ISA BUS

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• ISA BUS
• (Industry Standard Architecture)

Cahit Tarik Genç
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What is BUS ?

• A bus connects all the internal computer
  components to the CPU and Main memory.
• Every bus has a clock speed measured in MHz. A
  fast bus allows data to be transferred faster,
  which makes applications run faster. On PCs, the
  old ISA bus is being replaced by faster buses
  such as PCI.

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I/O BUS

• System Bus Connecting to CPU, memory and Cache.
• Address Bus
• Data Bus
• Control Bus
• I/O Bus Connecting to the above three buses is
  the "good old" standard I/O bus, used for slower
  peripherals (mice, modems, regular sound cards,
  low-speed networking) and also for compatibility
  with older devices. On almost all modern PCs this
  is the Industry Standard Architecture (ISA) bus.

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Types of I/O Buses

• PCI Local bus
• AGP
• PC-Card (PCMCIA)
• USB
• Fire Wire (IEEE-1394)

• ISA
• Micro Channel (MCA)
• EISA
• VESA Local bus (VL-bus)

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I/O BUS

• ISA BUS (8-16-bit)
• EISA BUS (32-bit)
• PCI BUS (32 or 64-bit)
• AGP (32-bit)

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ISA BUS
In 1982 when ISA BUS appeared on the first PC the
8-bit ISA bus ran at a modest 4.77 MHZ the
same speed as Intel 8088. ISA BUS is extremely
slow by today's standards and not suited to the
use of a graphical operating system like Windows.

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ISA BUS

• In 1984 the IBM AT was introduced using the Intel
  80286 at this time the bus was doubled to 16
  bits (the 80286's data bus width) and increased
  to 8 MHz (the maximum speed of the original AT,
  which came in 6 MHz and 8 MHz versions and 24
  address lines).

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ISA BUS
8-bit
16-bit
9
ISA BUS
8-bit ISA BUS
16-bit ISA BUS
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ISA BUS

• is used with sound cards, disk drives or most
  network and video cards.

8-bit ISA(XT) CARD
16-bit ISA CARD
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ISA BUS
ISA Card
12
ISA BUS
Additional connections converts to 16 bit ISA
Original 8 bit ISA connectors
VESA connectors
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ISA BUS
In the figure you can see the pinouts of the ISA
BUS. The BUS is divided into two sides. The first
side pins are named A1 to A31 and it is the
components side. It consists of the address and
data buses. The second side pins are named B1 to
B31 and it is the solder side. This side contents
the power pins and the signals related to
interrupts and DMA transfers.
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ISA BUS
More DMA lines
More DMA lines
IRQ lines 8 - 15
5, 12V Power G
IRQ lines 2-7
clock
Extra 8 bits of I/O bus
Extra 4 bits of Address bus

• 20 bits of Address bus

8 bits of I/O bus
Original 8 bit ISA
16 bit
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ISA BUS

• A0-A19 (pins A31 to A12) This twenty lines are
  the address BUS.They can address 1MB (220
  bytes).
• D0-D7 (pins A9 to A2) The data BUS consist of
  this eight data lines.
• AEN (pin B11) It is used for the DMA controller
  to take over the data and address buses in a DMA
  transfer.
• GND (pins B1, B10, B31) Connected to the ground
  of the computer.
• 5V (pins B3, B29) 5V DC output of the power
  source.
• -5V (pin B5) -5V DC output.
• -12V (pin B7) -12V DC output.
• 12V (pin B9) 12V DC output.
• MEMW (pin B11) The µP asserts this signal when
  doing a write to the memory.
• MEMR (pin B12) The µP asserts this signal when
  doing a read from the memory.
• IOW (pin B13) The µP asserts this signal when
  doing a write to a port.
• IOR (pin B14) The µP asserts this signal when
  doing a read from a port.
• DACK0-DACK3 (pins B15, B17, B19 and B26) The DMA
  controller sets this signals to let a device know
• that the DMA has the control of the buses.
• DRQ1-DRQ3 (pins B6, B16 and B18) Allow the
  peripheral boards to request the use of the
  buses.
• T/C (pin B27) The DMA controller sets this
  signal to let the peripheral know that the
  programmed number of bytes has been sent.
• IRQ2-IRQ7 (pins B4, B21, B22, B23, B24 and B25)
  Interrupt signals. The peripheral devices sets
  this signals to request for the attention of the
  µP.ALE (pin 28) This signal is used for the µP
  to lock the 16 lower address BUS in a latch
  during a memory (or port) input/output
  operation.CLOCK (pin 20) Is the system
  clock.OSC (pin 30) Is a high frequency clock
  which can be used for the I/O boards.

