1. Direct memory access

2. IO Command includes: buffer address buffer length read or write dada position in disk When IO complete, DMA sends an interrupt request to CPU.

3. reserve a free PCB reserve free memory space request a process ID for child process copy parent PCB to child’s PCB and modify the two PCBs copy parent’s memory image to child’s memory return child ID to parent and return 0 to child put the two process to ready state The work that OS should do for a system call fork():

4. {if (the specified target executable program is in current directory or in one of the created paths) {load the program; move current process to block state;} else {printf(“the specified target executable program does not exist”); call scheduler();}} The work that OS should do for a system call exec():

5. 1. close all files that have been opened by current process; 2. kill all its child processes which will recursively execute exit(); 3. withdraw memory allocated to this process; 4. withdraw process control block; The work that OS should do for a system call exit():