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?
COS318 Lec 2
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Next Reading Assignment
Sections 1.6-2.1 inclusive
 Keep up with what’s on home page
 I’ll try to remember to remind you


Coming up: x86 assembly for Proj 2
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About Quiz 0
It didn’t count, so don’t worry
 “Scores” are
5: 9 people
4: 14 people
3: 14 people
2: 8 people
1: 1 person

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Quiz 0 Questions
Register – everyone correct?
 Stack – data structure: last-in, first-out
 Stack frame – on procedure calls,
formal parameters, local variables,
return address all get pushed onto stack

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Question 4 (22 out of 46)
int *p = 0;
printf(“val is %d\n”, *p);
Type of p is “pointer to integer”.
Value of p is zero.
Deref of p is getting value at location 0, being
read as an integer.
Should cause an error on any sane system.
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Question 5 (29 out of 46)
Func( )
{
int *result;
CallSomeFunc(result);
}
Note: value of result can not change
What can change is data at location pointed to by result
To change value of result, you have to pass address of result
Normally, you declare result as a non-pointer, and pass in its
address by &result
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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
15
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
COS318 Lec 2
Disk 10Mx
Tape 100Mx
20
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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...
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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
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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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