Computer Organization

1
Computer Organization
2
Computer Organization

• This module surveys the physical resources of a
  computer system.
• Basic components
• CPU
• Memory
• Bus
• I/O devices
• CPU structure
• Instruction cycle
• Disk geometry

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Basic Architecture

• Processor (CPU)
• Main Memory
• volatile
• I/O devices
• secondary memory
• communications
• terminals
• System interconnection
• a bus is used to exchange data and control
  information

CPU
Memory
System Bus
Disk Controller
Network Controller
Serial Device Controller
4
Interconnection the bus

• Conceptually, a collection of parallel wires,
  each of which is dedicated to carrying one of
• data
• address
• control (of access to the bus)
• Only one component can write to a particular
  wire of the bus at a time

data address control
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CPU and the Memory

• The Central Processing Unit (CPU)
• responsible for instruction execution
• determines how the memory is to be modified
• contains a few data container called registers
• The Main Memory
• large collection of data containers
• each is labeled with a positive integer called
  its address
• For each instruction, the CPU fetches input data
  from registers or memory, then writes output to a
  register or memory location

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Central Processing Unit

• Arithmetic logic unit (ALU)
• performs arithmetic and logic operations
• Control unit
• reads and decodes instructions
• initiates execution ofinstruction by
  propercomponent
• Registers
• some have special purpose

CPU
ALU
Control
PC
PSW
CP
SP
DL
IR
AR
DP
CL
v0
a0
s0
s1
s2
s3
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Fetch-Decode-Execute Cycle

• The CPU is endlessly looping through these steps
• Actual steps will vary from processor to
  processor
• Simple model
• 1. fetch the next instruction
• 2. decode the instruction load its data from
  registers
• 3. execute the instruction
• 4. read from memory, or write to memory
• 5. write to registers

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Device Controllers

• Devices are not connected directly to the system
  bus
• Each device has a device controller between it
  and the system bus
• One controller may have multiple devices
• For example SCSI devices, IDE devices, USB
  devices

CPU
Memory
System Bus
Disk Controller
Network Controller
Serial Device Controller
9
I/O Devices

• Each device has a buffer which mediates data
  transfer.
• Transfer between memory and devices is limited by
  the size and speed of the data bus.
• For example, though a disk reads data to its
  buffer one block at a time, transfer to memory is
  one word at a time.

CPU
Memory
System Bus
buffer
Device Controller
10
Disk Structure

• Disk drives are addressed as large 1-dimensional
  arrays of logical blocks, where the logical block
  is the smallest unit of transfer
• Like a big random-access file where each record
  is a logical block
• The 1-dimensional array of logical blocks is
  mapped into the sectors of the disk sequentially.
• Sector 0 is the first sector of the first track
  on the outermost cylinder.
• Mapping proceeds in order through that track,
  then the rest of the tracks in that cylinder, and
  then through the rest of the cylinders from
  outermost to innermost.

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Disk Device

• Accessing data on disk requires waiting
• For the disk to spin to the proper location
• Rotational latency
• For the read/write heads to move to the proper
  location
• Seek time
• Disk access is around 100000 times slower than
  memory access

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Uniform sector mapping

• Sectors stored in the outer tracks use more space
  to store the same number of bits as the inner
  tracks
• This is somewhat wasteful
• The disk spins at a constant angular velocity
• 4200-10600 rpms

Track 0, Sector 0
Track 2, Sector 7
13
Zoned Bit Density

• The tracks are divided into zones
• There are more sectors in outer zones than in
  inner zones
• This leads to a more efficient use of disk space
• The disk still spins at a constant angular
  velocity
• But outer tracks have more bits per rotation, and
  so are read faster