System Calls

• System calls
• The file API
• Kernel entry/exit

User Mode vs Kernel Mode

Why system calls

Modern kernels offer many services to applications

function calls

regular function calls work like this:

• Caller pushes arguments on the stack
• Caller jumps to the line

IA-32 architecture

System calls

• From programmer's perspective, system calls are a lot like function calls

Trap Instructions

• Switches to kernel mode & jumps to a predefined address
  • Why does the kernel need a special, well-defined "entry point"

System calls steps

Kernel entry

PCB

Blocking

Context Switch

Scheduling

• Preemptive
• Non-preemptive

Priority

• Fixed
• Shortest Priority

Context switching and efficiency

The File API

File descriptors

Reading from & writing to streams

• int read(int fd, void *buf, size_t size)
• int write(int fd, void *buf, size_t size)

Working with multiple streams