20-755: The Internet
Lecture 2: Computer Systems I
David O’Hallaron
School of Computer Science and
Department of Electrical and Computer Engineering
Carnegie Mellon University
Institute for eCommerce, Summer 1999
Lecture 2, 20-755: The Internet, Summer 1999
1
Today’s lecture
•
•
•
•
Administrative issues (10 minutes)
Data (50 min)
Break (10 min)
Programs (40 min)
Lecture 2, 20-755: The Internet, Summer 1999
2
Systems
•
•
A system is a collection of interworking parts.
Examples:
–
–
–
–
–
•
•
the human body
the economy
a car
a stereo
a computer
Systems are often extremely complicated.
How do we understand complex systems?
– Abstraction is the key.
Lecture 2, 20-755: The Internet, Summer 1999
3
Abstraction
•
Example: a lighting system
light bulb
switch
A
A
B
B
Lecture 2, 20-755: The Internet, Summer 1999
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Abstraction
•
Here’s one way to understand how our
lighting system works:
“Closing the switch induces a voltage drop
between A and B, which causes current to
flow through the light bulb, which heats up
the filament, which causes the filament to
emit light. Opening the switch eliminates the
voltage differential, which stops the current,
which causes the filament to cool and stop
emitting light.”
Lecture 2, 20-755: The Internet, Summer 1999
5
Abstraction
•
Here’s another way to understand our lighting
system:
“Close the switch and light turns on. Open the
switch and the light turns off”
•
This is an example of an abstraction, where
we describe the behavior of a system in terms
of its inputs (the position of the switch) and
its outputs (whether it is emitting light or not).
Lecture 2, 20-755: The Internet, Summer 1999
6
Abstraction
•
•
Abstraction is one of the most powerful
weapons in the arsenal of computer science.
Useful because it hides complexity.
– High-level languages like C, Java, and Perl provide
abstractions for low-level machine instructions.
– Operating systems provide abstractions for resources
such as the CPU, memory, and I/O devices.
– TCP/IP provides an abstraction for collections of
interconnected heterogeneous networks.
•
However, abstractions are most useful if we
understand something about how things
work.
Lecture 2, 20-755: The Internet, Summer 1999
7
Typical computer system
Keyboard
Processor
Interrupt
controller
Mouse
Keyboard
controller
Modem
Serial port
controller
Printer
Parallel port
controller
Local/IO Bus
Memory
IDE disk
controller
SCSI
controller
Video
adapter
Network
adapter
Display
Network
SCSI bus
disk
disk
Lecture 2, 20-755: The Internet, Summer 1999
cdrom
8
Bits
•
•
All computer data (input, output, memory, and even
programs) are collections of 1’s and 0’s called bits
(binary digits).
“0” and “1” are abstractions for voltage levels.
– easy to store with bistable elements and can be reliably
transmitted on noisy and innacurate wires.
0
1
0
3.3V
2.8V
0.5V
0.0V
Lecture 2, 20-755: The Internet, Summer 1999
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Powers of 10
•
Def: 10p = 10 x 10 x ... x 10 (there are p 10’s)
– “ten to the power p” or “the pth power of 10”
•
Examples of powers of 10
100 = 1
101 = 10
102 = 10 x 10 = 100
103 = 10 x 10 x 10 = 1,000
Lecture 2, 20-755: The Internet, Summer 1999
10
Decimal (base 10) numbers
•
•
•
10 digits to choose from: 0, 1, ... , 8, 9.
Each digit corresponds to a different power of 10.
Examples:
345 = (3 x 102) + (4 x 101) + (5 x 100)
= 300 +
40
+ 5
132 = (1 x 102) + (3 x 101) + (2 x 100)
= 100 +
30
+
2
Lecture 2, 20-755: The Internet, Summer 1999
11
Powers of 2
•
Def: 2p = 2 x 2 x ... x 2 (there are p 2’s)
– “two to the power p” or “the pth power of two”
•
Examples of powers of 2
20 = 1
21 = 2
22 = 2 x 2 = 4
23 = 2 x 2 x 2 = 8
24 = 2 x 2 x 2 x 2 = 16
Lecture 2, 20-755: The Internet, Summer 1999
12
Binary (base 2) numbers
•
•
•
2 digits to choose from: 0, 1.
