A history of PC buses

International Test Instruments CorporationInternational Test Instruments Corporation

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A history of PC busesA history of PC buses

From ISA to PCI ExpressFrom ISA to PCI Express


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ISA (Industry Standard Architecture) - ISA (Industry Standard Architecture) - 1981 1981 Originated in the original IBM PC (8-bit / 4.77 MHz = 4 MB/s).Originated in the original IBM PC (8-bit / 4.77 MHz = 4 MB/s). The ISA standard is based on PC/AT (16-bit / 8 MHz = 16 MB/s) – 1984The ISA standard is based on PC/AT (16-bit / 8 MHz = 16 MB/s) – 1984 62 pins (8-bit PC ISA) or 98 pins (16-bit PC/AT ISA).62 pins (8-bit PC ISA) or 98 pins (16-bit PC/AT ISA). The ISA bus supported 1MB (PC/XT) / 16 MB (PC/AT) Memory and 64K The ISA bus supported 1MB (PC/XT) / 16 MB (PC/AT) Memory and 64K

I/O address spaces.I/O address spaces. Lacked bandwidth for graphical user interfaces (MS Windows).Lacked bandwidth for graphical user interfaces (MS Windows).

– VL Bus (VESA Local Bus) gave some bandwidth respite on newer PC VL Bus (VESA Local Bus) gave some bandwidth respite on newer PC motherboards but was married to the 486/33 CPU bus.motherboards but was married to the 486/33 CPU bus.

– VL Bus no longer worked when Pentium came out (60 MHz +) due to different VL Bus no longer worked when Pentium came out (60 MHz +) due to different CPU bus protocol.CPU bus protocol.

Later, IBM created MCA as future-proof, CPU-independent alternative.Later, IBM created MCA as future-proof, CPU-independent alternative.


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ISA (Industry Standard Architecture) – ISA (Industry Standard Architecture) – 1981 (cont.)1981 (cont.) 8-bit and 16-bit ISA slots8-bit and 16-bit ISA slots


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ISA (Industry Standard Architecture) – ISA (Industry Standard Architecture) – 1981 (cont.)1981 (cont.) 16-bit ISA slot signals (PC/AT)16-bit ISA slot signals (PC/AT)


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MCA (Micro Channel Architecture) – 1987 MCA (Micro Channel Architecture) – 1987

Created by IBM to address deficiencies of the ISA bus (CPU-Created by IBM to address deficiencies of the ISA bus (CPU-dependence and lack of performance).dependence and lack of performance).

Used by IBM PS/2 computers.Used by IBM PS/2 computers. 16/32 bits, 10 MHz, 20/40 MB/s16/32 bits, 10 MHz, 20/40 MB/s Software-based configuration (i.e. no IRQ, DMA jumpers).Software-based configuration (i.e. no IRQ, DMA jumpers). Was not an open standard so never caught on with PC Was not an open standard so never caught on with PC

manufacturers.manufacturers. Not backwards compatible with existing ISA expansion cards.Not backwards compatible with existing ISA expansion cards. Was used by advanced server platforms due to high overall Was used by advanced server platforms due to high overall

performance.performance.


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MCA (Micro Channel Architecture) – 1987 MCA (Micro Channel Architecture) – 1987 (cont.)(cont.) 32-bit MCA slots. Connector later reused by PCI.32-bit MCA slots. Connector later reused by PCI.


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EISA (Extended Industry Standard EISA (Extended Industry Standard Architecture) - 1988 Architecture) - 1988 Created by 3Created by 3rdrd-party PC clone vendors. -party PC clone vendors. 32-bit / 8 MHz = 32 MB/s.32-bit / 8 MHz = 32 MB/s. Software-based configuration (i.e. no IRQ, DMA jumpers).Software-based configuration (i.e. no IRQ, DMA jumpers). EISA slots accepted ISA cards but EISA cards would not work in ISA EISA slots accepted ISA cards but EISA cards would not work in ISA

slots.slots. Was Was nevernever popular because ISA + VLB carried the PC platform until popular because ISA + VLB carried the PC platform until

PCI came along.PCI came along. Did see use in higher-performance server systems.Did see use in higher-performance server systems.


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EISA (Extended Industry Standard EISA (Extended Industry Standard Architecture) – 1988 (cont.)Architecture) – 1988 (cont.)

