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Basic Computer Organization
• Block diagram of a typical computer
I/O bus
Memory bus
Processor (CPU)
Direct Memory Access (DMA)
Input/Output
Memory
– Bus – a collection of "wires" for transferring information
– Memory – holds data and instructions
– Without DMA, all I/O must pass through the CPU
– This is a logical organization. The physical organization may have a different
appearance
E.g., the I/O bus and memory bus may be physically located in one main system bus
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The CPU
Can be organized in various ways. Here is a simple CPU:
Memory bus
AC
ALU
PC
IR
Status
I/O bus
Control
Unit
• ALU – manipulates data
• Control unit
– Fetches and executes instructions
– Synchronizes the flow of data
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The CPU
Memory bus
AC
PC
IR
Status
ALU
I/O bus
Control
Unit
• CPU registers – are like memory cells but are local to the CPU
– AC (accumulator) – holds the result of an arithmetic or logical operation
– PC (program counter) – holds the memory address of the next instruction to
be executed
– IR (instruction register) – holds the currently executing instruction
– Status register – maintains info. about the most recent ALU operation and
certain system events
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Main Memory
• Contains
– instructions
– data
• Is composed of a number of memory cells or memory locations, each of the
same size
• Each memory cell has
– a unique address, and
– a value (or contents)
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Main Memory
• Units of information
– Bit (binary digit) – a 0 or 1
– Byte – a group of 8 bits (usually)
Historically, the no. of bits to represent one character ('A', '=', etc.)
– Word – a larger grouping of bits, such as 16 or 32 or 64
• Addressability
– Some machines are word-addressable (the smallest addressable storage unit is
the word), but these days, most are byte-addressable
• Machines are commonly word-organized, but byte-addressable
• Little-endian vs. big-endian
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Interaction of CPU and Memory
• The CPU can perform 2 operations on a memory cell:
– memory write (stores a new value into the memory cell)
– memory read (fetches the contents of the cell without altering it)
Main memory
Read
cell
0
cell
1
cell
2
Memory write
– CPU places address A on address bus
Write
Data bus
Memory read
– CPU places address A on address bus
– CPU activates Read control signal
– Memory sends contents of address A to
CPU on data bus
...
CPU
Control circuits
Address bus
Address
cell
n–2
cell
n–1
and value V on data bus
– CPU actives Write control signal
– Memory writes value V into cell whose
address is A, destroying previous
contents of A
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A Computer System As a Series of
Layers or Levels
• Topmost level: Viewed as a command processor
E.g., invoke text editor, invoke C compiler, start Web browser
• Lower: Viewed as a text manipulator, a C machine, a Web machine
• Lower: Viewed as a CPU, memory, I/O devices
• Lower: Viewed as ALU, control unit, registers, etc. (The machine level)
• Lower: ALU viewed as AND gates/OR gates, registers viewed as flipflops, etc. (The digital logic level)
• In this course, we'll focus first on the machine level, then on the digital
logic level and see their relationships to higher levels.
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Programming Language Hierarchy
(For Compiled Languages)
High-level language
C, C++, COBOL,
Pascal, Ada, …
Assembly language
Symbolic
machine language
Compiler
Assembler
Machine language
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Numeric
instructions
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Programming Language Hierarchy
•
High-level language
Machine language instructions
– Bit strings, with each instruction performing
a primitive operation
– Ex: 110001000101011 (Load contents of
mem. loc. 50 into AC)
Compiler
Assembly language
Assembler
Machine language
• Assembly language instructions
– Use mnemonic names for operations and symbolic names for memory
locations
– Ex: LAC 50 or LAC N
• High-level language instructions
– Typically translate into 5–10 machine language instructions
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Interpreted Languages
• Interpreters vs. compilers
– Some languages are interpreted rather than compiled
High-level
language program
Interpreter
Instruction
Machine lang.
Executed
Instruction
Machine lang.
Executed
Instruction

Machine lang.
Executed
– Original BASIC language was interpreted
– Disadvantage?
Advantage?
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Interpreted Languages
• Java
– The buzz: "Java programs run slowly because Java is an interpreted language"
– The truth: Java is a compiled language, not an interpreted language
However, Java uses a mixture of compilation and interpretation
Java program
Compiler
Interpreter
A generic
machine
language called
bytecode
__________
– Why do this?
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Instruction
Instruction
Instruction

Machine lang.
Machine lang.
Machine lang.
Executed
Executed
Executed
JVM
This interpreter is called the __________.
Each kind of machine (Mac, Intel, Sun, etc.) has
its own.
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