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Describing the Read operation of the ISA

• CPU sends out a high on the ALE signal, then
  sends out the A0-A19 lines. On the address of the
  target port to be read will be latched. Then the
  BUS takes the -IOR signal to a low level. So that
  the addressed device will take a data byte to the
  D0-D7 data bus. The microprocessor will read then
  the data bus and take the -IOR signal to a high
  again.

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Describing the Read/Write operation of the ISA

• The only difference between a memory read/write
  cycle and a port read/write cycle is that in a
  memory cycle the -MEMR and -MEMW signals will be
  asserted, working the same way as -IOR and -IOW
  do.

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ISA BUS
Memory
A0 to A31
Address bus(32 bit)
Keyboard
I/O bus (16 bit data)
D0 to D31

• Intel
• 80386DX
• CPU

Storage
Data bus (32 bit)
Printer
RD WR IO/M
Video
Control bus
Modem
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Bus Structure of Intel 486
ISA BRIDGE
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Bus Structure of Intel Pentium
North Bridge
South Bridge
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Bus Structure of Intel Pentium
NORTH BRIDGE
CPU Bus
PCI Bus
FRAME
Expansion Bus Controller
M/IO (memory or i/o address)
TRDY
D/C (Data or code)
IRDY
W/R (Write or Read)
STOP
REQ
AEN
GNT
BRDY (Burst Ready)
AD310
A31-A3 (Address Lines)
C/BE30
BE7 - BE0 (Byte Enable Lines)
D31-D0
CLK
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Bus Structure of Intel Pentium
SOUTH BRIDGE
PCI Bus
ISA Bus
MEMR
FRAME
Expansion Bus Controller
MEMW
TRDY
ALE
IRDY
IOR
STOP
IOW
REQ
INTA
GNT
A23-A0
AD310
C/BE30
D23-D0
CLK
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ISA BUS INTERRUPT SYSTEM

• 16-bit ISA bus chains two 8259As together.
• IRQ 9 is used to re-route anything trying to use
  IRQ 2.
• Incorporated in chip set.

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INTERRUPT CONTROLLER
(IRQ 2)
8259A
80x86
8259A
IRQ0
IRQ0 (8)
IRQ1
IRQ1 (9)
IRQ2
IRQ2 (10)
IRQ3
INTR
IRQ3 (11)
IRQ4
IRQ4 (12)
IRQ5
IRQ5 (13)
IRQ6
IRQ6 (14)
IRQ7
IRQ7 (15)
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Elimination of ISA Bus

• ISA Bus is slow, hard to use and bulky.
• ISA plug in cards to be replaced by either PCI
  plug-in cards or USB add-on peripherals
• Limited number of interrupts.
• No central registry.

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Elimination of ISA Bus

• The ISA bus is limited to 24 bits of address.
  224 16 MBytes. It means that an ISA card that
  uses DMA cannot physically access memory beyond
  16 MBytes of RAM. This is a limitation of the ISA
  bus.

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Elimination of ISA Bus

• Motherboard gets 32-bit data from ISA BUS at two
  times. Meanwhile at this time ISA BUS declares
  wait state to the motherboard. Therefore ISA
  BUS may reduce System Performance.

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Elimination of ISA Bus

• If you use a ISA based controller card such as
  Ultra SCSI - 40Mb / sec or SCSI-2
  Fast - 10 Mb / sec , you can expect no more than
  an 8Mb per second transfer rate from your
  controller card.

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Elimination of ISA Bus

• PCI cards have Plug and Play technology and can
  configure themselves, so Operating Systems
  distinguish them.
• ISA cards are more cumbersome to install than
  other cards because I/O addresses, interrupts and
  clock speed must be set using jumpers and
  switches on the card itself.

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Elimination of ISA Bus