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CS485G Spring 2015 42

47 System calls invoked by C library routines

1. Many C library routines include system calls.

(a) printf() calls write() which executes a system call.

(b) malloc() might call sbrk() to increase the space for the heap.

48 Processes

1. A process is an instance of a running program.

2. Each process has its own virtual memory, which it can treat as its

own private domain.

3. Each process may treat the CPU as its own as well.

4. The kernel hides that fact that the processes are sharing both memory

and the CPU.

5. The kernel schedules the processes so that they each get a “fair”

share of the CPU.

(a) Each process can pretend that it has the full use of the CPU: disk

and I/O operations seem immediate, and other processes don’t

interfere.

(b) In fact, the kernel often stops running process A and starts run-

ning process B. This process switch involves a context switch

from A to the kernel, a scheduling decision, and then a context

switch from the kernel to B.

Lecture 26, 4/3/2015

7. The kernel arranges memory maps so that the MMU hardware can

redirect all memory accesses that each process makes.

8. The fork() system call asks the kernel to create a new process.

(a) Lab 4 showed this material.

(b) Say A submits the call.

(d) B shares all of A’s code and has a new copy of all of A’s data and

stack. B’s program counter (PC) is identical to A’s.

(e) Therefore both A and B see a return from fork().

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Cirrus Logic CS485 Manual do Utilizador Página 42