Each digit corresponds to a different power of 2.
Examples (converting from binary to decimal)
1012 = (1 x 22) + (0 x 21) + (1 x 20)
=
4
+
0 + 1
= 5
0112 = (0 x 22) + (1 x 21) + (1 x 20)
=
0
+
2 + 1
= 3
Lecture 2, 20-755: The Internet, Summer 1999
13
Converting from decimal to binary
•
converting decimal 5 to binary
5 / 2 = 2 rem 1
2 / 2 = 1 rem 0
1 / 2 = 0 rem 1
•
first (righmost) binary digit is 1
second binary digit is 0
third binary digit is 1
So 5 = 1012
converting decimal 3 to binary
3 / 2 = 1 rem 1
1 / 2 = 0 rem 1
first (righmost) binary digit is 1
second binary digit is 1
So 3 = 112 = 0112
Lecture 2, 20-755: The Internet, Summer 1999
14
Counting in binary
•
With n bits, you can represent 2n numbers:
0, 1, ... , 2n - 1.
•
Example: 1 bit (21 = 2 numbers)
02
12
•
(0)
(1)
Example: 2 bits (22 = 4 numbers)
002
012
102
112
(0)
(1)
(2)
(3)
Lecture 2, 20-755: The Internet, Summer 1999
15
Counting in binary (cont.)
•
Example: 3 bits (23 = 8 numbers)
0002 (0)
0012 (1)
0102 (2)
0112 (3)
1002 (4)
1012 (5)
1102 (6)
1112 (7)
Lecture 2, 20-755: The Internet, Summer 1999
16
Practice problems
Powers of two:
Binary to decimal:
Decimal to binary
(a) 20 =
(a) 00102 =
(a) 8 =
(b) 21 =
(b) 01112 =
(b) 9 =
(c) 25 =
(c) 10012 =
(c) 12 =
(d) 11112 =
(d) 14 =
Lecture 2, 20-755: The Internet, Summer 1999
17
Practice problems: Counting in binary
(a) how many numbers can you represent using 4 bits?
(b) What is the largest number?
(c) write them out in binary, starting from 00002:
Lecture 2, 20-755: The Internet, Summer 1999
18
Two’s complement representation of
signed integers
sign bit:
0: positive
1: negative
positive
integers
negative
integers
Lecture 2, 20-755: The Internet, Summer 1999
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
(0)
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(-8)
(-7)
(-6)
(-5)
(-4)
(-3)
(-2)
(-1)
19
Interpreting two’s complement
numbers
•
To negate a two’s complement number:
– invert the bits (I.e., change each 0 to 1 and each 1 to 0).
– add 1 to the result.
•
•
Examples:
– To negate 0 = 00002 : 11112+ 1 = 00002
– To negate -2 = 11102 : 00012 + 1 = 00102
You can use this property to determine the
two’s complement representation of a
negative number.
– Example: -5 = inv(0101) + 1 = 1010 + 1 = 1011, where inv(x)
inverts the bits of x.
– Example: -12 = ?
Lecture 2, 20-755: The Internet, Summer 1999
20
Hex (base-16) representation of
binary numbers
Binary
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
Hex Decimal
0
1
2
3
4
5
6
7
8
9
a
b
c
d
e
f
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Lecture 2, 20-755: The Internet, Summer 1999
Hex is a compact way to represent
binary numbers. Each hex digit
represents a byte (8 bits) of data:
• 011010102 = 6a16
• 10111110111011112 = beef16
In Perl, use the “0x” prefix to denote a
hex number:
x = 0x6a;
y = 0xbeef;
21
Characters
•
•
ASCII characters are represented in 8-bit
chunks called bytes.
