COMP-1200 System Calls and Functions

Learning Objectives

• Compare and contrast system calls and function calls.
• Illustrate the behavior of a program using functions.
• Use a system call or another function.

System Calls

• Definition: System calls provide functions as part of the operating system (OS). They are included in the OS Kernel segment.
• Purpose: To safely and conveniently perform low-level, privileged operations.
• When a system call is made, control is passed to the operating system, and when it returns, the program continues execution from where it left off.

Understanding the Von Neumann Model

• Components:
  • CPU (Central Processing Unit)
  • Control Unit
  • Memory
  • I/O Devices (Keyboard, Mouse, Monitor, etc.)

Interacting with External Devices

• Certain operations, especially I/O operations, require specialized knowledge about I/O device registers:
  • Knowledge about what to do with registers.
  • Understanding the sequence of operations needed to use them.

Importance of the Operating System

• I/O resources are shared among multiple users/programs. A mistake in one program could adversely affect others, for example:
  • Printer accidentally printing 1,000 copies
  • Soundcard becoming unavailable due to errors.

System Calls in RISC-V

• The RISC-V architecture uses the ecall instruction to implement system calls.
• Use of Registers:
  • a7 Register: Chooses which system call to use.
  • a0, a1, … registers: Used to pass arguments to system calls.
  • Return Values: Returned via the a0 register.

Table of System Call Numbers in RARS

Example: Printing Integers

• Code Example:

# printInt(11)
li a0, 11  # a0 = 11
li a7, 1   # a7 = 1
ecall       # syscall(1)

# exit()
li a7, 10  # a7 = 10
ecall       # syscall(10)

• Equivalent Code:

printInt(11);  
exit();

Reading Integers Example

• Code Example:

# a0 = readInt()
li a7, 5   # a7 = 5
ecall       # syscall(5)
sub t0, zero, a0  # t0 = -a0
add t0, t0, t0     # t0 = t0 + t0

• Purpose: To read an integer input from the keyboard.

System Calls in Other Operating Systems

• System calls may differ across operating systems:
  • Example: In Linux, exit is syscall #94, not 10.
  • There is no readInt in Linux; instead, use read (syscall #63).

Functions Overview

• A function is a reusable block of code that performs a specific task, allowing code to be written once and reused multiple times.

Function Mechanics

• Functions can use both local and global variables.
• Arguments are passed using a-registers (a0, a1, …)
• Return values are typically stored in a0 or a1.
• Temporary registers (t0, t1): Assumed to be unavailable after a function call, used for brief calculations.
• Saved registers (s0, s1, …): Used for longer-term value storage during function calls.

Declaring Functions

• Functions start with a label (e.g., main, add) and conclude with a ret instruction to return to the original function.

Control Flow Instructions

• JAL (Jump And Link): Used to call a function by jumping to its label and saving the return address in ra.
• RET Instruction: Returns to the address saved in ra.

Example Function: add1

• Purpose: Add 1 to the integer provided as input.
• Code Example:

# int add1(int x)
add1:   addi a0, a0, 1
ret

• Equivalent in Java:

int add1(int x) {
  return x + 1;
}

Tracing add1

• Execution flow through registers:
  • On entry: Set a0 to 11, call add1, return to main.

Example Function: mult3

• Purpose: Multiply the integer by 3.
• Code Example:

# int mult3(int x)
mult3:   add t0, a0, a0
            add a0, a0, t0
            ret

• Equivalent in Java:

int mult3(int x) {
  return x + x + x;
}

What's Next?

• Topics to be covered:
  • Control Flow Instructions
  • Function Stack Management