Transcript Dr. Welch`s slides

CSCE 121, Sec 200, 507, 508
Fall 2010
Prof. Jennifer L. Welch
Simplified Model of a Computer
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How data is represented and stored
How data is operated on
How a program is stored
How the program is executed on the data
How programs are translated into machine
language
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How Data is Represented
• Fundamental unit of computer storage is a bit, data
entity that can take on two values, 0 and 1
• Given a sequence of bits (01010001), what does it
represent?
• Depends on the code
– Base 10 integer: 1,010,001
– Base 2 integer: 1*20 + 1*24 + 1*26 = 81 (in base 10)
– ASCII: The letter ‘Q’
• Must know the code to decode a sequence
• Other codes:
– Negative numbers (e.g., 2’s complement)
– Numbers with fractional parts (floating point)
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How Data is Stored
• Byte: a group of 8 bits; 28 = 256 possibilities
– 00000000, 00000001, …, 11111110, 11111111
• Memory: long sequence of locations, each big
enough to hold one byte, numbered 0, 1, 2, 3,…
• Address: the number of the location
• Contents of a location can change
• Use consecutive locations to store longer sequences
– e.g., 4 bytes = 1 word
1 1 0 0 1 1 0 0 0 0 1 1 0 0 1 1 0 1 0 0 1 1 0 0 1 1 0 …..
byte 0
byte 1
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byte 2
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Limitations of Finite Data Encodings
• Overflow: number is too large
– suppose 1 byte stores integers in base 2, from 0
(00000000) to 255 (11111111). If the byte holds
255, then adding 1 to it results in 0, not 256
• Roundoff error:
– insufficient precision (size of word): try to store
1/8, which is .001 in base 2, with only two bits
– nonterminating expansions in current base: try to
store 1/3 in base 10, which is .333…
– nonterminating expansions in every base:
irrational numbers such as π
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Kinds of Storage
The computer reads and writes the data stored
in it. From fastest to slowest:
• cache: super-fast
• main memory: random access (equally fast to
access any address
• disk: random access, but significantly slower
than main memory
• tape: sequential access, significantly slower
than disk
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How Data is Operated On
• Central processing unit (CPU) consists of:
• arithmetic/logic unit (ALU):
– performs operations on data (e.g., add, multiply)
• registers: special memory cells, hold data used
by ALU, even faster than cache
• control unit:
– figures out what the ALU should do next
– transfers data between main memory and
registers
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Machine Instructions
Goal: add the number stored in address 3 and the number stored in address
6; put the result in address 10.
Control unit does the following:
1. copies data from main memory address 3 into register 1:
LOAD 3,1
2. copies data in main memory address 6 into register 4:
LOAD 6,4
3. tells ALU to add the contents of registers 1 and 4, and put result in
register 3:
ADD 1,4,3
4. copies data in register 3 into main memory address 10:
STORE 3,10
LOAD, ADD and STORE are machine instructions.
How does the control unit know which instruction is next? The program!
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How a Program is Stored
• Program: list of machine instructions using
some agreed upon coding convention.
• Example:
ADD
1
4
0 0 1 0
0 0 0 1
opcode
1st operand
0 1 0 0
2nd operand
3
0 0 1 1
3rd operand
• Program is stored same way data is stored!
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How a Program is Executed
• The control unit has
– instruction register: holds current instruction to be executed
– program counter: holds address of next instruction in the
program to be fetched from memory
• Program counter tells where the computer is in the
program. Usually the next instruction to execute is the
next instruction in memory
• Sometimes we want to JUMP to another instruction
(e.g., if or while)
– unconditional JUMP: always jump to address given
– conditional JUMP: only jump if a certain condition is true
(e.g., some register contains 0)
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Machine Cycle
• fetch next instruction, as indicated by the
program counter (PC), and increment PC
• decode the bit pattern in the instruction
register – figure out which circuitry needs to
be activated to perform the specified
instruction
• execute the specified instruction by copying
data into registers and activating the ALU to
do the right thing
– a JUMP may cause the PC to be altered
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Diagram of Architecture
CPU:
PC:
R1:
IR:
R2:
R3:
control
unit
ALU
R4:
bus
0
main
memory:
1
2
3
4
5
data
…
first instr.
program
second instr.
…
third instr.
95
96
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98
99
100
…
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Evolution of Programming Languages
• Machine languages: all in binary
– machine dependent
– painful for people
• Assembly languages: allow symbols for operators and
addresses
– still machine dependent
– slightly less painful
– assembler translates assembly language programs into machine
language
• High-level languages: such as Fortran, C, Java, C++
– machine independent
– easier for people
– compiler translates high-level language programs into machine
language
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Compilation
Challenges faced by a compiler:
• one high-level instruction can correspond to
several machine instructions
– Ex: x = (y+3)/z;
• fancier data structures, such as
– arrays: must calculate addresses for references to
array elements
– structs: similar to arrays, but less regular
• fancier control structures than JUMP
– while, repeat, if-then-else, case/switch
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Compilation
Challenges faced by a compiler:
• functions (a.k.a. procedures, subroutines,
methods): must generate machine language
code to:
– copy input parameter values
– save return address
– start executing at beginning of function code
– copy output parameter values at the end
– set PC to return address
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Compilation Process
• Lexical analysis: break up strings of characters into
logical components, called tokens, and discard
comments, spaces, etc.
– Ex: “total = sum + 55.32” contains 5 tokens
• Parsing: decide how the tokens are related
– Ex: “sum + 55.32” is an arithmetic expression, “total = sum
+ 55.32” is an assignment statement
• Code generation: generate machine instructions for
each high-level instruction.
• The resulting machine language program, called
object code, is written to disk
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Linking
• Linker: combines results of compiling different
pieces of the program separately. If pieces
refer to each other, these references cannot
be resolved during independent compilation.
• Combined code is written to disk.
main
…
declares x
…
invokes p
…
function p
…
refers to x
…
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Loading
• Loader: when it’s time to run the program, the
loader copies the object code from disk into
main memory
– location in main memory is determined by the
operating system, not the programmer
• Loader initializes PC to starting location of
program and adjusts JUMP addresses
• Result is an executable
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