Two types of characters:
– printable characters: characters that can be typed on a
keyboard (e.g., ‘d’, ‘%’h)
– unprintable control characters (e.g., BEL,BS)
Lecture 2, 20-755: The Internet, Summer 1999
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ASCII character set
hex
00
01
02
03
04
05
06
07
08
09
0A
0B
0C
0D
0E
0F
10
11
12
13
14
15
16
17
18
19
char esc
NUL
SOH
STX
ETX
EOT
ENQ
ACK
BEL
BS
HT
LF
VT
FF
CR
SO
SI
DLE
DC1
DC2
DC3
DC4
NAK
SYN
ETB
CAN
EM
'\0'
'\a'
'\b'
'\t'
'\n'
'\v'
'\f'
'\r'
1A
1B
1C
1D
1E
1F
20
21
22
23
24
25
26
27
28
29
2A
2B
2C
2D
2E
2F
30
31
32
33
SUB
ESC
FS
GS
RS
US
SPACE
!
"
#
$
%
&
’
(
)
*
+
,
.
/
0
1
2
3
Lecture 2, 20-755: The Internet, Summer 1999
34
35
36
37
38
39
3A
3B
3C
3D
3E
3F
40
41
42
43
44
45
46
47
48
49
4A
4B
4C
4D
4
5
6
7
8
9
:
;
<
=
>
?
@
A
B
C
D
E
F
G
H
I
J
K
L
M
4E
4F
50
51
52
53
54
55
56
57
58
59
5A
5B
5C
5D
5E
5F
60
61
62
63
64
65
66
67
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
[
\
]
^
_
'
a
b
c
d
e
f
g
'\\'
68
69
6A
6B
6C
6D
6E
6F
70
71
72
73
74
75
76
77
78
79
7A
7B
7C
7D
7E
7F
h
i
j
k
l
m
n
o
p
q
r
s
t
u
v
w
x
y
z
{
|
}
~
DEL
23
Computer memory
Keyboard
Processor
Interrupt
controller
Mouse
Keyboard
controller
Modem
Serial port
controller
Printer
Parallel port
controller
Local/IO Bus
Memory
IDE disk
controller
SCSI
controller
Video
adapter
Network
adapter
Display
Network
SCSI bus
disk
disk
Lecture 2, 20-755: The Internet, Summer 1999
cdrom
24
Metrics for space and time
•
Space
– Kilobyte (KB) = 210
» approx. 102 , “a thousand”
– Megabyte (MB) = 220
» approx. 106, “a million”
– Gigabyte (GB) = 230
» approx. 109, “a billion”
– Terabyte (TB) = 240
» approx. 1012, “a trillion”
Lecture 2, 20-755: The Internet, Summer 1999
•
Time
– millisecond (ms) = 10-3 s
» .001 s
» “a thousandth of a sec”
– microsecond (us) = 10-6 s
» .000001 s
» “a millionth of a second”
– nanosecond (ns) = 10-9 s
» .000000001 s
» “a billionth of a second”
» 1 ns is the time it takes
light to travel about 12 in!
25
Computer memory
Bytes
•
•
•
Organized as a sequential
array of bytes.
Each byte has an integer
address (location).
Addresses start at 0.
Lecture 2, 20-755: The Internet, Summer 1999
Addr
0000
0001
0002
0003
0004
0005
0006
0007
0008
0009
0010
0011
0012
0013
0014
0015
26
Words
•
Machine has “word size”
– nominal size of numbers, including
addresses.
– Numbers, instructions, and
addresses typically fractions or
multiples of the word size.
– Words are addressed by first byte.
•
Addr
=
0000
PC-class machines are 32 bits
– Limits addresses to 4 GB.
– Becoming too small!
•
Words
Newer server-class machines
are 64 bits (e.g. DEC Alpha)
– Limits addresses to 4 TB.
– In 20 years, we’ll be complaining
about this too!
Lecture 2, 20-755: The Internet, Summer 1999
Addr
=
0008
Addr
=
0012
Bytes
Addr
0000
0001
0002
0003
0004
0005
0006
0007
0008
0009
0010
0011
0012
0013
0014
0015
27
Strings
•
•
•
A string is represented in
memory as a sequence of bytes
terminated by 0x00 (‘\0’).
Known as a “null-terminated
string”
Example (Perl):
– $x = “Hi Dave!\n”;
Bytes
‘H’
‘I’
SPACE
‘D’
‘a’
‘v’
‘e’
‘\0’
Lecture 2, 20-755: The Internet, Summer 1999
Addr
0000
0001
0002
0003
0004
0005
0006
0007
0008
0009
0010
0011
0012
0013
0014
0015
28
Data Representation Summary
•
Key concepts:
– It’s all just bits!