EISA slots – Accepts ISA cards too.EISA slots – Accepts ISA cards too.


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EISA (Extended Industry Standard EISA (Extended Industry Standard Architecture) – 1988 (cont.)Architecture) – 1988 (cont.) EISA card – Does not fit in ISA slot.EISA card – Does not fit in ISA slot.


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VLB (VESA Local Bus ) – 1992VLB (VESA Local Bus ) – 1992

32-bit, 33 MHz = 133 MB/s.32-bit, 33 MHz = 133 MB/s. Created by Video Electronics Standards Association (VESA) for attaching Created by Video Electronics Standards Association (VESA) for attaching

high-performance graphics cards to ISA, and later PCI, motherboards.high-performance graphics cards to ISA, and later PCI, motherboards. Custom slot that extends the 16-bit ISA slot (see next slide).Custom slot that extends the 16-bit ISA slot (see next slide). Directly tied to the 486/33 memory bus – Doomed for other CPUs due to Directly tied to the 486/33 memory bus – Doomed for other CPUs due to

different CPU memory bus protocol.different CPU memory bus protocol. The CPU memory bus only allowed one or two VLB card loads (due to The CPU memory bus only allowed one or two VLB card loads (due to

loading and SI issues).loading and SI issues). Difficult implementation, especially for 40/50 MHz 486 versions (instability Difficult implementation, especially for 40/50 MHz 486 versions (instability

common).common). 486DX2-66 and -100 used a 33 MHz front-side bus (x2/x3 internal clock) so 486DX2-66 and -100 used a 33 MHz front-side bus (x2/x3 internal clock) so

compatible with VLB.compatible with VLB.


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VLB (VESA Local Bus ) – 1992 (cont.)VLB (VESA Local Bus ) – 1992 (cont.)

32-bit VLB slot on ISA motherboard.32-bit VLB slot on ISA motherboard.


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VLB (VESA Local Bus ) – 1992 (cont.)VLB (VESA Local Bus ) – 1992 (cont.)

32-bit VLB Graphics Card32-bit VLB Graphics Card


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PCI (Peripheral Component Interconnect) PCI (Peripheral Component Interconnect) – 1992– 1992 Created by Intel (later PCI-SIG) to bring together best ideas Created by Intel (later PCI-SIG) to bring together best ideas

from prior bus architectures.from prior bus architectures. 32-bit, 33 MHz = 133 MB/s32-bit, 33 MHz = 133 MB/s Also supported 64-bit (while not commonly implemented) = Also supported 64-bit (while not commonly implemented) =

266 MB/s266 MB/s Full plug-and-play (cards report Memory, IRQ, I/O requirements Full plug-and-play (cards report Memory, IRQ, I/O requirements

– device drivers and operating system make sure the cards get – device drivers and operating system make sure the cards get the resources they need and that no resource clashes occur).the resources they need and that no resource clashes occur).

Growing used PCI bandwidth eventually necessitated separate Growing used PCI bandwidth eventually necessitated separate AGP graphics card bus.AGP graphics card bus.


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PCI (Peripheral Component Interconnect) PCI (Peripheral Component Interconnect) – 1992 (cont.)– 1992 (cont.) 4x32-bit PCI slots + 4x16-bit ISA slots.4x32-bit PCI slots + 4x16-bit ISA slots.


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PCI (Peripheral Component Interconnect) PCI (Peripheral Component Interconnect) – 1992 (cont.)– 1992 (cont.) PCI bus now decoupled from CPU bus.PCI bus now decoupled from CPU bus.


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PCI (Peripheral Component Interconnect) PCI (Peripheral Component Interconnect) – 1992 (cont.)– 1992 (cont.) PCI Bus interface signals (32/64 bit data).PCI Bus interface signals (32/64 bit data).


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PCI (Peripheral Component Interconnect) PCI (Peripheral Component Interconnect) – 1992 (cont.)– 1992 (cont.) Configuration Space Registers & PnPConfiguration Space Registers & PnP


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PCI-X (Peripheral Component PCI-X (Peripheral Component Interconnect - Extended) - 1998Interconnect - Extended) - 1998 32/64-bit, 66/133/266/533 MHz (up to 4,266 MB/s).32/64-bit, 66/133/266/533 MHz (up to 4,266 MB/s). PCI-SIG (IBM, Compaq, HP). Note: Intel wanted a better option.PCI-SIG (IBM, Compaq, HP). Note: Intel wanted a better option. Improved protocol over PCI (Split-Response Transactions, MSI signals Improved protocol over PCI (Split-Response Transactions, MSI signals

interrupts via memory writes in host PCI bridge rather than dedicated interrupts via memory writes in host PCI bridge rather than dedicated INTx interrupt lines.).INTx interrupt lines.).

