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Lecture 2: IBM PC/AT and ISA Bus. 80386- based PC/AT-Compatible System. DRAM. 82345 Data Buffer. 386 DX+82340 chip set. 386 DX. SYSTEM BUS. LOCAL BUS. 82346 System Controller. 82341 Peripheral Combo. 387 DX. 82344 ISA Controller. 82385 DX Cache Controller. Cache. BIOS.

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80386 based pc at compatible system
80386-based PC/AT-Compatible System

DRAM

82345

Data Buffer

386DX+82340 chip set

386DX

SYSTEM BUS

LOCAL BUS

82346

System

Controller

82341

Peripheral

Combo

387 DX

82344

ISA

Controller

82385DX

Cache

Controller

Cache

BIOS

Industry Standard Architecture(ISA) Bus

pentium processor 82430 pciset isa
Pentium Processor/82430 PCIset ISA

Host Bus

Pentium Processor

CNTL

ADDR

DATA

Latch

82434

PCMC

82433

LBX

DRAM

SRAM

PCI BUS

CNTL

ADDR/DATA

82378 SIO

Grahics

PCI devices

ISA BUS

block diagram of the system board
Block Diagram of the System Board
  • All signal lines are TTL compatible. Fan-out are two low power Shottkey(LS) TTLs.
  • SA0 through SA19: System Address Bus:(I/O)
    • to address memory and I/O devices; 16MB of memory with LA17 through LA23
    • input when CPUHLDA is high and MASTER* is low; output at all other times
    • SA bus driven by CPU when CPUHLDA is low; SA bus driven by 8237 DMA controller when CPUHLDA is high
    • latched with an internally generated ALE signal

External

Master

data

CPU

addr

ISA Bus

Memory

addr

data

MEM

DMA

I/O

slide7
LA17 through LA23 (Latchable Address Bus); I/O
    • the same as SA19-SA0
  • MEMR* (Memory Read, active low); I/O
    • Input when CPUHLDA is high and MASTER* is low
    • it is driven from the 288 bus controller when CPUHLDA is low and MASTER* is high
    • it is driven by the 8237 DMA controller when CPUHLDA is high and MASTER* is high
    • requires an external 10K pull-up registor
  • MEMW*(Memory Write, active low): I/O
    • Input/output determination: the same as MEMR*
    • requires an external 10K pull-up registor
  • SMEMR*(Memory Read): I/O
    • Input/Output determination: the same as MEMR*
    • active on memory read cycles to addresses below 1 MB.
    • requires an external 10KW pull-up registor
  • SMEMW*(Memory Write):I/O
    • Input/Output determination: the same as MEMR*
    • active on memory read cycles to addresses below 1 MB.
    • requires an external 10K pull-up registor
  • SBHE*(System Byte High Enable) : I/O
    • controlled the same way as the SA bus
slide8
REFRESH*(Refresh signal); I/O
  • SYSCLK(System CLock) : O
    • this output is half the frequency of the BUSCLK input
    • BALE, IOR*, IOW*, MEMR*, MEMW* are synchronized to SYSCLK
  • OSC(Oscillator): I-TTL
    • the buffered inout of the external 14.318 MHz oscillator.
  • RSTDRV(Reset Drive): O
  • BALE(Buffered Address Latch Enable): O
    • A pulse which is generated at the beginning of any bus cycle initiated from the CPU.
  • AEN (Address Enable): O
    • goes high anytime the inputs CPUHLDA and MASTER* are both high
    • DMA controller has control when this signal is active
  • T/C (Terminal Count): O
    • indicates that one of the DMA channels terminal count has been reached
  • DACK7*- DACK5*, DACK3*- DACK0* (DMA Acknowledge): O
  • DRQ7-DRQ5, DRQ3-DRQ0 (DMA Request) : I
    • DRQ0-DRQ3 : from 8-bit I/O adapters to/from system memory
    • DRQ5-DRQ7: from 16-bit I/O to/from system memory
    • DRQ4 is not available externally as it is used to cascade the two DMA controllers together.
slide9
IRQ15-IRQ9, IRQ7-IRQ3, IRQ1 (Interrupt Request) : I
    • inputs for the 8259 megacells
    • IRQ0, IRQ2, IRQ8 ; not available as external inputs
  • MASTER* (Master) : I
    • used by an external device to disable the internal DMA controllers and get access to the system bus
    • when asserted it indicates that an external bus master has control of the bus.
  • MEMCS16* (Memory Chip Select 16-bit) : I
    • used to determine when a 16-bit to 8-bit conversion is needed for CPU addresses
    • A 16 to 8 conversion is done anytime the System Controller requests a 16-bit memory cycle and MASTER* is sampled high.
  • IOCS16* (I/O Chip Select 16-bit) : I
    • functions the same way as MEMCS16* signals
  • IOCHK* (I/O Channel Check): I
    • used to indicate that an error has taken place on the I/O bus
  • IOCHRDY (I/O Channel Ready) : I
    • pulled low in order to extend the read or write cycles of any bus access when required
    • the default number of wait states for cycles initiated by the CPU;
      • four wait states for 8-bit peripherals
      • one wait state for 16-bit peripherals
      • three wait states for ROM cycles
    • One wait state is inserted as the default for all DMA cycles
slide10
WS0* (Wait State 0) : I
    • pulled low by a peripheral on the bus to terminate a CPU controlled bus cycle earlier than the default values