As can be seen, at the bottom level is the microkernel, which is about 4,000 lines of code (mostly in C, plus a small amount of assembly language). It handles interrupts, scheduling, and message passing. It also supports an application programming interface (API) of about 30 kernel calls that authorized servers and drivers can make. User programs cannot make these calls. Instead, they can issue POSIX system calls which send messages to the servers. The kernel calls perform functions such as setting interrupts and copying data between address spaces.