The document discusses a configurable operating system (COS) for embedded systems on system-on-a-chip (SoC) devices. COS allows customization at compile-time by removing unnecessary functionality. It has a hardware abstraction layer (HAL) that maps operations to specific hardware. The COS kernel provides core functionality like threading and scheduling. Developers can implement interrupt handling, timers, scheduling algorithms, and memory management through COS interfaces. A case study modified the Linux kernel to include COS interfaces and components. Evaluations showed the performance impact of memory management configurations and cache usage. The main advantages of COS are its configurability, which avoids unwanted code, and its portability across architectures.

1. COS: A Configurable OS for Embedded
SoC Systems
28 January 2015
by,
Prateek Anand

2. CONTENTS
 INTRODUCTION
 CONFIGURABLE OS ON SOA
 IMPLIMENTATION OF COS
 CASE STUDY
 ADVANTAGES OF ECOS

3. 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.

4. 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,.

5. CONFIGURABLE OS ON SOA

6. 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

7. oScheduleing, determining which thread should
currently be running
oIntegration with the system’s support for interrupts and
exceptions
Contd…

8. C. User space
The user space is a software layer provided for the
facility of application programs and user interface
software.

9. 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.

10. 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.

11. 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].

12. Case study
A. Implementation
We modify the ARM Linux kernel to accommodate the COS
interfaces and construct service components.
Service Component Config. Method
Interrupt Handling Top/Bottom Half
Top/Bottom Half w/ tasklet
Compile Time
Scheduler Rate Monotonic
Earliest Deadline First
Pinwheel
Round Robin
Run-Time
Memory
Management
Paging with MMU
Paging without MMU
Partition and Block
Bitmap
Compile Time
File System EXT2
RAMFS
VFAT
Run-Time
TABLE I
COMPONENTS OF THE COS IMPLEMENTATION

13. 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

14. 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