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let divisible n m = match n mod m with 0 -> true | _ -> false

let crackle_pop n =
  let n_is_divisible_by = divisible n in

  match (n_is_divisible_by 3, n_is_divisible_by 5) with
  | true, true -> "CracklePop"
  | true, _ -> "Crackle"
  | _, true -> "Pop"
  | _ -> string_of_int n

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Processes

• abstraction of running program
• supports ability to have psuedo-concurrent operation
  • In a uniprogramming model, no notion of a process is needed, because you only have one at all times

Key elements of a process

• program counter
• registers

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System Calls

• The interface between user programs and the operating system is primarily about dealing with abstractions

• a syscall library is provided in many languages to make system calls from the language

• making a system call is a special type of procedure call except that syscalls enter kernel while procedure calls do not

  • procedure library in C executes some built-in api function, which traps to kernel with the specified params

• TRAP instruction jumps to a fixed address, then this piece of code examines the syscall number and dispatches to the appropriate syscall handler (via id-handler indexed table)

• Once syscall is completed its work, control might be returned to the user space procedure, and the stack is cleaned up

• N.B. If the syscall is blocking, it will not return control to the user space procedure! ex. is a keyboard watiing for input

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Practice Questions - Part 1

1. How is a system call different from a function call?

• A system call executes a piece of code in the kernel (protected) portion of the OS. A function call executes a piece of code in the user portion of the OS.

2. How is system call processing different from interrupt servicing?

• The OS processes system calls from the user while the OS processes interrupts from hardware or other processes in the OS

shardulbee

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
#include <fcntl.h>


int main() {
  int pipefd[2];
  pipe(pipefd);