Task-Switching

1. Task-Switching How the x86 processor assists with context-switching among multiple program-threads

2. Program Model • Programs consist of data and instructions • Data consists of constants and variables, which may be ‘persistent’ or ‘transient’ • Instructions may be ‘private’ or ‘shared’ • These observations lead to a conceptual model for the management of programs, and to special processor capabilities that assist in supporting that conceptual model

3. Conceptual Program-Model runtime library Shared Instructions and Data (persistent) STACK Private Data (transient) created during runtime heap BSS Uninitialized Data (persistent) DATA created at compile time Initialized Data (persistent) TEXT Private Instructions (persistent)

4. Task Isolation • The CPU is designed to assist the system software in isolating the private portions of one program from those of another while they both are residing in physical memory, while allowing them also to share certain instructions and data in a controlled way • This ‘sharing’ includes access to the CPU, whereby the tasks take turns at executing

5. Multi-tasking IDT GDT IDTR TR GDTR TSS 1 TSS 2 supervisor-space (ring0) user-space (ring3) shared runtime library STACK STACK SP SS heap heap BSS BSS DATA DATA DS TEXT TEXT IP CS Task #2 Task #1

6. Context-Switching • The CPU can perform a ‘context-switch’ to save the current values of all its registers (in the memory-area referenced by the TR register), and to load new values into all its registers (from the memory-area specified a new Task-State Segment selector) • There are four ways to trigger this ‘switch’ operation on x86 processors

7. How to cause a task-switch? • Use a ‘jmpf’ instruction (far jump): jmpf #0, #task_selector • Use a ‘callf’ instruction (far call): callf #0, #task_selector • Use an ‘int-n’ instruction (with a task-gate): int 0x80 • Use an ‘iret’ instruction (with NT=1): iret

8. ‘jmpf’ and ‘callf’ • These instructions are similar – they both make use of a ‘selector’ for a Task-State Segment descriptor Base[31..24] P D P L 0 type Base[23..16] Base[ 15..0 ] Limit[ 15..0 ] TSS Descriptor-Format type: 16bitTSS( 0x1=available or 0x3=busy) or 32bitTSS( 0x9=available or 0xB=busy)

9. When to use ‘jmpf’ or ‘callf’? • Use ‘jmpf’ to switch to a different task in case you have no intention of returning • Use ‘callf’ to switch to a different task in case you want to ‘return’ to this task later

10. No Task Reentrancy! • Since each task has just one ‘save area’ (in its TSS), it must not be possible for a task to be recursively reentered! • The CPU enforces this prohibition using a ‘busy’ bit within each task’s TSS descriptor • Whenever the TR register is loaded with a new selector-value, the CPU checks to be sure the task isn’t already ‘busy’; if it’s not, the task is entered, but gets marked ‘busy’

11. Task-Nesting • But it’s OK for one task to be nested within another, and another, and another… TR TSS #4 LINK TSS #3 LINK TSS #2 LINK TSS #1 LINK callf callf callf current TSS

12. The NT-bit in FLAGS • When the CPU switches to a new task via a ‘callf’ instruction, it sets NT=1 in FLAGS (and leaves the old TSS marked ‘busy’) • The new task can then ‘return’ to the old task by executing an ‘iret’ instruction (the old task is still ‘busy’, so returning to it with a ‘callf’ or a ‘jmpf’ wouldn’t be possible)

13. Task-switch Semantics

14. Task-Gate Descriptor • It is also possible to trigger a task-switch with a software or hardware interrupt, by using a Task-Gate Descriptor in the IDT P D P L 0 type (=0x5) Task-State Segment Selector Task-Gate Descriptor Format

15. ‘Threads’ versus ‘Tasks’ • In some advanced applications, a task can consist of multiple execution-threads • Like tasks, threads take turns executing (and thus require ‘context-switching’) • CPU doesn’t distinguish between ‘threads’ and ‘tasks’ – context-switching semantics are the same for both • Difference lies in ‘sharing’ of data/code

16. A task with multiple threads TSS 1 TSS 2 Each thread has its own TSS-segment supervisor-space (ring0) user-space (ring3) STACK 1 STACK 2 STACKS (each is thread-private) heap DATA 1 DATA 2 DATA (some shared, some private) CODE 1 CODE 2 TEXT (some shared, some private)

17. Demo program: ‘twotasks.s’ • We have constructed a simple demo that illustrates the CPU task-switching ability • It’s one program, but with two threads • Everything is in one physical segment, but the segment-descriptors create a number of different ‘logical’ segments • One task is the ‘supervisor’ thread: it ‘calls’ a ‘subordinate’ thread (to print a message)

18. Thread #2 uses an LDT • To support isolation of memory-segments among distinct tasks or threads, the CPU allows use of ‘private’ descriptor-tables • Same format for the segment-descriptors • But selectors use a Table-Indicator bit 15 3 2 1 0 Descriptor-table index field T I RPL Format of a segment-selector (16-bits) TI = Table-Indicator (0 = GDT, 1 = LDT) RPL = Requested Privilege-Level

19. In-class Exercise #1 • In our ‘twotasks.s’ demo, the two threads will both execute at privilege-level zero • An improved version of this demo would have the ‘supervisor’ (Thread #1) execute in ring 0 and the ‘subordinate’ (Thread #2) execute in ring 3 • Modify this demo-program, to incorporate that suggested improvement!

20. More enhancements? • The demo-program could be made much more interesting if it used more than one subordinate thread, and if the supervisor thread took turns repeatedly making calls to each subordinate (i.e., time-sharing) • You can arrange for a thread to be called more than once by using a ‘jmp’ after the ‘iret’ instruction (to re-execute the thread)

21. In-class Exercise #2 • Modify the demo so it has two subordinate threads, each of which prints a message, and each of which can be called again and again (i.e., add a jmp-instruction after iret): begin: ; entry-point to the thread . . . iret jmp begin