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Simple version of OS organization:

• Apps
  • Abstract layer — unaware of the rest
    • Client/server with send/receive primitives
      • • Harder modularity
      • • No fast transfer of data
    • Virtualization
      • Machine + extra instructions
      • Extra: system calls
      • Pro: same memory
• Kernel
• Hardware

Kernel + Hardware: Abstract machine

System Calls with x86-64 Linux

• Use a SYSCALL instruction—privileged
• To invoke, use rdi, rsi, rdx, r10, r8, r9
• To point to memory, point to address, next register is the size
• rax is the system call number
• Note that memory is not used, just registers
• The kernel uses the SYSRET call to return to user code
• rax returns the syscall value (also r11 due to technical reasons)
• Note values from -1 to -4095 means error. The negation of this number is the errorno exposed in C/C++
• Cons
  • More hassle than function calls
  • More state needs to be saved

fd = open("f", O_RDONLY);

int open(char const* f, int flags) {
	// move f into %rdi
	asm("movqsl %eax, %rax");
	// move flags into %rsi

	asm("movq %12, %rax");
	asm("SYSCALL");
}
	

Operating System Organization

When creating a large application, what approach should we take?

• An object-oriented program?
  • Slow?
  • Monolithic (all or nothing, one part breaks, everything does)