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COS: A Configurable OS for EmbeddedSoC Systems
28 January 2015
by, Prateek Anand
CONTENTS
INTRODUCTION
CONFIGURABLE OS ON SOA
IMPLIMENTATION OF COS
CASE STUDY
ADVANTAGES OF ECOS
EMBEDDED SYSTEMS
An embedded system is a combination of computer hardware and software, and perhaps additional mechanical or other parts, designed to perform a dedicated function.
CONFIGURABILITY
An embedded system must lend itself to flexible configuration so that only the functionality needed for a specific application and hardware suite is provided.
NEED FOR CONFIGURABILITY
OSes can be customized for application specific purposes.
Time and effort to understand and modify is less compared to monolithic kernel.
Unnecessary functionalities and features can be easily removed from the kernel.
Enable embedded system developers to work within a familiar and proven environment,.
CONFIGURABLE OS ON SOA
A. Hardware Abstraction Layer (HAL)
The HAL is software that presents a consistent API to the upper layers and maps upper-layer operations onto a specific hardware platform. Thus, the HAL is different for each hardware platform.
B. COS Kernel The COS kernel provides the core functionality
needed for developing multithreaded applications:
ocreate new threads in the system
oControl over the various threads in the system
oScheduleing, determining which thread should currently be running
oIntegration with the system’s support for interrupts and exceptions
Contd…
C. User space
The user space is a software layer provided for the facility of application programs and user interface software.
IMPLIMENTATION OF COS
A. Interrupt Handling Developers can register/remove an ISR of a specified IRQ (Interrupt ReQuest line) and also execute all ISRs (Interrupt Service Routines) when necessary via the interface. functions are used to initialize, enable/disable, mask, clear the IRQs on the system.
B. Timer The COS timer interface acts as part of hardware abstraction layer between kernel and system hardware timers. Developers can initialize, start/stop, set interval and operating mode of a timer.
C. Scheduler The COS kernel can register/unregister tasks to a scheduler, Therefore scheduling algorithms can be independently implemented as a scheduler component without detail knowledge and modification to the OS kernel.
D. Memory Management The COS memory management interface defines initialization, allocate/free memory operations. It also maintains the number of total, used, and free memory and pointers to free and used memory pool for kernel usage.
E. File System Linux uses VFS (Virtual File System) as an abstraction layer for file systems to enable kernel to perform operations on various underlying file systems through same interface. VFSdefines general file operations including open(), read(), write(), llseek(), etc [2].
Case studyA. Implementation We modify the ARM Linux kernel to accommodate the COS interfaces and construct service components.
Service Component Config. Method
Interrupt Handling Top/Bottom HalfTop/Bottom Half w/ tasklet
Compile Time
Scheduler Rate MonotonicEarliest Deadline FirstPinwheelRound Robin
Run-Time
Memory Management
Paging with MMUPaging without MMUPartition and BlockBitmap
Compile Time
File System EXT2RAMFS VFAT
Run-Time
TABLE I COMPONENTS OF THE COS IMPLEMENTATION
B. Performance Evaluation In order to evaluate the performance of COS, we measure the overhead of proposed interfaces.
Mem Manage. Component Allocate (μs) Access (μs)
Paging with MMU 316 137
Paging without MMU
208 994
Paging with MMU (D Cache disabled)
742 876
TABLE II PERFORMANCE IMPACT OF MMU AND D CACHE
MAIN ADVANTAGES OF ECOS
Configurability: It can be configured in great detail at compile time, which avoids the need to add unwanted code to the library to be linked with the application code.
Portability eCos is designed to be portable to a wide range of target architectures and platforms, including 16, 32 and 64 bit architectures, microcontrollers and DSPs
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