Process and Memory

Xv6 time-share processes. Save its CPU registers when process not running, and restoring them back when it resumes. Process can create another process using fork

wait returns the pid of the exited child of the current process. wait waits until one child to exit.

Parent process and child process are executing with different memory and different registers. Changing a value does not affect another.

exec system call replaces the calling process’ memory with a new memory image loaded from a file. The file is ELF format.

Xv6 Shell calls getcmd to reads a line of user input, then call fork, which creates a copy of the shell process. The parents calls wait, while the child is running the command.

Process wants more memory could call sbrk(n).

Questions: Why fork and exec not combine together?

Answer:

If they are separate, the shell can fork a child, use open, close, dup in the child to change the standard input and output file descriptors, and then exec! Executed program does not need to understand how to redirect I/O. It is simple.