CSE Fall Introduction - 1 What is an Embedded Systems  Its not a desktop system  Fixed or semi-fixed functionality (not user programmable)  Lacks some or all traditional human interfaces: screen, keyboard, pointing device, audio  May have stringent real-time requirements (Hard and Soft)  Usually has sensors and actuators for interface to physical world  Figures of Merit  Reliability – it can never crash  Safety – Involves things that move and can harm/kill a person  Power Consumption – may run on limited power supply. Want slowest possible clock, least amount of memory. You will always be resource constrained!  Cost – Engineering Cost, Mfg Cost, Schedule tradeoffs  Product life cycle issues: maintainability, upgradability, servicability  Performance

CSE Fall Introduction - 2 Features of an Embedded Operating System  Live Without  User Interface  Dynamic Linking and Loading  Virtual Memory, Protection Modes (Processes)  Disk  Device Drivers?  Instead we have  Low Latency Interrupts  Tasks (threads)  Task communication primitives  Data sharing primitives  Device Drivers?

CSE Fall Introduction - 3 Embedded OS Features  Task Scheduling/Multithreading  Memory Management (Threads v. Processes)  Synchronization and Communication Primitives  Multitasking Overhead (Context Switch)  Interrupt Latency  Do Timer 1 interrupts trigger during operating system calls (? (use simulator to find out, or look at source code)

CSE Fall Introduction - 4 Multitasking – state maintained for each task Running Ready BlockedDisabled time-slice wait() signal() delete() create() delete() only one task in this state at a time For TINY, this takes cycles per update…is this okay?

CSE Fall Introduction - 5 Interrupt Priorities  Key question: Can Tone Generator Interrupt the OS? OS Task1 Task2 ISR Why is this important?

CSE Fall Introduction - 6 Preemptive vs. Non preemptive OS T1/hi T2/lo ISR Pre-emptive: All tasks have a different priority… hi priority task can preempt low priority task. Highest priority task always runs to completion (wait). Advantage: Lower latency, faster response for high priority tasks. Disadvantage: Potential to starve a low priority task Tiny: no priority, round robin only. No starvation. signal T2 signal T1, preempt T2 T2 Completes

CSE Fall Introduction - 7 Sharing Code between Tasks  Shared functions are useful! deq? enq? next (same for head or tail)? Q declaration and initialization?  Shared data issues  What happens when one of these functions is interrupted by the OS unsigned char next(unsigned char current, unsigned char size) { if (current == size-1) return 0; else return (current+1); } it depends on where the parameters and the automatic variables are stored

CSE Fall Introduction - 8 Encapsulation and Re-use typedef struct qstruct { unsigned char head; unsigned char tail; unsigned char *array; unsigned char size; } qtype; // global variable: qtype Q; void taskID(void) _task_ 0{ unsigned char d; unsigned char i; unsigned char array[QSIZE]; unsigned char x; initq(&Q, array, QSIZE); while (1) { do { ET0 = 0; x = enq(&Q,i++); ET0 = 1; } while (!x); do { ET0 = 0; x = deq(&Q,&d); ET0 = 1; } while (!x); } shared functions are okay if we disallow swapping during calls. why? re-entrant stack not protected by Tiny OS. But shared C libraries are okay. Why? not sure yet.

CSE Fall Introduction - 9 Ways to Protect a Critical Section  Disable Interrupts … be as specific as possible  Semaphores wait() – signal()  each system call is guaranteed to be atomic:  wait block until the flag is set, and clears the flag before returning.  signal sets the flag.  most OS allow you to declare a semaphore as a data type. As many as you want. Tiny limits us to one/task, and it is used to signal the task. Therefore we use semaphores to protect shared data/critical sections.  Disable task switching via a system call…about the same as disabling interrupts to the OS.

CSE Fall Introduction - 10 Embedded System Types  Control Dominated Systems  Software State machines, state synchronization, etc … nuclear power plan  Data flow dominated Systems  our music player  a network router  queues, messages, packets, routing,

CSE Fall Introduction - 11 Homework for Friday/Monday  Propose what our systems should do, and a general architecture for doing it.  Can we handle multiple streams to/from the pilot  Can we stream music while sending a song to another player?  Can we receive a tune while at the same time?  What do we have to do in the system  What do we have to do in the protocol with the host?

CSE Fall Introduction - 12 General Binary Semaphores

CSE Fall Introduction - 13 Comparative Real Time OSes  Compare Tiny to Full to VxWorks

CSE Fall Introduction - 14 Orchestra Functions  Time of day  Get data from net, send to player or to pilot  Send music to net  Get music from net  Play music  Task to play music from net  Task to create/delete other tasks (master)

CSE Fall Introduction - 15 Benefits

CSE Fall Introduction - 16 Embedded Software  Software States v. Finite State Machines  Hierarchical State  Thread/Process Communication  Critical Sections  Synchronization  Messaging and Signaling

CSE Fall Introduction - 17 Live without?  Live Without  User Interface  Dynamic Linking and Loading  Virtual Memory, Protection Modes  Disk  Processes  Instead we have  Real Time Kernel (very small OS)  Tasks (threads)  Task communication primitives  ADC  Timers  Event Capture  PWM