ECE 424 Embedded Systems Design Operating System Overview

ECE 424 Embedded Systems Design Operating System Overview Chapter 7 Ning Weng

Operating System • Abstractions ─ Uninterrupted Computation: No Interrupts ─ Infinite Memory: just an illusion. ─ Simple I/O: Avoid dealing directly with devices (simple writes/reads)

Uninterrupted computation • Underlying mechanisms § § § Context switching Scheduling Protection • Flavors of “process” - increasing complexity § Interrupt handlers, threads, processes

Infinite memory via virtual memory • Via virtual memory Page mapping (avoids finding contiguous locations). § Demand paging (use more space than memory) Slow DRAM lookup avoided with fast TLB Protection by allowing only OS to modify page tables. § • •

Simple I/O using system calls • • For abstraction alone, I/O could be libraries. For security, I/O handled by device drivers. System calls, trap to kernel protection levels. More expensive function call, because of privilege escalation.

Four Types of Operating Systems • • Single-user, single task - Designed to manage the computer so that one user can effectively do one thing at a time. Ex: Apple i. Phone Single-user, multi-tasking - Type of operating system most use on desktop and laptop computers today. Windows 7 and the Mac. OSX are examples of OS that let a single user have several programs in operation at the same time. Multi-user - Allows many users to obtain the computer's resources simultaneously. OS must make sure that each program being used has sufficient and separate resources so that a problem with one user doesn't affect the other users. Ex: Unix Real-time operating system (RTOS) - Main task is to manage computer’s resources so a particular operation executes in precisely the same amount of time every time it occurs.

Embedded Operating Systems Characteristics • Designed to perform a dedicated function • Real-Time Operating Systems (RTOS) • Examples: ─ i. OS, Android, Blackberry OS ─ Vx. Works (Boeing 787 Dreamliner and many spacecraft)

Service Calls Ning Weng ECE 424 8

Service Call Design Patterns Ning Weng ECE 424 9

Tasks & Threads & Process Ning Weng ECE 424 10

RTOS States Transitions Ning Weng ECE 424 11

Nonpreemptive FIFO Scheduling Ning Weng ECE 424 12

RR Scheduler with Priority & Preemption Ning Weng ECE 424 13

Memory Allocation • Malloc() and Free() ─ Allocates and frees memory from system heap to an application, respectively • Problem with the allocation system: ─ Forgets sequence of memory and free allocations • Can cause fragmentation (small contiguous section + lots of free memory) • Solution: ─ Allocator algorithm • Maximize size of contiguous blocks over time

Fragmentation Example

Fragmented Heap Ning Weng ECE 424 16

Power of two heap Ning Weng ECE 424 17

Virtual Memory • Memory Management Unit (MMU) manages translation between code, data, and heap process memory to physical memory • Virtual addresses are unique to accessing process • Physical addresses are unique to the hardware (i. e. ram)

Address Space Mapping Ning Weng ECE 424 19

Freeing and Swapping Memory • Freeing Memory ─ Memory allocated by a task may not be automatically freed upon deletion of that task • Must keep track of allocated memory • Swapping Memory ─ Utilized in Linux to allocate more virtual memory to applications than the total amount of physical memory ─ Rarely used in embedded systems • Additional wear on system (normally based on solid-state drives)

Clocks and Timers • Synchronous Execution ─ Used to specify timeouts • Sleep() and yield() are also used to delay execution ─ If time is long enough, task is de-scheduled and moved to ready queue • Asynchronous Execution ─ Callback functions – Indicates that a timeout has occurred to other threads • Callbacks with expiry time less than or equal to current time count are called • Can release semaphores • Often called because of timer interrupt – Interrupt – mechanism used to inform CPU that an asynchronous event has occurred

Time of Today • Time source ─ Real time Clock • Hardware timer backup with battery ─ CPU when it is running • Seed source ─ RTC ─ NTP (network time protocol) ─ Cellular radio network Ning Weng ECE 424 22

Mutual Exclusion/Synchronization • Goal: to serialized atomic access to share resources Ning Weng ECE 424 23

• Difficulties of Concurrency Sharing of global resources ─ Writing a shared variable: the order of writes is important ─ Incomplete writes a major problem • Optimally managing the allocation of resources • Difficult to locate programming errors as results are not deterministic and reproducible.

