1. CS 3214 Computer Systems
Lecture 9
Godmar Back

2. Announcements Exercise 4 will be posted today Project 3 coming
CS 3214 Fall 2011

3. Part 1
Threads and Processes
CS 3214 Fall 2011

4. Processes Def: An instance of a program in execution
OS provides each process with key abstractions
Logical control flow
1 flow – single-threaded process
Multiple flows – multi-threaded process
Private address space
Abstracted resources: e.g., stdout/stdin file descriptors
These abstractions create the illusion that each process has access to its own
CPU (or CPUs for multi-threaded processes)
Memory
Devices: e.g., terminal
CS 3214 Fall 2011

5. Context Switching
Historical motivation for processes was introduction of multi-programming:
Load multiple processes into memory, and switch to another process if current process is (momentarily) blocked
This required protection and isolation between these processes, implemented by a privileged kernel: dual-mode operation.
Time-sharing: switch to another process periodically to make sure all processes make equal progress
Switch between processes is called a context switch
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6. Dual-Mode Operation Two fundamental modes:
“kernel mode” – privileged
aka system, supervisor or monitor mode
Intel calls its PL0, Privilege Level 0 on x86
“user mode” – non-privileged
PL3 on x86
Bit in CPU – controls operation of CPU
Privileged operations can only be performed in kernel mode. Example: hlt
Must carefully control transition between user & kernel mode
int main() {
asm(“hlt”); }
CS 3214 Fall 2011

7. Mode Switching User  Kernel mode External (aka hardware) interrupt:
For reasons external or internal to CPU
External (aka hardware) interrupt:
timer/clock chip, I/O device, network card, keyboard, mouse
asynchronous (with respect to the executing program)
Internal interrupt (aka software interrupt, trap, or exception)
are synchronous
can be intended (“trap”): for system call (process wants to enter kernel to obtain services)
or unintended (usually): (“fault/exception”) (division by zero, attempt to execute privileged instruction in user mode, memory access violation, invalid instruction, alignment error, etc.)
Kernel  User mode switch on iret instruction
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8. A Context Switch Scenario
Timer interrupt: P1 is preempted, context switch to P2
I/O device interrupt: P2’s I/O complete switch back to P2
Process 1
Process 2
user mode
kernel mode
System call: (trap): P2 starts I/O operation, blocks context switch to process 1
Timer interrupt: P2 still has
time left, no context switch
Kernel
CS 3214 Fall 2011

9. Context Switching, Details
intr_entry:
(saves entire CPU state)
(switches to kernel stack)
intr_exit:
(restore entire CPU state)
(switch back to user stack)
iret
Process 1
Process 2
user mode
kernel mode
Kernel
switch_threads: (in)
(saves caller’s state)
switch_threads: (out)
(restores caller’s state)
(kernel stack switch)
CS 3214 Fall 2011

10. System Calls
User processes access kernel services by trapping into the kernel, executing kernel code to perform the service, then returning – very much like a library call. Unless the system call cannot complete immediately, this does not involve a context switch.
Process 1
user mode
kernel mode
Kernel
Kernel’s System Call Implementation
CS 3214 Fall 2011

11. Syscall example: write(2)
32-bit Linux
/* gcc -static -O -g -Wall write.c -o write */ #include <unistd.h>
int
main() {
const char msg[] = "Hello, World\n";
return write(1, msg, sizeof msg); }
/usr/include/asm/unistd.h: ….
#define __NR_write
a <__write_nocancel>:
805005a: push %ebx
805005b: 8b mov 0x10(%esp),%edx #arg2
805005f: 8b 4c 24 0c mov 0xc(%esp),%ecx # arg1
: 8b 5c mov 0x8(%esp),%ebx # arg0
: b mov $0x4,%eax # syscall no
805006c: cd int $0x80
805006e: 5b pop %ebx
805006f: 3d 01 f0 ff ff cmp $0xfffff001,%eax
: 0f e jae ed0 <__syscall_error>
805007a: c ret
CS 3214 Fall 2011

12. Kernel Threads
Most OS support kernel threads that never run in user mode – these threads typically perform book
keeping or other supporting tasks. They do not service system calls or faults.
Process 1
Process 2
user mode
kernel mode
Kernel
Kernel Thread
Careful: “kernel thread” not the same as kernel-level thread (KLT) – more on KLT later
CS 3214 Fall 2011

13. Context vs Mode Switching
Mode switch guarantees kernel gains control when needed
To react to external events
To handle error situations
Entry into kernel is controlled
Not all mode switches lead to context switches
Kernel decides when – subject of scheduling policies
Mode switch does not change the identity of current process/thread
See blue/yellow colors in slide on ctxt switch details
Hardware knows about modes, does not (typically) know about contexts
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14. Bottom Up View: Exceptions
An exception is a transfer of control to the OS in response to some event (i.e., change in processor state)
User Process OS
event
current
exception
next
exception processing
by exception handler
exception
return (optional)
CS 3214 Fall 2011