» everything in a computer is represented as a
collection of bits that are interpreted in different
ways.
– memory is organized as a sequence of bytes
» each byte in memory has its own address.
– each machine has nominal word size
» numbers are fractions or multiples of words
» the address of a word is the address of its first byte
•
floating point representation is used for nonintegral numbers:
– e.g., 3.14159
– too complex for us to study in this course.
Lecture 2, 20-755: The Internet, Summer 1999
29
Break time!
(10 mins)
Lecture 2, 20-755: The Internet, Summer 1999
30
Today’s lecture
•
•
•
•
Administrative issues (10 minutes)
Data (50 min)
Break (10 min)
Programs (40 min)
Lecture 2, 20-755: The Internet, Summer 1999
31
Programs
Keyboard
Processor
Interrupt
controller
Mouse
Keyboard
controller
Modem
Serial port
controller
Printer
Parallel port
controller
Local/IO Bus
Memory
IDE disk
controller
SCSI
controller
Video
adapter
Network
adapter
Display
Network
SCSI bus
disk
disk
Lecture 2, 20-755: The Internet, Summer 1999
cdrom
32
Processor components
Processor
Memory
Addresses
P
C
ALU
Register
File
Data
Instructions
Object Code
Program Data
• PC (Program Counter)
» Contains address of the next instruction
• ALU (Arithmetic/Logic Unit)
– addition, subtraction, etc.
• Register File
» Small fast internal memory for heavily
used program data, typically 16, 32, or
64 locations (called registers), each of
which holds 1 word.
Register
name Register file
R0
R1
R2
...
R30
R31
• Memory
» Contains both object code and data
Lecture 2, 20-755: The Internet, Summer 1999
33
Object code
•
•
•
Processor simply executes one machine
language instruction after another until you
unplug it.
A program (or code) is a related set of these
instructions.
Key idea: Programs are stored in memory as
simply another kind of data
– variously called object code, machine code, object
program.
•
Example:
– on the DEC Alpha, the word 0x42110401 is the bit pattern for
an instruction that adds the contents of two registers and
stores the result in a third register.
Lecture 2, 20-755: The Internet, Summer 1999
34
Assembly Language
•
machine language instructions are
represented in ASCII text form as assembly
language instructions.
•
– Add the integers in registers 16
and 17 and store the result in
register 1.
addq r16,r17,r1
•
0x1200012d0: 0x42110401b
Lecture 2, 20-755: The Internet, Summer 1999
Assembly
Object Code
– 32-bit pattern
– Stored at address
0x1200012d0
35
Basic processor operation
•
Fetch
– load an instruction from memory, using the address
contained in the PC.
•
Execute
– load word from memory to register file
– store word from register file to memory
– perform an arithmetic operation (e.g., addition) on the
contents of two registers and store the results in a
register.
•
Update PC
– if Branch instruction
» set PC to some new address
– if not Branch instruction
» increment PC to point to next sequential instruction
» ex: PC <-- PC + 4
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36
Basic instructions: load
•
Move a word from memory to a register.
– example: ld r1, addr
– move word at address addr (Mem[addr]) to register r1.
– increment PC with address of next instruction.
Processor
Memory
addr
P
C
ALU
Register
File
PC <-- PC + 4
Lecture 2, 20-755: The Internet, Summer 1999
Mem[addr]
Object Code
Program Data
Mem[PC]
“ld r1, addr”
37
Basic instructions: store
•
Move a word from a register to memory
– example: st r1, addr
– move contents of register r1 (Reg[r1]) to memory
address addr.
– increment PC with address of next instruction
Processor
Memory
<addr>
P
C
ALU
Register
File
PC <-- PC + 4
Lecture 2, 20-755: The Internet, Summer 1999
Reg[r1]
Object Code
Program Data
Mem[PC]
“st r1, addr”
38
Basic instructions: arithmetic
operations
•
Add the contents of two registers and store
the result in a third register
– example: add r16,r17,r1
– add contents of r16 and r17 and store the results in r1.
