01 Intro

Software¶

a set of programs

Types of Softwares¶

Operating System¶

System software that acts as an interface between users and the hardware resources of a computing system.

• Resource allocator
  • Keeps track of occupied/empty portions of the primary and secondary memory
  • Status of I/O Devices
• control program
• Kernel of a computing system Kernel is the most important part of any computing system/programming environment
  • Kernel is system software which is part of operating system
  • Kernel provides interface between hardware and software components

Types of Operating System¶

• Mobile
• Personal Computer
• Real Time
• Distributed

Hardware Resources¶

• Memory
  • Primary is volatile
  • Secondary is non-volatile
• Processor
• I/O Devices

Example¶

flowchart LR

gc[Google Chrome] -->
CPU((CPU)) -->
Screen[Output Screen]

HD[(Hard Disk)] -->
|Operating System <br/> loads| pm[[Primary Memory]]

Device¶

flowchart LR
OS <-->
dd[Device Driver] <-->
dc[Device Controller] <-->
Device

Controller¶

• Data Register/Local buffer
• Command Register

Driver¶

• Interface between OS & Device
• Understands the device controller & device

Daemons¶

Background system processes, that are not in direct control of user

Interrupts¶

Asynchoronous request to the CPU, handled by the OS using the ISR (Interrupt Service Routine).

They can be initiated anytime without reference to the system clock.

Could be hardware-generated or software-generated.

ISR & IVT¶

ISR and IVT(Interrupt Vector Table) are stored in fixed location in memory.

ISR is accessed using IVT, which contains the starting address of the ISRs.

8086 has 256 vectored interrupts

Each ISR requires 4 bytes

• 2 bytes of IP followed by
• 2 bytes of CS

If the starting address of IVT is \(\text{00000_H}\), then the ending address is \(\text{003FF_H}\)

Graph¶

(take from slides)

Interrupt latency¶

Time taken to service an interrupt

Advantages¶

• Save processor time
  • Processor resources are very valuable, as it can be used for some other task
• Avoid polling
  • Going around asking I/O devices; wasting processor time

flowchart LR

subgraph Write Operation
direction LR
up2[User Program] -->
OS2[OS] -->
dd2[Device Driver] -->
dc2[Device Controller] <-->
od2[Output Device]

dc2 -->|Status| dd2
end

subgraph Read Operation
direction LR
up[User Program] -->
OS -->
dd[Device Driver] -->
dc[Device Controller] <-->
id[Input Device]

dc -->|Interrupt| dd
end

Program Counter¶

Similar to instruction pointer of x86

• Initially points to 1^st instruction
• Subsequently, points to the address of the next instruction to be executed

Interrupt Handling¶

1. An interrupt that occurs in between an instruction can only happen after fetch, decode, execute, write-back of that instruction is first complete.

2. When interrupt occurs, we need to push the following values into stack

   • PC so that we can return to the same point after finishing the interrupt

   • CPU state Contents of all CPU and flag registers

   • Service the interrupt using ISR
   • Restore processor state
   • Load the saved return address into the program counter
   • Resume interrupted computation

Storage of Multi-Byte data¶

Little Endian = Lower Byte is stored first then Higher Byte. This is what 8086 uses

Big Endian = Higher Byte is stored first then Lower Byte

Storage Structure¶

• Bit (0/1) [most basic unit]
• Byte = 8 bits
• Word = Group of bytes

Types Of Storage Devices¶

flowchart TB
Memory -->
cpu[CPU Registers] & Cache & Primary & Secondary & Tertiary

Primary["Primary<br/>(Volatile)"] --> RAM
Secondary["Secondary<br/>(Non-Volatile)"] --> HDD & SSD
Tertiary["Tertiary<br/>(Non-Volatile Backup)"] --> CD & DVD & Backup

flowchart TB

subgraph While Using
CPU2[CPU] -->|1| PM2[PM] -->|2| CPU2
end

subgraph First Time
direction LR

CPU1[CPU]
CPU1 -->
|1| PM1[PM] -->
|2| SM1[(SM)]

SM1 -->|3|PM1

PM1 -->|4| CPU1
end

Booting¶

Process of loading OS Kernel into the primary memory

Steps¶

• Starting address of Bootstrap program is stored into the Program Counter
• Bootstrap loader loads the OS using Boostrap program

Bootstrap Program¶

• In Intel architecture, it’s called as BIOS(Basic Input Output System)
• In Unix architecture, it’s called as GRUB(GRand Unified Bootloader)
• In Android, LK(Little Kernel)

Tasks¶

1. Run POST(Power-On Self Test) diagnostics
2. Initialize and check peripheral devices
3. Initializes other aspects of the system, such as the registers
4. Locates and loads the kernel

ROM¶

Read-Only Memory

EEPROM¶

Electrically Erasable Programmable Read-Only Memory

It is firmware (combination of hardware and software that can hold code)

Bootstrap program is burnt into EEPROM

System Programs loaded to PM, System process, Deamons, Program in its execution, Printer network, Background Process

In linux, the first program is systemd

Software Interrupt¶

Trap/exception

flowchart LR

UP[User Program]
OS
Tasks

subgraph Tasks
    direction LR
    io[I/O Management]
    Memory
end

UP -->|Request| OS --> Tasks

Programming Types¶

Uni was before.

Modes of Operations¶

This mode bit will change continuously.

Privileged Execution of Instruction¶

Privilege signifies the access level of a program

I/O, memory, timer, CPU, and Interrupts are privileged

Privilege level varies from

• 0 (high privilege)
• 3 (low privilege)

If a program trying to access memory is privileged, then it is checked if it is in Kernel mode or User mode.

If it is in user mode, a trap is generated.

Lab code¶

Lab

I missed something (Sep 14 1^st Hour)¶

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