Personal Computer Memory and Buses

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Personal Computer Memory and Buses

• Semester 1, Week 3, Lect 1

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Memory

• Computer memory refers to computer components,
  devices and recording media that retain data
  (storage of data) for some interval of time.
• The amount of time might be a nanosecond to many
  years.
• Computer storage is one of the core functions of
  the modern computer and is the one that makes
  computers so useful.
• The Central Processing Unit (CPU) and storage,
  together, make up the basic Von Neumann computer
  model or architecture.

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Memory (2)

• Memory types in a hardware system
• Registers
• Cache (Level 1 and Level 2)
• RAM (Random Access Memory) - main memory
• ROM (Read Only Memory)
• Also
• Flash Memory
• Hard disk ('C Drive')
• CD - CD ROM or DVD

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Internal Memory Diagram

• Internal memory block diagram with external hard
  disk

CPU
Data bus
Address bus
Chipset
BIOS
I/O bus
Memory
Hard disk
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Registers

• In a computer, a register is one of a small set
  of holding places that are part of a central
  processing unit (CPU).
• A register may hold a software instruction, a
  storage address or other data, such as single
  characters or a bit sequence for a symbol, pixel
  anything.

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Registers (2)

• In a 32-bit instruction computer a register must
  be 32 bits in length to hold a complete
  instruction.
• Some chip designs allow for half-registers, for
  shorter instructions.
• Depending on CPU design and systems software
  language, registers may be numbered or have
  specific names.

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Registers (3)

• So, the processor uses registers as high-speed
  working storage areas - they store no more than a
  few bytes.
• The operating system needs time to control the
  hardware so registers are needed to facilitate
  the processing and movement of data and
  instructions between
• RAM,
• the control unit and
• the arithmetic-logic unit (ALU).

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Registers (4)

• Two types of register are
• Control and Status registers
• User-visible registers
• There are other register types in or with
  components on a motherboard their names and
  types vary with different hardware systems. E.g.
  registers for a Celeron processor will not be
  identical in quantity and type to an Intel
  Pentium

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Control and Status Registers

• This type of register is generally not available
  to user programs.
• Some are used by the CPU to control its
  operation(s) and some are used by the operating
  system to control program execution.

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A figure showing registers ON a processor
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Control and Status Registers (3)

• Examples of Control and Status registers
• Program counter (PC) which contains the address
  of the next instruction to be fetched.
• Instruction register (IR) which contains the
  instruction most recently fetched.
• Program status word a register or group of
  registers containing
• Condition codes and status information bits,
• Interrupt enable/disable bit,
• Supervisor operating system/user mode bit.

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User-Visible Registers

• User-visible registers are available to operating
  system and user programs.
• They hold data, addresses and some condition
  codes associated with program instructions.

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User-Visible Registers (2)

• Examples of user-visible registers
• Data registers, which can be assigned by the user
  program to perform operations on data.
• Address registers, which contain memory addresses
  of data and instructions. They may contain a
  portion of the address that is used to calculate
  the complete address.

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32 Registers for Instructions

• Thirty two registers
• Registers are denoted in the literature as ri,
  where
• 0 lt i lt 31.
• So there are thirty-two of them.
• Registers r0 - r3 and r28 - r31 are special
  registers and are used for control and status.
  Registers r4 - r27 can be said to be 'user
  visible'.

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Cache Memory

• Cache memory is Random Access Memory (RAM) that a
  computer CPU can access more quickly than it can
  access 'main memory' RAM.
• As the microprocessor processes data, it looks
  first in the cache memory and if it finds data
  there, where it was placed from a previous
  reading of data, the CPU saves time in not having
  to transfer data from RAM.

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Cache Memory (2)

• Cache memory is sometimes described in levels of
  closeness and accessibility to the CPU. A Level 1
  (L1) cache is on the same chip as the CPU and
  can, typically, store 32 kilobytes.
• Level 2 (L2 cache) is usually a Static RAM (SRAM)
  chip, near to the CPU but separate from it. The
  CPU can access the L2 if the L1 happens to be
  empty, still saving the transfer from RAM.

