1 Jan 1998
The I/O Bus
A personal computer may transfer data from disk to CPU, from CPU to memory, or from memory to the display adapter. A PC cannot afford to have separate circuits between every pair of devices. A mechanical switch, like the old phone systems used, would be too slow.
The solution is a Bus. The Bus is simply a common set of wires that connect all the computer devices and chips together. Some of these wires are used to transmit data. Some send housekeeping signals, like the clock pulse. Some transmit a number (the "address") that identifies a particular device or memory location. The computer chips watch the address wires and respond when their identifying number is transmitted. They then transfer data on the other wires.
From the first IBM PC up through the first PS/2 computers (introduced in 1987) a computer had one bus and all of its devices and chips ran at the same speed. On those systems, additional computer memory was often added by plugging an adapter card into the same slots that held I/O adapters. Starting with machines that used the 386 CPU, the memory and CPU of the system ran faster than the I/O devices. The solution was to separate the CPU and memory from all the I/O. Today, memory is only added by plugging it into special sockets on the main computer board.
Whether there is more than one Bus, or one Bus with different speeds, is a matter of perspective. A car drives down the local streets at 25 miles per hour. Then it turns onto a highway ramp and accelerates to 55. Is there one road system, or two? The important thing is that there is a connection that allows a flow of traffic between the two speed zones. Within the PC, data can flow from any chip to any other chip, but different parts of the path run at different speeds.
Analogy Alert: Electricity flows through wire at the same speed everywhere (at about the speed of light). So a "faster" bus is not one where the electrons move faster, but rather one in which the time between meaningful events (the "clock speed") is faster. On a faster bus the chips have to react more quickly, and the engineering has to be more rigorous so that voltage signals can be clean and tight. If you built a road system the same way, the "speed limit" would increase by squeezing down the maximum length of each car and packing them more tightly bumper to bumper as they drove down the road.
In a modern PC, there may be a half dozen different Bus areas. There is certainly a "CPU area" that still contains the CPU, memory, and basic control logic. There is a "High Speed I/O Device" area that is either a VESA Local Bus (VLB) or an PCI Bus. An very low cost home computer may have no high speed devices. A more typical desktop system connects the high speed bus on the mainboard to the display adapter and IDE disk interface chip. Then one or two extra I/O slots may allow adapter cards to connect to the high speed bus. The remaining I/O device slots support standard "ISA" bus cards. Some computers will also provide sockets for a number of PCMCIA "credit card" adapters commonly found in laptop computers.
History
In 1984 IBM was shipping its PC AT model. The CPU, memory, and I/O bus all shared a common 8MHz clock. This became the basis for all subsequent clone computers. The term "AT" is a registered trademark of IBM, so this I/O bus became known as the ISA (Industry Standard Architecture) bus.
Every currently marketed PC supports some ISA interface slots. The bus and matching adapter cards are simple and cheap. ISA is a 16-bit interface, which means that data can be transferred only two bytes at a time. More importantly, the ISA bus runs at only 8 MHz and it typically requires two or three clock ticks to transfer those two bytes of data. This is not a problem for devices that are inherently slow like the COM port (modem), the printer port, the sound card, or the CD-ROM. However, the ISA bus is too slow for high performance disk access and therefore is not acceptable in Servers. It is also too slow for modern Windows display adapters.
In 1987 IBM introduced a new Microchannel (MCA) bus. It had clear advantages over the previous PC bus. It's 10 MHz clock was slightly faster. The cards could be automatically configured with a utility program instead of setting physical switches and jumpers. The bus can transfer four bytes of data at a time and, in some configurations and with some cards, it can transfer data every clock tick. However, the Microchannel itself was expensive, the adapter cards were more expensive, and the technology remained encumbered by IBM licensing.
The other vendors developed an extension of the older ISA interface called EISA. An EISA slot contained the older ISA interface, and then an extra socket with additional connections. The user could plug either an old ISA card or a new EISA card into the slot. The newer cards supported a 32-bit data interface and could therefore transfer four bytes of data per operation. However, to remain compatible with the old card, EISA still ran at 8MHz. And the extra logic pushed up the cost of both the EISA system and each adapter card.
As the 486 CPU chip became popular, the idea of running I/O devices at 8 or 10 MHz collided with a mainboard that ran everything else at 33 MHz. It was also clear that it should not require an extra $500 to transfer data at 32-bits.
