Operating System
Structures
Vivek Pai
Princeton University
COS318 Lec 2
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Gedankenexperiment

What does this program do?
static void Loop(void)
{
static char *startAddr;
char local;
printf(“diff is %d\n”, startAddr – (&local));
startAddr = &local;
Loop( );
}
int main(int argc, char *argv[ ])
{
Loop( );
}
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Mechanics
Have you:
 Subscribed to pu.cs.318?
 Sent me mail with your details?
– Hey, it was an assignment after all…

Sent me a picture of yourself?
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More Mechanics
Syllabus is now “complete”, along with
goals for each lecture
 Preparatory readings are listed

– External readings are not yet included

Projects are being finalized
– Regular lab not yet operational

How do we want to handle quizzes?
– Scheduled or random?
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Reading Assignment
Sections 1.0-1.3 inclusive
 Keep up with what’s on home page
 I’ll try to remember to remind you

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A Typical Computer
from a Hardware Point of View
CPU
Memory
...
CPU
Chipset
I/O bus
Network
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A Typical Computer System
CPU
..
.
Memory
Programs and data
Operating System Software
CPU
OS
Apps
Data
Network
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Typical Unix OS Structure
Application
Libraries
User space/level
Kernel space/level
Portable OS Layer
Machine-dependent layer
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Typical Unix OS Structure
Written by programmer
Compiled by programmer
Uses function calls
Application
Libraries
Portable OS Layer
Machine-dependent layer
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Typical Unix OS Structure
Written by elves
Provided pre-compiled
Defined in headers
Input to linker (compiler)
Invoked like functions
May be “resolved” when
program is loaded
Application
Libraries
Portable OS Layer
Machine-dependent layer
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Typical Unix OS Structure
Application
Libraries
Portable OS Layer
“Guts” of system calls
All “high-level” code
Machine-dependent layer
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Typical Unix OS Structure
Application
Libraries
Portable OS Layer
Machine-dependent layer
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Bootstrap
System initialization
Interrupt and exception
I/O device driver
Memory management
Kernel/user mode
switching
Processor management
12
Another Look: Unix “Onion”
Applications
User and Kernel
boundary
OS Service
Device
Hardware
Driver
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What’s An Application?
Four parts (“segments”)
 Code/Text – instructions
 Data – initialized global variables
 Stack
 Heap
What’s a stack and heap?
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OS Service Examples

Examples that are not provided at user level
– System calls: file open, close, read and write
– Control the CPU so that users won’t stuck by
running
while ( 1 ) ;
– Protection:
• Keep user programs from crashing OS
• Keep user programs from crashing each other

Examples that can be provided at user level
– Read time of the day
– Protected user level stuff
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Processor Management

Goals
– Overlap between I/O and
computation
– Time sharing
– Multiple CPU allocations

CPU
I/O
CPU
CPU
CPU
I/O
Issues
CPU
– Do not waste CPU
resources
I/O
– Synchronization and mutual CPU CPU
exclusion
– Fairness and deadlock free
CPU
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Memory Management

Goals
– Support programs to run
– Allocation and
management
– Transfers from and to
secondary storage

Register
L2 10x
Memory 200x
Issues
– Efficiency & convenience
– Fairness
– Protection
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Disk 10Mx
Tape 100Mx
17
x86 Architecture Registers
31
15
87
AL
BL
CL
DL
AH
BH
CH
DH
BP
SI
DI
SP
0 16-bit
AX
BX
CX
DX
32-bit
EAX
EBX
ECX
EDX
EBP
ESI
EDI
ESP
15
0
CS
DS
SS
ES
FS
GS
Segment registers
General-purpose registers
EFLAGS register
EIP (Instruction Pointer register)
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x86 Memory
232-1
31
24 23 16 15
87
0
.
.
.
Byte 7 Byte 6 Byte 5 Byte 4
Byte 3 Byte 2 Byte 1 Byte 0
0
Byte order is little endian
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I/O Device Management

Goals
– Interactions between
devices and applications
– Ability to plug in new
devices

Issues
– Efficiency
– Fairness
– Protection and sharing
User 1
...
User n
Library support
Driver
Driver
I/O
device
I/O
device
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Window Systems

All in the kernel (Windows)
– Pros: efficient?
– Cons: difficult to develop new services

All at user level
– Pros: easy to develop new apps
– Cons: protection

Split between user and kernel (Unix)
– Kernel: display driver and mouse driver
– User: the rest
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File System

A typical file system
– Open a file with
authentication
– Read/write data in
files
– Close a file

Can the services be
moved to user level?
User 1
...
User n
File system services
File
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...
File
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Bootstrapping






Power up a computer
Processor reset
– Set to known state
– Jump to ROM code
Load in the boot loader
from stable storage
Jump to the boot loader
Load the rest of the
operating system
Initialize and run
Boot
loader
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Boot
loader
OS
sector 1
OS
sector 2
.
.
.
OS
sector n
23
System Boot
Maps to FFFFFFF0h= 232-16

Power on (processor waits until Power Good
Signal)
 Processor jumps on a PC (“Intel Inside”) to
address FFFF0h
• 1M= 1,048,576= 220 =FFFFFh+1
• FFFFFh=FFFF0h+16 is the end of the (first 1MB of)
system memory
• The original PC using Intel 8088 had 20 address lines :-)

(FFFFFFF0h) is a JMP instruction to the
ROM BIOS startup program
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ROM BIOS startup program (1)

POST (Power-On Self-Test)
• If pass then AX:=0; DH:=5 (586: Pentium);
• Stop booting if fatal errors, and report

Look for video card and execute built-in ROM
BIOS code (normally at C000h)
 Look for other devices ROM BIOS code
• IDE/ATA disk ROM BIOS at C8000h (=819,200d)

Display startup screen
• BIOS information

SCSI disks: must often
provide their own BIOS
Execute more tests
• memory
• system inventory
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ROM BIOS startup program (2)

Look for logical devices
– Label them
• Serial ports
– COM 1, 2, 3, 4
• Parallel ports
– LPT 1, 2, 3
– Assign each an I/O address and IRQ

Detect and configure PnP devices
 Display configuration information on screen
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ROM BIOS startup program (3)

Search for a drive to BOOT from
– Floppy or Hard disk
• Boot at cylinder 0, head 0, sector 1

Load code in boot sector
 Execute boot loader
 Boot loader loads program to be booted
• If no OS: "Non-system disk or disk error - Replace and
press any key when ready"

Transfer control to loaded program
 Is it okay to boot at first sector on the floppy
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or disk?
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Ways to Develop An Operating
System
A hardware simulator
 A virtual machine
 A good kernel debugger

– When OS crashes, always goes to the
debugger
– Debugging over the network
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