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RAM (Random Access Memory) or Main Memory

• RAM Random Access Memory serves as a
  short-term memory for the computer. (The hard
  disk is usually described as the long-term memory
  for the computer.)
• While the computer is in use, the application
  program instructions and data pass continuously
  through the processor.
• They are stored, most immediately and for a short
  time, in RAM.

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RAM (Random Access Memory) or Main Memory (2)

• RAM also stores many operating system
  instructions during this same time.
• The main RAM is usually a Dynamic RAM (DRAM)
  chip.
• DRAM is 'dynamic', unlike Static RAM that is used
  in the cache. DRAM needs to have its storage
  cells refreshed with electrical charge every few
  milliseconds.

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RAM (Random Access Memory) or Main Memory (3)

• Static RAM does not need refreshing because it
  uses continually moving current, switched in one
  of two directions. DRAM cells hold a charge in
  place. Static RAM can be accessed more quickly
  than DRAM.

A photo of RAM
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ROM (Read Only Memory)

• ROM is 'built-in' computer memory containing data
  and instructions that normally can only be read,
  not written to.
• ROM code holds start-up instructions - i.e. ROM
  contains the programming that allows your
  computer to be "booted up" or regenerated each
  time you turn it on.
• ROM chips contain instructions which are specific
  for that particular motherboard. Those programs
  and data will remain in the PC throughout its
  life usually they are not altered.

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ROM (Read Only Memory) (2)

• Unlike RAM, the data in ROM is not lost when the
  computer power is turned off.
• The ROM is sustained by a small long-life battery
  in the computer.

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Personal Computer Buses

• A bus is a common pathway across which data can
  travel within a computer. This pathway can be
  established between two or more computer
  elements.
• These pathways are on the computer's motherboard,
  interconnecting the microprocessor with
  attachments to the motherboard in expansion slots
  - such as the hard disk drive, CD-ROM drive,
  graphics card, sound card

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Personal Computer Buses (2)

• There are a number of types of buses, including
  the following
• System or I/O bus (Input/Output bus)
• Processor bus
• Memory bus
• Address bus

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The System Bus

• The system bus, also known as the I/O bus or the
  expansion slot bus is the one over which most
  data flows.
• Any signal that goes to or from any device such
  as a video system, disk drives, and printer -
  travels over this bus.
• The busiest I/O pathway is, typically, to and
  from a video card.

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Types of System or I/O Buses

• Many I/O buses are introduced to give better
  system performance for
• Faster CPUs
• Increasing software demands
• Greater multimedia requirements.

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System (I/O) Bus

• Many modern PC systems still incorporate the same
  basic bus architecture as the old IBM PC/AT of _at_
  1984 - but also include a second high-speed local
  I/O bus such as
• VL-Bus (VESA Local Bus, popular in mid 1990s) or
• PCI (Peripheral Component Interconnect- a more
  modern architecture),
• which offer much greater performance for adapters
  that need it.

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System (I/O) Bus (2)

• You can identify different types of I/O buses by
  their architecture. E.g.
• ISA (Industry Standard Architecture),
• PCI Local Bus (Peripheral Component
  Interconnect),
• FireWire,
• USB (Universal Serial Bus).

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System (I/O) Bus (3)
CPU
Video
Memory
I/O
System Bus
FIG. 1 ISA (Industry Standard Architecture)
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System (I/O) Bus (4)

• Each bus architecture is implemented by a chipset
  that is connected to the processor bus.
  Typically, this chipset also controls the memory.

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Processor Bus

• The processor bus is the communication pathway
  between the CPU and immediate support chips.
• Support chips Chipsets.
• This bus is used to transfer data between the CPU
  and the main system bus, for example, or between
  the CPU and an external memory cache.

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Processor Bus (2)

• Figure 2 shows how this bus fits into a typical
  PC system.

FIG. 2  The processor bus
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Processor Bus (3)

• The processor bus is required
• to get information to/from the CPU at fastest
  possible speed -
• so this bus operates at a much faster rate than
  any other bus on a PC.
• The processor bus consists of electrical circuits
  for data, for addresses (the address bus), and
  for control purposes. The same control as the
  CPUs Control Unit.

