Computer Architecture

1
Computer Architecture
2
Review

• Representation and interpretation are inverse
  operations
• How do you represent in binary
• Letters?
• Images?
• Sounds?
• Smells?
• Is there ever a time you wouldnt want to use
  compression?

3
Learning Objectives

• Understand and explain basic computer
  architecture
• CPU
• Memory
• I/O
• Demystify computer hardware
• Opening up the black box (literally in section)

4
Real Computers

• Like the LMC
• Processor
• Memory
• Stores instructions and data
• Depending on IC, may be treated as instruction
• Input/Output
• But different
• Processor (CPU) can understand a few more
  instructions
• Instruction execution is automatic no little
  man
• Memory and processor are binary, not decimal
• Memory is bigger
• Can have several input/output devices
• All components are a lot faster!

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Subsystem 1The CPU

• CPU Central Processing Unit
• Internal clock ticks very fast (e.g., 2Ghz 2
  billion ticks per second)
• activities are synchronized to start on a clock
  tick
• some activities take more than one clock tick
• Instruction execution is automatic
• (tick) find memory address of next instruction
• (tick) retrieve instruction from memory
• (tick) decode the instruction
• (tick) fetch argument from memory if necessary
• (tick) execute instruction
• (tick) store result in memory if necessary

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Instruction Set

• The instructions that a CPU can execute
• LMC add, store, get, etc.
• Real CPUs a few more, more complicated
• E.g., add and store result in memory location
• RISC vs. CISC
• RISC has fewer, simpler instructions
• Simpler to implement in hardware
• Faster execution
• More instructions to accomplish same program
• Each CPU has own instruction set
• Sometimes deliberately compatible
• Pentium can execute all 486 instructions, plus
  some
• Pentium IV, AMD Athlon execute Intel Pentium
  instructions, plus some more
• Thus, Pentium IV and Athlon are both
  backwards-compatible with 486

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Subsystem 2Memory

• Like the LMC, the CPU cant remember anything
  that isnt stored in memory.
• Several types of memory/storage, which differ in
  terms of
• Performance
• Cost
• Most systems include a mix of memory types
• Cache, RAM, hard drive, removable media, etc.

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Addressing and Memory Capacity

• Contents of memory look like this
• But its easier to think of it as a series of
  fixed bit-length locations, each having a unique
  identifier
• In LMC, specify a two-digit decimal address
• How many possible locations?
• In binary computer, you might have 16-bit, 24-bit
  or 32-bit address
• How many possible locations in each of these
  systems?

01010000100000100101001100000000100101001100000000
0110010101001110
Remember that 210 is about a thousand, and that
216 26 x 210 64 x thousand
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Semiconductor Memory

• Volatile RAM (Random Access Memory)
• Can access any location equally fast
• Name is a bit misleading, since ROM works this
  way, too
• Static (SRAM) vs. Dynamic (DRAM) affects power
  consumption, speed
• SRAM typically used for cache lower power,
  faster
• DRAM typically used as main system memory
• Many types of DRAM in use today, e.g., SDRAM, DDR
  SDRAM, RDRAM, etc.
• Non-volatile ROM (Read Only Memory)
• Can access any location equally fast
• Retains memory even without power
• What kinds of information might a system designer
  want to store in ROM?

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Other Types of StorageHard Disks and Floppies

• Slower than main memory
• Bits stored as magnetic field of different
  polarity
• Magnetized surface of disk rotates under a
  magnetized head (read/write mechanism)

Sector

• Disk divided into tracks, at different radius
  from the center
• Seek time is how long it takes to move the head
  to a track
• Tracks are divided into sectors
• Latency is how long it takes for the desired
  sector to rotate under the disk
• Drive performance also influenced by speed of
  interface (IDE, SCSI, Firewire, etc.)

Track
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Disk Arrays (RAID)

• RAID Redundant Array of Independent (or
  Inexpensive) Disks
• Instead of better (faster, more reliable) disks,
  use more disks
• Several levels of RAID
• Level 0 offers improved performance
• Level 5 offers big performance improvement and
  good fault tolerance
• How does RAID do this?

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Disk Arrays (RAID)

• Redundancy
• Store data on more than one disk
• If one disk fails, can still access all data
• Example disk mirroring
• Data Striping
• Spread data across several disks
• Can access all disks in parallel
• Increases throughput (data transfer rate)
• Decreases reliability, unless

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Parity Bit Redundancy Through Striping

• Consider storing the bit string 01110
• Suppose each bit stored on different disk
• Define parity(bit-string) as equal to
• 1 if odd number of ones in the bit-string,
• else 0
• Store parity(01110) on a sixth disk
• What is parity (01110)?
• If any one disk fails, can recover the string

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Parity Bit Exercise
01110

• What should the parity bit be?
• What happens if the third drive fails?
• What happens if the fifth drive fails (instead of
  the third)?
• What if 3 and 5 fail?
• Work on your own, then discuss.

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The Memory Hierarchy

• Want to achieve balance between price and
  performance
• Small quantity of fast, expensive, temporary
  (dependent on power) semiconductor-based memory
  (e.g., cache, RAM)
• Larger quantity of slower, less expensive storage
• Doesnt require power to preserve data
• Sometimes permanent (e.g., ROM), sometimes
  alterable (e.g., hard drive, CDR, etc.)
• May be removable (e.g., floppy disks)
• The memory hierarchy
• Registers and Cache
• Main Memory
• Magnetic disks
• Tapes, CD-ROMs, etc

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Caching

• Caching means storing a copy of a subset of the
  slower memory in the small, fast memory
• e.g., copying data from main memory to CPU cache.
• Why do this?
• If you want to add something to the cache but
  its full, what do you get rid of?

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Other Caches

• Principle applies whenever there is small, fast
  access memory and large, slow memory
• Main memory cache of hard disk
• Hard disk cache of CD-ROM
• Proxy server cache of web sites
• Non-computer examples?

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Subsystem 3I/O Devices

• Input
• Keyboard
• Mouse
• Hard Disk
• Floppy Disk
• What else?

• Output
• Printer
• Screen
• Speakers
• What else?

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I/O Bus

• One set of wires connect all devices and CPU
• One device at a time transmits
• Transport of information is shared (public)
• Hence called a bus (public transportation for
  bits)
• Bus characteristics
• Clock speed
• of bits (width)
• (Form factor)

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I/O Interrupts

• Certain events cause interrupt signals
• E.g., keypress
• CPU stops what it is doing
• CPU starts executing a program to handle the
  interrupt
• E.g., passes keypress to an application, which
  displays character on screen
• CPU goes back to what it was doing before

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Conclusion

• Real computers consist of three interacting
  subsystems.
• CPU
• Memory
• I/O
• The performance and capabilities of a computer
  are dictated by the interaction of these
  resources.
• Next time Operating Systems