– increment PC with address of next instruction
Processor
P
C
ALU
Memory
Register
File
PC <-- PC + 4
Object Code
Program Data
Mem[PC]
“add r16, r17, r1”
Lecture 2, 20-755: The Internet, Summer 1999
39
Basic instructions: branch
•
branch to a new location in the program
– example: branch addr
– set the PC to addr
Processor
P
C
ALU
Memory
Register
File
PC <-- addr
Lecture 2, 20-755: The Internet, Summer 1999
Object Code
Program Data
Mem[PC]
“branch addr”
40
Altering the control flow
•
•
Changing the default value of the PC (I.e.,
pointing to the next instruction in memory) is
called altering the control flow.
There are two mechanisms for altering the
control flow:
– executing branch instructions
– exceptions
» crucial mechanism for modern multitasking
operating systems.
Lecture 2, 20-755: The Internet, Summer 1999
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Exceptions and interrupts
Keyboard
Processor
Interrupt
controller
Mouse
Keyboard
controller
Modem
Serial port
controller
Printer
Parallel port
controller
Local/IO Bus
Memory
IDE disk
controller
SCSI
controller
Video
adapter
Network
adapter
Display
Network
SCSI bus
disk
disk
Lecture 2, 20-755: The Internet, Summer 1999
cdrom
42
Exceptions
•
An exception is a transfer of control to the OS in
response to some event (i.e. change in processor state)
User Process
event
Operating System
exception
exception
return (optional)
Lecture 2, 20-755: The Internet, Summer 1999
exception processing
by exception handler
(also called a device
driver or an interrupt
handler)
43
Internal (CPU) exceptions
•
Internal exceptions occur as a result of
events generated by executing instructions.
– Execution of a SYSCALL instruction.
» allows a program to ask for OS services (e.g., timer
updates)
– Execution of a BREAK instruction
» used by debuggers
– Errors during instruction execution
» arithmetic overflow, address error, parity error,
undefined instruction
– Events that require OS intervention
» virtual memory page fault
Lecture 2, 20-755: The Internet, Summer 1999
44
External (I/O) exceptions
(or I/O interrupts)
•
External exceptions occur as a result of
events generated by devices external to the
processor (managed by interrupt controller).
– I/O interrupts
» hitting ^C at the keyboard
» arrival of a packet from a network
» arrival of data from a disk
– Hard reset interrupt
» hitting the reset button
– Soft reset interrupt
» hitting ctl-alt-delete on a PC
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45
High-level languages
•
High-level languages like C/C++, Java, and
Perl provide an abstraction for the low-level
details of machine-language programs.
C Code
long foo(long a, long b, long c)
{
long sum = (a+a+b)*c;
return(sum);
}
Lecture 2, 20-755: The Internet, Summer 1999
Corresponding
machine/assembly code
foo:
0x42100400
0x40110400
0x4c120400
0x6bfa8001
addq
addq
mulq
ret
a0,a0,v0
v0,a1,v0
v0,a2,v0
zero,(ra),1
46
Procedures
•
•
Procedures (or functions) are named
collections of commonly executed instruction
sequences.
Crucial abstraction mechanism in every
programming language at every level.
– take some input, produce some output
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47
Perl procedure example
# definition of function say
# prints first parameter followed by second
sub say {
print “$_[0], $_[1]!\n”;
}
#
# main body of Perl program
#
From
“Learning
Perl”,
page 95.
# first invocation of s
say(“hello”,“world”); # prints “hello, world!”
# second invocation of say
$x = “hi there”;
$y = “Dave”;
say($x, $y); # prints “hi there, Dave!”
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Compiled vs interpreted programs
•
Compiled programs:
– translated in a series of steps by a compiler, assembler,
and linker from an ASCII source program written in a
high level language to a binary executable...
– and then loaded into memory and executed by a loader.
– Examples: C/C++ compilers, Java Just-in-Time (JIT)
compilers
•
Interpreted programs:
– executing interpreter reads the ASCII source program and
executes its statements.
– Examples: Java virtual machine (JVM), Perl interpreter.
•
Compiled object code is to processor as
interpreted source program is to interpreter.
