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 iPhone
Single-user, multi-tasking - Type of operating system most use on
desktop and laptop computers today. Windows 7 and the MacOSX 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:
─ iOS, Android, Blackberry OS
─ VxWorks (Boeing 787 Dreamliner and many
spacecraft)
Service Calls
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Service Call Design Patterns
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Tasks & Threads & Process
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RTOS States Transitions
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Nonpreemptive FIFO Scheduling
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RR Scheduler with Priority & Preemption
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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
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Power of two heap
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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
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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 all 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
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Mutual Exclusion/Synchronization
• Goal: to serialized atomic access to share resources
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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 P1
.
chin = getchar();
.
chout = chin;
putchar(chout);
.
.
Process P2
.
.
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 (semWait)
─ increment. (semSignal)
OS Required Semaphore
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Semaphore
• Binary
• Couting
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Execution Environment
MATH
LIBRARY
APPLICATION (mpg123)
•
Program
•
Libraries
•
Kernel
subsystems
•
Hardware
STANDARD C
LIBRARY
Networking
Memory
Management
Filesystems
Device
Control
Process
Management
Character
Devices
Architecture
Dependent
Code
b
OPERATING
SYSTEM
Network
Subsystem
Memory
Manager
File System
Devices
Disk
Network Interfaces
Memory
CPU
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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.
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Service, Driver and Device
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Scatter Gather Structures
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Direct Memory Access (DMA)
• What, why and how/
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Transmit Descriptor Ring
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Network Stack and Device Driver
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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
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Files Open and Read
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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
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Brownout Events
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Real Time
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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, VxWorks
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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