Due to higher cost, normally only server PCs used 64-bit PCI-X.Due to higher cost, normally only server PCs used 64-bit PCI-X. Desktop systems migrated directly from PCI to PCI Express, bypassing Desktop systems migrated directly from PCI to PCI Express, bypassing

PCI-X.PCI-X. Wide buses resulted in difficult length matching of the bus signals (in Wide buses resulted in difficult length matching of the bus signals (in

worst case, violating Tsu/Th).worst case, violating Tsu/Th). PCI-X reached practical frequency limit due to loading and skew of the PCI-X reached practical frequency limit due to loading and skew of the

wide buseswide buses The wide buses tied up most I/O pins on the chipsets, limiting overall chip The wide buses tied up most I/O pins on the chipsets, limiting overall chip

functionality.functionality.


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PCI-X (Peripheral Component Interconnect - PCI-X (Peripheral Component Interconnect - Extended) – 1998 (cont.)Extended) – 1998 (cont.)


2020

PCI-X (Peripheral Component PCI-X (Peripheral Component Interconnect - Extended) – 1998 (cont.)Interconnect - Extended) – 1998 (cont.) Fast and wide buses result in Tsu/Th timing skew between signals within Fast and wide buses result in Tsu/Th timing skew between signals within

the buses.the buses.– If the skew is too high, the interface will break!If the skew is too high, the interface will break!


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PCI Express – 2004 PCI Express – 2004

Actually not a bus but a point-to-point link.Actually not a bus but a point-to-point link. High-speed bi-directional serial link (2.5 / 5.0 / 8.0 Gbps per High-speed bi-directional serial link (2.5 / 5.0 / 8.0 Gbps per

lane, 1 to 32 lanes).lane, 1 to 32 lanes). Clock 8b/10b encoded within serial data stream.Clock 8b/10b encoded within serial data stream. Maintains software backwards compatibility of Configuration Maintains software backwards compatibility of Configuration

Space registers (Plug-and-Play).Space registers (Plug-and-Play). Also software backwards compatible with regards to I/O and Also software backwards compatible with regards to I/O and

Memory-mapped device registers.Memory-mapped device registers. No length matching between lanes needed (separate lane-to-No length matching between lanes needed (separate lane-to-

lane de-skew built into receiver).lane de-skew built into receiver).


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PCI Express – 2004 (cont.)PCI Express – 2004 (cont.)

PCI Express vs. PCI slots.PCI Express vs. PCI slots.


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PCI Express bandwidth comparison.PCI Express bandwidth comparison.


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PCI Express 1.0a/1.1: 250 MB/s per lane (max 8 GB/s for 32 Lanes) – PCI Express 1.0a/1.1: 250 MB/s per lane (max 8 GB/s for 32 Lanes) – 2003/2005.2003/2005.

PCI Express 2.0: 500 MB/s per lane (max 16 GB/s for 32 Lanes) - 2007.PCI Express 2.0: 500 MB/s per lane (max 16 GB/s for 32 Lanes) - 2007. PCI Express 3.0: 800 MB/s per lane (max 26 GB/s for 32 Lanes) - 2010.PCI Express 3.0: 800 MB/s per lane (max 26 GB/s for 32 Lanes) - 2010. PCI Express 4.0: Again doubling transfer rate? Expected to be released PCI Express 4.0: Again doubling transfer rate? Expected to be released

2014/2015.2014/2015. Efficient and physically compact enables use for all platforms (mobile, Efficient and physically compact enables use for all platforms (mobile,

desktop, server).desktop, server). No longer need for separate AGP graphics bus slot (lots of available No longer need for separate AGP graphics bus slot (lots of available

bandwidth). bandwidth).


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A history of PC busesA history of PC buses

This completes the overview of the legacy PC buses.This completes the overview of the legacy PC buses. For a continued description of the PCI Express bus For a continued description of the PCI Express bus

architecture, see the presentation “PCI Express 101”.architecture, see the presentation “PCI Express 101”.

A history of PC buses

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