System calls: how programs ask the operating system for help

0 · Recall Warm-up

Answer from memory first. Answers are in the Answer Vault at the bottom.

1. Is the system-call (trap) instruction privileged? from Lesson 0.1.2
2. 2³²/2¹²? from Lesson 0.1.1

1 · The Hook

When a C program calls printf("hi"), the letters eventually reach your screen, but the program never touches the screen itself; it is not allowed to. Somewhere a request crosses from user mode into the kernel. That doorway is a system call. GATE asks what happens during one, and which familiar functions are system calls and which are not.

3 · Core Concept

Predict First — the answer

Once. printf is a library function that buffers text in user mode; only the single underlying write system call enters the kernel.

In plain words

A system call is a program's request to the operating system: "open this file", "give me more memory", "start a new process". The program puts its request in agreed places, executes a special trap instruction, the CPU jumps into the kernel, the kernel does the work, and control returns with an answer.

1. The steps of a system call

1. The program calls a library wrapper (e.g. read(fd, buf, n)).
2. The wrapper puts the system-call number and the parameters in agreed places.
3. It executes the trap instruction: the CPU switches to kernel mode and jumps to the OS's trap handler.
4. The kernel looks up the number in the system-call table, runs the matching function, and puts the result in a register.
5. The CPU switches back to user mode and the program continues.

Each system call therefore costs two mode switches (user → kernel → user).

2. Passing parameters

In registers (fast, few); in a block of memory whose address is in a register; or on the stack.

3. Kinds of system calls (UNIX examples)

4. System calls vs library functions

• System calls: fork, exec, read, write, open, close, exit, brk/sbrk.
• Library functions (run in user mode, may or may not call the kernel): printf (calls write), malloc (sometimes calls brk/mmap), strlen, sqrt (never calls the kernel).

5. Kernel structures (names to recognise)

• Monolithic kernel: all OS services in one big kernel (traditional UNIX, Linux).
• Microkernel: only the essentials in the kernel; file systems and drivers run as user-level servers (more switches, more robust).

4 · Visual

read(fd, buf, 100) step by step

 USER MODE                          KERNEL MODE
 ---------                          -----------
 program calls read()
   library puts number + params
   in registers, executes TRAP  ---> trap handler
                                     look up syscall table
                                     sys_read(): copy data
                                     result -> register
   program continues            <--- return-from-trap (back to user mode)

5 · Worked Examples

Example 1 Easy

Is strlen a system call?

Solution

No: it just counts characters in user memory.

Example 2 Medium

How many mode switches does one system call cause?

Solution

2: user → kernel, then kernel → user.

Example 3 GATE

A program makes 5 system calls. How many times does the CPU switch from user mode to kernel mode because of them?

Solution

5 (and 5 switches back).

6 · GATE Traps

• printf and malloc are library functions; write and brk are the system calls underneath.
• A system call is entered by a trap instruction, not an ordinary function call.
• The trap switches to kernel mode; the return switches back.
• Not every library call reaches the kernel (sqrt never does).

7 · Memory Hook

"Library wraps, trap enters, kernel works, return leaves."

8 · Connect the Dots

Phase 0 is complete. Phase 1 looks at the most important thing the kernel manages: processes, how they are created with the fork system call, and what states they move through.

9 · Self-Test

GATE format. Try every question before opening the vault.

Q1 · MCQ · 1 mark(s) — System calls

Which of these is a system call?

1. (A) strlen
2. (B) sqrt
3. (C) write
4. (D) printf

Q2 · NAT · 1 mark(s) — Mode switches

A program performs 7 system calls. Total number of mode switches they cause?

Numerical answer: ________

Q3 · MCQ · 1 mark(s) — Mechanism

A system call transfers control to the kernel using:

1. (A) a privileged instruction executed in user mode
2. (B) a trap (software interrupt) instruction
3. (C) disabling interrupts
4. (D) an ordinary function call

Q4 · MSQ · 2 mark(s) — Categories

Which are process-control system calls?

1. (A) exec
2. (B) fork
3. (C) wait
4. (D) lseek

Q5 · MCQ · 1 mark(s) — Kernel design

In a microkernel, file systems and device drivers usually run:

1. (A) in the BIOS
2. (B) on the disk controller
3. (C) inside the kernel
4. (D) as user-level server processes

10 · Rule Card

• System call: wrapper → parameters + number → trap → kernel handler via system-call table → result → return to user mode (2 mode switches).
• Parameters: registers, memory block, or stack.
• Categories: process (fork, exec, exit, wait), file (open, read, write, close, lseek), device, information, communication.
• printf, malloc, strlen are library functions; write, brk are system calls.
• Monolithic kernel vs microkernel.

11 · Concept Ledger

Unlocked in this lesson. From here on, these terms may be used.

system callsystem-call wrappersystem-call number and tabletrap instructionparameter passingmode switches per system callprocess-control, file, device, information and communication callslibrary function vs system callmonolithic kernelmicrokernel