X86 Computer Bus Subsystems IBM PC/XT Architecture ('82, '83)

X86 Computer Bus Subsystems
EXPANSION BUSSES – Allow Addition of System Devices “Adapters”
ISA - Industry Standard Architecture (8,16-bit D-Bus)
MCA - Microchannel Architecture (32-bit D-Bus)
EISA - Extended ISA (32-bit D-Bus)
LOCAL BUSSES – Connect Adapters Directly to x86 System Bus
VLB - VESA Local Bus
PCI - Peripheral Component Interconnect
EISA - Advanced Graphics Port
PERIPHERAL BUSSES – Dedicated to Peripheral Communication
SCSI - Small Computer System Interface (parallel)
USB - Universal Serial Bus (serial)
IBM PC/XT Architecture (’82, ’83)
(XT is “extended” PC – 4.77 MHz Bus)
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ISA (8-bit Version)
(Typical 4.77 MHz Bus – IBM PC, IBM XT)
IBM AT Architecture (’84)
(AT is “advanced technology” – 6 MHz)
ISA “Standard” was Modified to 16-bit Bus
2

ISA (16-bit Version)
(aka The “AT Bus”)
AT Bus Standard
Due to “clones” became so common it is a “standard”
• 16-bit Data Bus
• 24-bit Address Bus
• 11 Interrupt Request Lines
– IRQ2-7, IRQ10-12, IRQ14, IRQ15
• 7 DMA Channels
• 8.33 MHz Max. Bus Frequency
3

Plug and Play
– Each New Adapter May Require:
• DMA Channels
• IRQ Lines
• Parallel I/O Addresses
– Adding New Adapter to ISA Bus Can Cause Conflicts
1) User Must MANUALLY Change DIP Switch
2) Automatic Configuration
– “Plug and Play”
• “Standard” Developed by Intel/Microsoft
• Adapters Contain Special Registers for OS Query
• Adapters Accept Commands to Change Values or Deselect
• Need a PnP Aware ROM-BIOS (sequence during boot-time)
Microchannel Architecture (MCA)
Proprietary IBM Adapter Bus - 1987
• 8.33 MHz, 16-bit AT Bus Causes Sever I/O Bottleneck
– Lots’ of Graphics versus Character Mode Data
– 25 MHz 80386DX Available – CPU BW Far Exceeds Bus BW
• MCA – 10 MHz at Selectable 8, 16, 32 bits
• Totally Incompatible with ISA
• IBM Proprietary – Must License to Use
– Pay IBM $$$$ to Make Adapters
• 9 Popular Computer Manufacturers Form Consortium
– “Gang of Nine” – EISA is Born!
• MCA is no longer popularly used
4

EISA
• Compatible With ISA and an OPEN Standard
• 32-bit Address and Data Bus Width
• Supports Automatic Adapter Configuration
– CFG (driver file) with each Adapter
– Writes Special Information in CMOS RAM for boot time
• 15 IRQs Supported
• 7 (32-bit) DMA Channels Supported
• 8.33 MHz Bus Speed
EISA Structure
5

EISA
ISA ISA EISA
EISA Pinout
Local Busses – VLB, PCI, AGP
• EISA BW Still Too Slow For Video
• Need Direct Access to x86 System Bus
– for VRAM write/read
• VESA Developed VLB (1992)
– designed to operate “with” ISA or EISA or MCA
– use IRQ and DMA support of Adapter Bus
– VLB portion operates at system bus speed (50 or 66 MHz)
– no automatic configuration support
– x86 only bus
• PCI
– developed with ISA Compatibility in mind
– New Bus – Extension in the Sense of Compatibility
– Requires a “Bridge” Circuit
• AGP
– Advanced Graphics Port
– Extension to PCI Standard 66MHz
6

PCI Based Computer
PCI Characteristics
• 0-33 MHz (0 MHz Important for Laptops)
• 32-bit OR 64-bit Data Bus
• Burst Mode Support
– Northbridge Transforms Mult. Sys. Req. into PCI Burst
• Bus Master Support
– Similar to DMAC Capability
• Sharable IRQs
– ISRs (drivers) Chained Together
– Each Checks to Determine Which Should Execute
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PCI Pinout
I/O (Peripheral) Busses
• NOT System Bus Adapters
• SCSI (Small Computer System Interface)
- high speed, parallel
- HD, Tape Drive, CD-ROM, etc.
• USB (Universal Serial Bus)
- medium/low speed, serial
- mice, keyboard, modems
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SCSI (“scuzzy”)
• Many Versions – Original:
– 8-bit Data
– 1-bit Parity
– 9-bit Control
• Intelligent – SCSI Controller Accepts High Level
Commands
– e.g. HD can backup to Tape Drive while CPU does “other things”
• Daisy Chained with Device IDs
• Must Terminate – Prevent Reflections
USB
• 1.5 Mb/s and 12 Mb/s Data Rates
• Simplifies Task of Connecting Peripherals
• Uses Notions of “Nodes” and “Hubs”
• Devices Connect to Nodes present at a Hub
• All Hubs Daisy-Chained to “Host Root Hub”
• Host Allows upto 127 Different USB Devices
9

USB
Typical PCI Based x86 Computer Architecture
L3
Cache
AGP SDRAM
PCI Adapter 1 PCI Adapter 2
XD Bus 8-bit
EIDE 1
EIDE 2
Flash Mem BIOS RTC and CMOS
System Bus 64-bit
North Bridge
PCI Bus 32-bit
South
Bridge
L2
Cache
L1
Cache
ISA Bus 16-bit
x86
CPU
Common Package
PCI Adapter 3 PCI Adapter 4
USB Root Hub 1
USB Root Hub 2
KBD, PS/2, LPT,
UART1, UART2, Floppy
ISA Adapter 1 ISA Adapter 2
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