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Compiled programs
ASCII text files
C program (p1.c p2.c)
“source program”
Compiler
ASCII text files
Asm program (p1.s p2.s)
Assembler
binary files
Object program (p1.o p2.o)
libraries (.a)
Linker
binary file
Executable object program (p)
“executable program”
Loader
memory
Executing process (p)
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50
Interpreted programs
Perl script (foo.pl)
Executing Perl interpreter
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51
Pros and cons of compiled and
interpreted programs
•
Efficiency
– compiled programs are more efficient
» instructions are executed directly by hardware
» interpreted programs can be orders of magnitude
slower than compiled programs.
» one of the current problems with Java.
•
Ease of Use
– Interpreted programs are easier to write
» compiled code: edit, compile, link, execute cycle
» interpreted code: edit, execute cycle
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52
•
Processes
A process is an instance of a running program
– runs concurrently with other processes (multitasking)
– managed by a shared piece of OS code called the kernel
» kernel is not a separate process, but rather runs as part of
some user process
– each process has its own data space and process id (pid)
– data for each process protected from other processes
Process A
Process B
user code
Time
kernel code
user code
Just a stream
of instructions!
kernel code
user code
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Unix process hierarchy
[0]
init [1]
Daemon
e.g. httpd
shell
child
child
grandchild
Lecture 2, 20-755: The Internet, Summer 1999
child
grandchild
54
Unix Startup: Step 1
1. Pushing reset button loads the PC with the address of a small
bootstrap program.
2. Bootstrap program loads the boot block (disk block 0).
3. Boot block program loads kernel (e.g., /vmunix)
4. Boot block program passes control to kernel.
5. Kernel handcrafts the data structures for process 0.
[0]
init [1]
Lecture 2, 20-755: The Internet, Summer 1999
process 0: handcrafted kernel process
process 1: user mode process
fork() and exec(/sbin/init)
55
Unix Startup: Step 2
[0]
/etc/inittab
Daemons
e.g. snmp
init [1]
getty
Lecture 2, 20-755: The Internet, Summer 1999
init forks new processes as per
the /etc/inittab file
forks a getty (get tty or get terminal)
for the console
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Unix Startup: Step 3
[0]
init [1]
login
Lecture 2, 20-755: The Internet, Summer 1999
getty execs a login program
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Unix Startup: Step 4
[0]
init [1]
tcsh
Lecture 2, 20-755: The Internet, Summer 1999
login gets user’s login and password
if OK, it execs a shell
if not OK, it execs another getty
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Running programs from the
Unix shell
[0]
The shell displays a prompt
(% in this example) and
waits for the user to type a
command such as ls, cd, or
the name of a program to
execute.
init [1]
keyboard
shell (tcsh)
“%”
screen
%
Lecture 2, 20-755: The Internet, Summer 1999
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Running programs from the
Unix shell
[0]
When the user types in the
name of program to run (e.g.
foo), the shell creates a new
child process and runs the
program within that process,
transferring control of
the keyboard and display to the
child.
init [1]
keyboard
“foo”
shell (tcsh)
“foo”
child(foo)
Lecture 2, 20-755: The Internet, Summer 1999
screen
% foo
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Running programs from the
Unix shell
[0]
init [1]
shell (tcsh)
keyboard
child (foo)
Lecture 2, 20-755: The Internet, Summer 1999
While the child is running, it
reads input from the keyboard
and writes output to the screen.
screen
% foo
<output from foo>
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Running programs from the
Unix shell
[0]
init [1]
keyboard
shell (tcsh)
“%”
The shell waits for the child
process (foo) to finish and then
prints another prompt,
Indicating that it is ready to
read another command from
the keyboard.
screen
% foo
<output from foo>
%
Lecture 2, 20-755: The Internet, Summer 1999
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Programs summary
•
Key concepts:
– programs exists at different levels of abstraction
» machine code, assembly code, high-level source
– programs are just data in memory
– program instructions are just bits!
– programs can be executed by a processor or by an
interpreter.
– processes are instances of running programs.
– modern OS’s allow multiple processes to run
independently at the same time.
Lecture 2, 20-755: The Internet, Summer 1999
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