Processes

What is a simple Mini-OS?

• user programs

• use system call interface (API)

• API connects to kernel

• kernel manages:

  • process management

  • memory management

  • file system management

  • I/O control

• kernel controls hardware:

  • CPU

  • RAM

  • devices

What are CPU registers?

• very small, very fast storage inside CPU

• faster than main memory

• used during program execution

• each register has a specific role

• OS uses them in process management and system control

What are the main types of CPU registers?

• general-purpose registers

  • temporary data

  • arithmetic

  • addresses

  • calculation results

• special-purpose registers

  • PC / IP = address of next instruction

  • SP = top of stack

  • BP = references variables in stack

  • flags register (FR) = stores status flags like zero, carry, overflow

What are the different sets of CPU registers?

• segment registers

  • store addresses of memory segments

  • e.g. code CS, data DS, stack SS

• floating-point registers

  • used for decimal / floating-point arithmetic

• vector registers (SIMD)

  • process multiple data values at the same time

• control registers

  • control CPU/system behavior

  • used for system control, interrupts, modes

What are the main general-purpose registers and their roles?

• EAX (Extended Accumulator Register)= main accumulator, arithmetic default register

• EBX (Extended Base Register)= base register, memory addressing

• ECX (Extended Counter Register)= counter register, loops/iterations

• EDX (Extended Data Register) = data register, extra arithmetic data/results

What are the additional general-purpose registers?

• ESI (Extended Source Index)= source index

• EDI (Extended Destination Index) = destination index

• EBP (Extended Base Pointer) = base/frame pointer

• ESP (Extended Stack Pointer)= stack pointer (top of stack)

What are protection rings, user mode/kernel mode, and system calls?

• 2 modes:

  • user mode = applications run here

  • kernel mode = OS/kernel runs here

• privileged operations

  • only allowed in kernel mode

  • if a user program tries them → CPU fault/error

• protection rings (x86)

  • Ring 0 = highest privilege, kernel

  • Ring 3 = lowest privilege, applications

  • Rings 1 and 2 exist, but modern OSs mainly use Ring 0 and Ring 3

• system calls

  • way for a program to request services from the kernel

  • interface between process and operating system

  • examples:

    • hardware access

    • create/run processes

    • scheduling / kernel services

**A process is a program that is currently running.

How do system calls work?

• application runs in Ring 3

• when it needs OS service, it makes a system call

• CPU switches to Ring 0 / kernel mode

• kernel dispatcher/handler executes the request

• control returns to the application

What are Linux system calls used for?

• read data

• write data

• open file

• close file

• get file information

How is a syscall invoked? (read() example)

• user program calls read()

• wrapper puts:

  • syscall number in RAX

  • arguments in registers like RDI, RSI, RDX

• special instruction / trap enters kernel

• kernel syscall dispatcher handles it

• result is returned to user space

What is a process and what is its structure?

• process = abstraction of a running program

• program is loaded into memory

• OS schedules the process on the CPU

• process structure / address space:

  • text = program code

  • data = static variables/data

  • heap = dynamic memory created during execution

  • stack = function calls, local variables, LIFO

• process address space:

  • process uses virtual memory

  • mapped to physical memory

  • mapping done via page table

  • handled by MMU and OS memory management

What are process states, PCB, multiprogramming, and context switching?

• process states:

  • new = process created

  • ready = waiting for CPU

  • running = currently executing

  • blocked = waiting for I/O/event

  • terminated = finished

• PCB (Process Control Block):
  stores important process info

  • program counter

  • registers

  • stack pointer

  • program status word

  • process ID

  • file descriptors

• multiprogramming:

  • multiple processes in system at same time

  • CPU switches quickly between them

  • creates illusion of parallelism

• context switching:

  • OS saves state of one process

  • loads state of another process

  • CPU changes from one process to another

What are POSIX and the main POSIX process-creation functions?

• POSIX = Portable Operating System Interface

• standard interface for Unix-like OSs

• gives compatibility/portability

• helps software run on different systems with similar system calls

• process creation (POSIX):

  • fork() = creates a child process

  • exec() = replaces process memory with a new program

  • wait() = parent waits for child to finish

  • exit() = terminates process

• fork() return values:

  • < 0 = error

  • = 0 = child process

  • > 0 = parent gets child PID

What is a process tree?

• parent-child hierarchy of processes

• root often init (pid 1)

• children can create more children

How does a process terminate?

• exit() after finishing

• exit due to handled error

• exit due to unhandled error

• parent can terminate child with kill()

• OS removes process and frees resources

What are the process memory segments?

• text = program instructions, read-only

• data = initialized global/static data

• bss = uninitialized global/static data

• heap = dynamic memory, grows up

• stack = local variables/function calls, LIFO, grows down

• top area also has command-line arguments and environment variables

What is the stack segment?

• RAM region used during function calls

• LIFO structure

• uses PUSH and POP

• stores:

  • function arguments

  • local variables

  • return address

• divided into stack frames

• grows toward lower addresses

• stack pointer (SP) stores address of top element

What is a stack frame?

