A mini kernel: dual mode and a system-call table

Simulate how a CPU with dual-mode operation executes a user program that uses system calls.

The kernel has this system-call table (like Linux x86-64):

The program's process has PID 1204. Input: one instruction per line until the end of input:

• add, load, store — ordinary instructions (allowed in user mode);
• hlt, cli, out, settimer — privileged instructions;
• syscall NAME — trap into the kernel with the call's number.

Execute them in order, starting in user mode. Print one line per instruction:

• ordinary: user: add
• privileged in user mode: EXCEPTION: privileged instruction 'cli' in user mode -> process 1204 terminated, and stop (no further instructions run);
• syscall NAME with a known name: trap -> kernel: syscall 1 (write) ... return to user except:
  • getpid prints trap -> kernel: syscall 39 (getpid) returns 1204 ... return to user;
  • fork prints trap -> kernel: syscall 57 (fork) returns child pid 1205 ... return to user (each fork creates the next PID: 1205, 1206…);
  • exit prints trap -> kernel: syscall 60 (exit) -> process 1204 exits and stops.
• unknown system call: trap -> kernel: unknown syscall 'NAME' returns -ENOSYS ... return to user.

At the end print Mode switches: N (each trap into the kernel and each return to user mode counts as one switch; a trap that terminates the process counts only the entry) and Instructions executed: M (including the one that stopped the program).

Input:

load
syscall getpid
add
syscall write
syscall fork
cli
store

Output:

user: load
trap -> kernel: syscall 39 (getpid) returns 1204 ... return to user
user: add
trap -> kernel: syscall 1 (write) ... return to user
trap -> kernel: syscall 57 (fork) returns child pid 1205 ... return to user
EXCEPTION: privileged instruction 'cli' in user mode -> process 1204 terminated
Mode switches: 7
Instructions executed: 6