1. Safe Queue? … while (next(head) == tail); // block if full
queue[head] = data; // write data
ET0 = 0; // disable T-0 interrupts
head = next(head); // update pointer
ET1 = 1; // enable T-1 interrupts
unsigned char next(unsigned char ptr) {
if (ptr == QMX) ptr = 0;
else ptr++
return(ptr); }

2. Consider the compiler C code for blocking queue Compiler output?…
while (next(head) == tail);
queue[head] = data;
ET0 = 0;
head = next(head);
ET1 = 1;
unsigned char next(unsigned char ptr) {
if (ptr == 9) ptr = 0;
else ptr++
return(ptr); }
Compiler output?
MOV R0,TAIL
MOV R1,HEAD
LOOP: ACALL NEXT SUBB A,R0
JZ LOOP
MOV R3,#QUEUE
ADD R3,R1
CLR ET0
ACALL NEXT
MOV HEAD,A
SETB ET0
Assume next() returns value in accumulator, leaves R1 as is

3. I think we’re safe now volatile data unsigned char tail;…
while (next(head) == tail);
queue[head] = data;
ET0 = 0;
head = next(head);
ET1 = 1;
unsigned char next(unsigned char ptr) {
if (ptr == 9) ptr = 0;
else ptr++
return(ptr); }
MOV R1,HEAD
LOOP: ACALL NEXT
MOV R0,TAIL ;refresh
SUBB A,R0
JZ LOOP
MOV R3,#QUEUE
ADD R3,R1
CLR ET0
ACALL NEXT
MOV HEAD,A
SETB ET0
*compiler knows that sfr’s are volatile (ports, flags)

4. Embedded Software Architectures
No Operating System
Round robin: sequential polling for events
Round robin w/ interrupts
Function Queue Scheduling
Real Time Operating System

5. Maestro w/ Round Robin Only
void main (void) {
if (TF0) music_task(); // process timer overflow
if (R1) serial_input_task(); // process serial input }
Would this work for maestro (lab4)?
How do we know?

6. Task Diagram Worst case: character comes one cycle before TF0
Worst Case Latency = max run time of all other tasks
serial task
main
serial
music
main
w.c. latency
deadline
What is the worst
case serial processing
time?
Depends on response message length--could be bad!
How much latency can we
tolerate? Practically none
timer0 overflow occurs (TF0)
char arrives

7. Round Robin w/ Interrupts
volatile bit fEvent;
void isr(void) {
time_critical_processing();
fEvent = TRUE; }
void main (void) {
if (R1) serial_input_task();
if (fEvent) {
housekeeping();
fEvent = FALSE;
Why not put housekeeping into the ISR too?
Then our worst case latency to a separate time critical event would be poor
Would this work for maestro? See next slide

8. Maestro in RR+INT
What are the time critical functions? Flipping channels, change tones at end of timeslice
What are the housekeeping functions?
TNE count down and stream processing (TNE, Tones)
Do these have hard time constraints too?
Yes, one time slice (18ms)…good enough? Not necessarily…serial is undefined!
volatile bit fEndOfSlice;
void isr(void) {
process_tones();
if (!--sliceCount) {
changeTones();
sliceCount = SliceSize
fEndOfSlice = TRUE; }
main () {
if (R1) serial_input_task();
if (fEndOfSlice) {
if (--TNE==0)
process_next_event(); fEndOfSlice = FALSE;

9. Task Diagram Worst case analysis: character comes one cycle before TF0
Problem: housekeeping can still miss its deadline, depends on serial task execution time
time slice start
time slice end
isr
serial
housekeeping
main
deadline
timer0 overflow occurs (TF0)
char arrives
Can we do better?

10. Benefits Modularization of the Application
Some Virtual Machine Features
Device Drivers (Virtual Devices)

11. Embedded Software Software States v. Finite State Machines
Hierarchical State
Thread/Process Communication
Critical Sections
Synchronization
Messaging and Signaling