• memory block for one function call

• created on top of the stack

• contains:

  • arguments

  • return address

  • base pointer (BP)

  • local variables

What is the base pointer (BP)?

• CPU register: BP / EBP / RBP

• points to a fixed position in current stack frame

• used as reference for:

  • function parameters

  • local variables

• helps with organized stack access

• useful for debugging/backtracking

How do function calls and returns work on x86

• call instruction starts function

• pushes return address onto stack

• return address = instruction after the call

• ret instruction pops return address

• execution continues in calling function

How are function parameters and local variables stored on the stack?

• arguments are above EBP

• local variables are below EBP

• return address and old base pointer are also in stack frame

• EBP is fixed reference point inside the frame

What is a buffer overflow?

• writing more data than a buffer can hold

• can overwrite nearby stack data

• may affect:

  • local variables

  • base pointer

  • return address

• dangerous security problem

What are the basic types of inter-process communication (IPC)?

• message passing

• shared memory

What is message-passing IPC?

• processes communicate with send() and receive()

• messages go through a message queue

• OS/kernel controls the communication

• needs system calls for each message

• slower, but simpler and more isolated

What is shared-memory IPC?

• both processes access the same shared memory region

• memory is mapped into both processes’ virtual memory

• after setup, OS is less involved in communication

• faster

• but more complex and more error-prone

Message passing vs. shared memory?

• message passing: send/receive, slower, simpler, isolation yes, easier

• shared memory: write/read shared area, faster, more complex, isolation no

1. What is the primary purpose of an Operating System?

A) To translate source code into machine code

B) To manage system resources and ensure process isolation

C) To execute only user programs

D) To control user access levels

Correct answer: B

• OS manages system resources

• OS ensures process isolation

• A = compiler job

• C too limited / wrong

• D is only one part, not the main purpose

2. Why are processes divided into user mode and kernel mode?

A) To improve application performance

B) To reduce memory consumption

C) To protect the system from unauthorized access by applications

D) To allow direct access to hardware

Correct answer: C

• user mode / kernel mode exist for protection

• apps in user mode cannot do privileged operations directly

• this prevents unauthorized or dangerous access to the system/hardware

• A not the main reason

• B wrong

• D opposite: modes exist to restrict direct hardware access

What is the main function of the EAX register in x86 architecture?

A) Manages function pointers B) Serves as an accumulator for arithmetic operations

C) Holds the next instruction address

D) Stores system parameters

Correct answer: B

• EAX = main accumulator register

• commonly used for arithmetic operations

• C would be IP / EIP / RIP

• D not its main role

• A wrong

4. What is the purpose of the wait() system call in POSIX systems?

A) To create a new process

B) To terminate the parent process

C) To block the parent process until the child processes complete

D) To replace the memory space of the process

Correct answer: C

• wait() makes the parent process block/wait

• it waits until the child process finishes

• A = fork()

• B wrong

• D = exec()

5. Which memory segment grows toward lower addresses?

A) Heap

B) Stack

C) Data segment

D) Text segment

Correct answer: B

• stack grows toward lower addresses

• heap usually grows toward higher addresses

• data and text do not grow like that in normal execution

6. During a context switch, what is the most critical operation performed by the OS?

A) Pausing CPU clock temporarily

B) Saving only the heap data

C) Saving CPU registers and the stack pointer into the Process Control Block (PCB)

D) Restarting the process from the beginning

Correct answer: C

• during a context switch, the OS must save the process CPU state

• especially registers and stack pointer in the PCB

• later it can restore them and continue the process

• A wrong

• B not enough

• D wrong, process continues, not restarts

7. Which statement correctly describes the behavior of exec() in UNIX?

A) Creates a new process without affecting the parent

B) Replaces the current process memory space with a new program

C) Suspends the parent process D) Forces a transition to kernel mode

Correct answer: B

• exec() does not create a new process

• it replaces the current process memory/image with a new program

• usually used after fork()

• A = more like fork()

• C wrong

• D not its meaning

8. Which memory segment holds uninitialized global variables?

A) Heap

B) Data segment

C) BSS (Block Started by Symbol)

D) Stack

Correct answer: C

• BSS stores uninitialized global/static variables

• data segment stores initialized global/static variables

• heap = dynamic memory

• stack = local variables / function calls

9. When does a buffer overflow occur?

A) When a variable is unused

B) When a program writes more data than the allocated buffer size

C) When a process fails to call exec()

D) When fork() completes without wait()

Correct answer: B

• buffer overflow happens when a program writes more data than the buffer can hold

• this can overwrite nearby memory

• dangerous because it may affect variables, base pointer, or return address

• A, C, D are unrelated

10. What is the main advantage of shared-memory IPC over message-passing IPC?

A) It is safer because the OS controls all communication

B) It is faster since it avoids repeated system calls for every data exchange

C) It requires no synchronization

D) It is simpler to implement

Correct answer: B

• shared memory is faster

• after setup, processes can exchange data without a system call for every message

• message passing is usually slower because the OS/kernel is involved each time

• A describes message passing more

• C false, shared memory does need synchronization

• D false, shared memory is usually more complex