1. CSE 466 – Fall 2000 - Introduction - 1
Threads and Stacks
Process: entire address space is private
Threads: heap is common, but stack is private. What if stack wasn’t private?
DATA
DATA
stack
stack
task 1
context ?
task 1
context sp
globals
globals sp
context
swap OS OS
regs
regs
CSE 466 – Fall Introduction - 1

2. How TINY manages the stack
worst case stack size is sum of worst case
for each task, plus ISR’s.
One stack/thread
stack 3
stack 3
stack 3
stack 2
stack 2
stack 1
stack 2
stack 1
stack 1 sp
globals sp
globals sp
globals OS OS OS
regs
regs
regs
CSE 466 – Fall Introduction - 2

3. CSE 466 – Fall 2000 - Introduction - 3
Reentrant Stack
re-stack 3
I don’t know why
the compiler doesn’t
use the regular stack for
re-entrant function
calls!
Probably so that user can
put re-entrant stack into
external data memory
this is inefficient, so limit use of
re-entrant functions.
re-stack 2
re-stack 1
re-sp
stack 3
stack 2
stack 1 sp
globals OS
regs
CSE 466 – Fall Introduction - 3

4. Is this safe (this is about parameter passing, not stack stuff)
int function average(int a, int b, int c, int d, int e, int f) {
return ((a+b+c+d+e+f)/6); }
void task1 _task_ 1 (void) {
int a,b,c,d,e,f; …
disable();
average(a,b,c,d,e,f);
enable()
CSE 466 – Fall Introduction - 4

5. CSE 466 – Fall 2000 - Introduction - 5
Embedded System Types
Data flow dominated Systems
our music player
a network router
queues, messages, packets, routing
Control Dominated Systems
Software State machines, distributed state management/synchronization, e.g. power plant, autopilot, etc.
CSE 466 – Fall Introduction - 5

6. CSE 466 – Fall 2000 - Introduction - 6
Design Meeting
Music Packet format: TNE.CHAN,(CHAN==0?OPCODE:TONE),[TONE,[TONE]]
even if CHAN == 0, TNE is valid. That’s why we can’t use TNE field for OPCODE. TNE is not always 31 when CHAN==0.
From last week: player has “modes”.
IDLE, all commands from the pilot are in the form of ASCII characters: ‘S’ and ‘T’?
STREAMING all commands come in the form of music opcodes:
OPCODE == 0 means stop. This is a good choice because, if we set TNE == 0, then okay if interpreted as TONE, we will start turning off the music anyway. So: x.0,0 means stop. The player should send at least 0’s to stop the player. Is it safe to set TNE == 0? Check your software!! Stop means return to IDLE mode.
OPCODE == 1 means CONTINUE, and TNE is valid
OPCODE == 2 means PAUSE (was stop). Okay?? if we are paused… how do we resume? What if we want to clear the buffer and start over? do we need more commands? Do we need to define a new mode? -- PAUSED?
Do we need any other opcodes?
TRANSMITTING
How to we enter that mode?
what form do commands take? is PAUSE supported?
What is the format for a complete song?
What does the pilot send to the player to transmit a song?
CSE 466 – Fall Introduction - 6

7. Mode (State) Diagram for the Player
IDLE
STREAM
what is
the difference
between IDLE
and PAUSED?
Modes are like states in an FSM…but they are not static. What does that mean?
There is an execution sequence associated with each mode.
Mode determines response to events and choice of next mode Tx
PAUSED
CSE 466 – Fall Introduction - 7

8. An Alternative: Is this equivalent?
‘S’/clearb()
‘R’
STREAM
IDLE
STOP
STOP
‘T’/clearb() Tx
CSE 466 – Fall Introduction - 8

9. Communication and Synchronization
Semaphores
Queues: Messages, Mailboxes, and Pipes
Events
Rendezvous
CSE 466 – Fall Introduction - 9