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Processor Bus (4)

• In a Pentium-based system, the processor bus has
  64 data lines, 32 address lines, and associated
  control lines.
• The processor bus operates at the same base clock
  rate as the CPU does externally. The processor
  bus is tied to the external processor pin
  connections and can transfer one bit of data per
  data line every one or two clock cycles.
• Thus, for example, a Pentium, Pentium Pro, or
  Pentium II can transfer 64 bits of data at a time.

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Memory Bus

• The memory bus is used to transfer information
  between the CPU and main memory - the RAM in a
  personal computer system.
• A memory bus may be part of the processor bus
  itself, but in most cases is implemented
  separately by a dedicated chipset for
  transferring information between the processor
  bus and the memory bus.

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Memory Bus (2)

• Note
• In virtually all systems that are 16MHz or
  faster, there will be a special memory controller
  chipset that controls the interface between the
  faster processor bus and the slower main memory.
• This chipset typically is the same chipset that
  is responsible for managing the I/O bus.

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Memory Bus (3)

• Figure 3 shows how the memory bus fits into your
  PC.

FIG. 3  The memory bus
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Memory Bus (4)

• Information on the memory bus is transferred at a
  much slower rate than the information on the
  processor bus.
• The chip sockets (or the slots) for memory
  SIMMs/DIMMs are connected to the memory bus in
  much the same way that expansion slots are
  connected to the I/O bus.
• (SIMMS/DIMMS Single/Dual Inline Memory Modules)

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Address Bus

• This bus is actually a subset of the processor
  and memory buses.
• Since a system bus often consists of 64 data
  lines, 32 (or 36) address lines, and a few
  control lines, these address lines constitute the
  address bus.
• In most block diagrams, this bus is actually
  considered a part of the processor and memory
  buses.

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Address Bus (2)

• The address bus is used to indicate what address
  in memory or what address on the system bus are
  to be used in a data transfer operation.
• The address bus indicates precisely where the
  next bus transfer or memory transfer will occur.
• The size of the memory bus also controls the
  amount of memory that the CPU can address
  directly.

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Programmed I/O

• The programmed input/output (PIO) interface was
  the original method used to transfer data between
  the CPU, through the ATA controller, and an ATA
  device.
• (ATA Advanced Technology Attachment is a
  standard interface for connecting storage devices
  such as hard disks and CD-ROM drives inside
  personal computers.)

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Programmed I/O (2)

• The Programmed I/O interface is grouped into
  different modes for different transfer rates.
• This was a slow means of data transfer around
  3.5 Megabytes per second.

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Memory Mapped I/O

• I/O addresses are like bi-directional post
  boxes in the computers memory device.
• When the information from a device needs to be
  placed into the PC it needs to have a place for
  the data.
• For this reason each device might have its own
  small area of memory to use. The name for this is
  memory-mapped I/O.

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Memory Mapped I/O (2)

• When the device has a byte of data to send it
  sits device's designated I/O address space.
• When the CPU is ready to process the data it gets
  it from this area.
• When it later wants to send data to or through
  the device it uses this address again (or another
  one near it). This is a simple way of dealing
  with information exchange between devices.

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Memory Mapped I/O (3)

• Memory-mapped I/O uses the same bus to address
  both memory and I/O devices.
• The CPU instructions used to read and write to
  memory are also used in accessing I/O devices.

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Memory Mapped I/O (4)

• So, instead of having "real" memory (RAM) as an
  address,for an I/O device the device has its own
  address space.
• An address can be placed an I/O device.
• Thus, communicating with an I/O device can be the
  same as reading and writing to memory addresses
  devoted to the I/O device.

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Web Sites

• Try these Web page links. If one or two do not
  open apologies
• http//home.cfl.rr.com/eaa/MemoryTypes.htm
• http//www.pcguide.com/ref/ram/types.htm
• http//computer.howstuffworks.com/ram.htm
• http//www.extremetech.com/article2/0,3973,92994,0
  0.asp

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Next

• Boolean Algebra
• (How the logic of computers can be represented by
  binary digits)