chapter 6: Limited Direct Execution

Question 1

challenges of time sharing

Answer 1

performance

control

Question 2

what does “direct execution” mean

Answer 2

running the program directly on the CPU

Question 3

direct execution protocol (how it works) (not limited)

Answer 3

OS:
create entry for process list
allocate memory for program
load program into memory
set up stack with argc/argv
clear registers
execute call main()

program: 
run main()
execute return from main

OS:
free memory of process
remove from process list

Question 4

what is the problem with direct execution?

Answer 4

problem 1: restricted operations

Question 5

what are the 2 modes to prevent restricted operations

Answer 5

user mode : cant issue IO request

kernel mode : can do whatever

Question 6

what should a user process do when it wishes to perform some kind of privileged operation such as reading from disk?

Answer 6

user program performs a system call

Question 7

what is trap and return-from-trap instruction that the OS performs?

Answer 7

trap: jumps into kernel mode

return-from-trap: returns to user mode

Question 8

how does OS make sure that the trap knows what to run?

Answer 8

the OS initializes a trap table that is mapped to syscall handlers

Question 9

how does user code execute system call?

Answer 9

it specifies a system-call number which maps to a system call and let the kernel do the thing.

Question 10

what are the 2 phases in the LDE(limited direct execution) protocol?

Answer 10

1. at boot time, kernel initializes trap table
2. kernel sets up a few things. when process wishes to perform syscall, it traps back into OS, performs it, and return from trap to the process.

Question 11

if a process is running, the OS isnt. how can OS regain control so that it can switch between processes?

Answer 11

there are 2 ways, cooperative and noncooperative approach

Question 12

how does the cooperative approach work?

Answer 12

the OS regains control of the CPU by waiting for a system call or an illegal operation to take place.
note: if process goes in infinite loop here, the only way is to reboot the machine

Question 13

how does the non-cooperative approach work?

Answer 13

a timer interrupt!
when the interrupt is raised, the process is halted and the OS regains control.
note: hardware needs to save state of process to resume running it later

Question 14

what is a context switch?

Answer 14

save a few registers on current running process
and restore registers for the soon to be executing process
note: it may hinder performance