincating between CPU and the devices

1. I/O = Polling

• I/O-only memory space shared by the CPU and a device
• CPU and device communicate by reading / writing to the virtual memory space
• Problems
  • CPU must mediate all transfers
    • Case) Transfer data from disk to memory
    • CPU must copy value from the I/O port to memory
  • All I/O must be synchronous
    • Case) Disk wants to send data
    • but the CPU isn't reading the I/O port

2. DMA: Direct Memory Access

• Parts of RAM shared by the CPU and a device
• I/O can access to memory directly
• After transfer is completed, I/O generate interrupt to the CPU

3. Interrupts

• Signal from a device to the CPU
• Interrupt causes the OS to switch to handler code (from the running code to an interrupt handler)
  • Each interrupt (=each device) is assigned a number
  • Number acts as an index into the Interrupt Vector Table
• Interrupt causes a context switch
  • State of the CPU must be saved before the switch
  • and restored after the handler completes

Context means the Current task status = register value

Interrupt Timeline

• Interrupt enables the CPU and devices to work in parallel

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Booting up

1. Start the BIOS

BIOS: Basic Input/Output System

• Code from the BIOS gets copied to RAM at a low address (e.g. 0xFF)
  • BIOS is stored in ROM so that BIOS doesn't need to be copied to RAM
  • Practically, BIOS code gets copied to RAM because ROM is too slow
• JMP 0xFF written to RAM at 0xFFFF0
• x86 CPU always start with 0xFFFF0 in the EIP(Instruction Pointer) register

RAM 0xFF 에 BIOS code 가 복사되고 CPU 가 항상 시작하는 위치인 0xFFFF0 에는 0xFF 로 JMP 하라는 명령어가 존재

Essential goals of the BIOS

• Scan storage media for a Master Boot Record (MBR)
  • Load the boot record into RAM
  • Tell the CPU to execute the loaded code

2. Load setting from CMOS

3. Initialize devices

Scans and initializes hardware

• CPU and memory
• Keyboard and mouse

Builds the Interrupt Vector Table

Runs POST test

4. RUN POST (Power-On Self Test)

5. Initiate the bootstrap sequence

Bootstrapping

• BIOS identifies all potentially bootable devices
• MBR has code(bootloader) that can load the actual OS

Master Boot Record

• 512 byte file written to sector 1
  • 446 bytes of executable code
  • Entries for 4 partitions
  • The magic number (55AA) serves to identify the MBR
    

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Applications

System Call Interface

System call

• Interface between OS and Applications

System call table

• Each OS API is given a specific index in the table

Traps: Software Interrupts

Software can also generate interrupts

• 1 is system call number
• CPU calls 80 interrupt
• On x86, system calls are initiated via an interrupt
  • On x86, int 0x80 is the system call interrupt
  • EAX holds the table index of the desired API

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Kernel

Kernel

• The one program running at all times on the computer

Kernel features

Device Management

• Required: CPU and Memory
• Optional: disks, keyboards, etc.

Loading and executing programs

System calls and APIs

Protection and Security

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Kernel architecture

Monolithic kernels

• All funcitonality is compiled together
• All code runs in privileged kernel-space

Pros

• Single code base
• Robust APIs
• Fast performance

Cons

• Large code base $\rightarrow$ hard to manage
• Bugs crash the entire kernel

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Micro kernels

Pros

• Small code base $\rightarrow$ easy to manage
• Service may crash, but the system will remain stable
• Easy to add new functionality

Cons

• Slow performance (Many context switches)
• No stable APIs

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Hybrid kernels