RTS: Kernel Design
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Kernel
&
Device
drivers
Servers (application ~, web ~, component ~)
Shell
XWin
Thread lib
ftp
User applications
System call interface
Process, memory, file system, network managers.
Kernel
Device drivers
Hardware/controller
Devices
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Simple kernels
 Polled loop: Say a kernel needs to process packets that are
transferred into the DMA and a flag is set after transfer:
for(;;) {
if (packet_here)
{
process_data();
packet_here=0;
}
}
Excellent for handling high-speed data channels, a processor is
dedicated to handling the data channel.
Disadvantage: cannot handle bursts
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Simple kernels: cyclic executives
 Illusion of simultaneity by taking advantage of
relatively short processes in a continuous loop:
for(;;) {
process_1();
process_2();
process_3();
…
process_n();
}
Different rate structures can be achieved by repeating tasks in the list:
for(;;) {
process_1();
process_2();
process_3();
process_3();
}
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Cyclic Executives: example:
Interactive games
 Space invaders:
for(;;) {
check_for_keypressed();
move_aliens();
check_for_keypressed();
check_collision();
check_for_keypressed();
update_screen();
}
}
check_keypressed() checks for three button pressings: move tank
left or right and fire missiles.
If the schedule is carefully constructed we could achieve a very
efficient game program with a simple kernel as shown above.
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Finite state automata and Co-routine
based kernels
void process_a(void){
for(;;) {
switch (state_a) {
case 1: phase_a1(); |
case 2: phase_a2(); |
….
case n: phase_an();}}}

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state_a and state_b are state
counters;
Communication between coroutines
thru’ global variables;
Example: the fanous CICS from IBM
: Customer Information Control
System
IBM’s OS/2 uses this in Windows
presentation management.
void process_b(void){
for(;;) {
switch (state_b) {
case 1: phase_b1(); |
case 2: phase_b2(); |
….
case n: phase_bn();}}}
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Interrupt driven systems
 Main program is a simple loop.
 Various tasks in the system are schedules via
software or hardware interrupts;
 Dispatching performed by interrupt handling routines.
 Hardware and software interrupts.


Hardware: asynchronous
Software: typically synchronous
 Executing process is suspended, state and context
saved and control is transferred to ISR (interrupt
service routine)
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Interrupt driven systems: code
example
void main() {
init();
while(TRUE);
}

Foreground/background systems
is a variation of this where main
does some useful task in the
background;
void int1(void){
save (context);
taks1();
retore (context);}
void int1(void){
save (context);
taks1();
restore (context);}
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Process scheduling
 Scheduling is a very important function in a real-time
operating system.
 Two types: pre-run-time and run-time
 Pre-run-time scheduling: create a feasible schedule
offline to meet time constraints, guarantee execution
order of processes, and prevents simultaneous
accesses to shared resources.
 Run-time scheduling: allows events to interrupt
processes, on demand allocation of resources , and
used complex run-time mechanisms to meet time
constraints.
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Task characteristics of real
workload
 Each task Ti is characterized by the following temporal

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parameters:
Precedence constraints: specify any tasks need to precede
other tasks.
Release or arrival time: ri,j: jth instance of ith task
Phase Φi: release time of first instant of ith task
Response time: time between activation and completion
Absolute deadline: instant by which task must complete
Relative deadline: maximum allowable response time
Period Pi: maximum length of intervals between the release
times of consecutive tasks.
Execution time: the maximum amount of time required to
complete a instance of the task assuming all the resources are
available.
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More on Cyclic Executives
 Simple loop cyclic executive
 Frame/slots
 Table-based predetermined schedule cyclic
executive
 Periodic, aperiodic and interrupt-based task
 Lets design a cyclic-executive with multiple
periodic tasks. (See notes given in class)
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The basic systems
 Several functions are called in a prearranged
sequence
 Some kind of cooperative scheduling
 You a have a set of tasks and a scheduler
that schedules these tasks
 Types of tasks: base tasks (background),
interrupt tasks, clock tasks
 Frame of slots, slots of cycles, each task
taking a cycle, burn tasks to fill up the left
over cycles in a slot.
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Blind Bingo
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A
c
b
g
k
V
n
m
L
s
E
t
y
w
f
D
v
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x
e
Display();
Read input();
Loop:
update display();
If all done exit();
Read input();
End Loop;
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Period, Frame and Hyper-period:
Cyclic Executive Design
 See class notes
 Design the slots
 Table-driven cyclic executive
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