A Simple Example void echo() { chin = getchar(); chout = chin; putchar(chout); }

A Simple Example: On a Multiprocessor Process P 1 Process P 2. chin = getchar(); . chout = chin; putchar(chout); . . chin = getchar(); chout = chin; . putchar(chout); .

Competition among Processes for Resources Three main control problems: • Need for Mutual Exclusion ─ Critical sections • Deadlock • Starvation

Disabling Interrupts • Uniprocessors only allow interleaving • Interrupt Disabling ─ A process runs until it invokes an operating system service or until it is interrupted ─ Disabling interrupts guarantees mutual exclusion ─ Will not work in multiprocessor architecture

Pseudo-Code while (true) { /* /* } disable interrupts */; critical section */; enable interrupts */; remainder */;

• Special Machine Instructions Compare&Swap Instruction ─ also called a “compare and exchange instruction” • Exchange Instruction

Compare&Swap Instruction int compare_and_swap (int *word, int testval, int newval) { int oldval; oldval = *word; if (oldval == testval) *word = newval; return oldval; }

Semaphore • Semaphore: ─ An integer value used for signalling among processes. • Only three operations may be performed on a semaphore, all of which are atomic: ─ initialize, ─ Decrement (sem. Wait) ─ increment. (sem. Signal)

OS Required Semaphore Ning Weng ECE 424 33

Semaphore • Binary • Couting Ning Weng ECE 424 34

Execution Environment 35 • Program • Libraries • Kernel subsystems • Hardware

Device Driver • The driver controls the hardware and provides an abstract interface to its capabilities. • The driver ideally imposes no restrictions (or policy) on how the hardware should be used by applications. • Scatter gather list: ─ a mechanism defined and supported by OS to represent a list of data is not physically contiguous. Ning Weng ECE 424 36

Service, Driver and Device Ning Weng ECE 424 37

Scatter Gather Structures Ning Weng ECE 424 38

Direct Memory Access (DMA) • What, why and how/ Ning Weng ECE 424 39

Transmit Descriptor Ring Ning Weng ECE 424 40

Network Stack and Device Driver Ning Weng ECE 424 41

Storage File System • present logical (abstract) view of files and directories ─ hide complexity of hardware devices • facilitate efficient use of storage devices ─ optimize access, e. g. , to disk • support sharing ─ provide protection Ning Weng ECE 424 42

Files Open and Read Ning Weng ECE 424 43

Synchronization File System • File write is asynchronous ─ Write call function does not block • Data may not have been in disk • Consolidating and scheduling writing requests for performance • Synchronization is required before ─ Shut down, restart or disk removal ─ Example: Stop a USB disk (safe remove) • Two challenges of power interactions ─ Un-notified power removal • Large capacitor and sensing circuits ─ Brownout • Voltage drop Ning Weng ECE 424 44

Brownout Events Ning Weng ECE 424 45

Real Time Ning Weng ECE 424 46

Real Time • Real time: required to complete its task on time ─ Usually have deterministic time bound ─ Hard or soft • Features of RTOS ─ Scheduling, resource allocation, interrupt handing and etc ─ Example, Vx. Works Ning Weng ECE 424 47

In a Hard RTOS • Thread priorities can be set by the client • Threads always run according to priority • Kernel must be preemptible or bounded • Interrupts must be bounded • No virtual memory

In a Soft RTOS… • Like a hard RTOS: ─ Priority scheduling, with no degradation ─ Low dispatch latency ─ Preemptible system calls ─ No virtual memory (or allow pages to be locked) • Linux: guarantees about relative timing of tasks, no guarantees about syscalls