The first solution was the VESA Local Bus (VLB), which became popular at the start of 1993. VESA is a consortium of companies making displays and display adapters. Desktop machines began to include one or two Local Bus slots to support a high speed video card and, perhaps, one other high speed device. A few vendors produced VESA SCSI adapter cards, or Local Bus LAN adapters. Nevertheless, VESA remained largely a display standard.
PCI - The Current Standard
The PCI bus was developed by Intel. Although it is mostly known for its CPUs, Intel also has a historical association with Ethernet, multimedia, and some disk interfaces. So Intel was unhappy with the VLB concentration on just the video interface and wanted to develop a general purpose bus. The objective was an interface that was fast and inexpensive. It did not have to be simple (advances in chip technology took care of that) and could achieve a low cost by high volume production.
PCI is a 64 bit interface in a 32 bit package. Figuring this out requires a bit of arithmetic. The PCI bus runs at 33 MHz and can transfer 32 bits of data (four bytes) every clock tick. That sounds like a 32-bit bus. However, a clock tick at 33 MHz is 30 nanoseconds, and memory only has a speed of 70 nanoseconds. When the CPU fetches data from RAM, it has to wait at least three clock ticks for the data. By transferring data every clock tick, the PCI bus can deliver the same throughput on a 32 bit interface that other parts of the machine deliver through a 64 bit path.
The PCI bus has all the signals of the old ISA bus. This allows a PCI adapter card to emulate older equipment. For example, a PCI disk controller can respond to the same addresses and generate the same interrupts as the older disk controllers that the BIOS understands. However, PCI devices can also be self-configuring and operate in a Plug and Play mode.
The PCI bus connects at one end to the CPU/memory bus and at the other end to a more traditional I/O bus. The PCI interface chip may support the video adapter, the EIDE disk controller chip, and maybe two external adapter cards. A desktop machine will have only one PCI chip, and so it will add a number of extra ISA only slots. A server may add additional PCI chips, and extra server slots will usually be EISA.
While ISA and EISA are exclusively PC interfaces, the PCI bus is now used in Power Macintosh systems and PowerPC machines. It may be attractive for minicomputers and other RISC workstations.
PC Card (formerly PCMCIA) and CardBus
Laptop computers typically have two slots for "credit card" adapters. Originally this interface was called "PCMCIA" but that proved too technical for wide acceptance. Today there is an effort to rename the interface as "PC Card."
A credit card adapter is much smaller than the adapter cards that plug into the ISA or PCI slots of a laptop computer. They are also more expensive and slower. Although PC Card slots have been offered as an option in some models of desktop computers, the clear disadvantage over full sized card restricts their use to laptops.
The credit card slots are bridged to the main I/O bus of the computer. Laptops have been built to use an ISA, Microchannel, or VESA Local Bus internally. The most modern laptops now come with an internal PCI bus. Credit card adapters can be connected to any of these systems.
A full sized card can come with switches or jumpers that can be used to configure its I/O address or IRQ. More advanced cards may be configured with a utility program. Since it is small project to remove the cover from a desktop system and switch cards, a desktop adapter is designed with the assumption that it will stay put.
A PC Card adapter is sealed in a metal case. It has no configuration switches. They are easy to insert and remove, since a user may need a LAN card for use in the office and a modem card for use at home or when traveling. The PCMCIA standard was developed late enough to incorporate an early version of "plug and play."
In the laptop, each socket is itself an I/O device. A PC Card adapter can be plugged into the system at any time, even when the power is on and a system is running. The socket can query the card for identifying information, and the adapter can be configured by the operating system to use available I/O addresses, IRQs and similar resources.
Full support for PCMCIA was too complicated (and required too much memory) to easily fit into the old DOS operating system. Laptops were not an important platform for Windows NT, so it has a very limited support for this architecture. The best PCMCIA support is found in Windows 95. It is possible to plug a new adapter card into a running Windows 95 machine, have the operating system recognize it immediately, and have the system dynamically configure new driver support.
The main problem with the PCMCIA bus is performance. Current systems support only a 16 bit interface to adapter cards. Adapter cards transfer data at a rather low clock speed, and there is no provision for Busmaster data transfer. The maximum data transfer rate from a PC Card adapter to the CPU or memory is only 2 megabytes per second. A PCI adapter, in contrast, can burst data at 133 megabytes per second.
A 32 bit version of PCMCIA has been created under the label "CardBus". It is currently available only on the most powerful and expensive laptop systems. If you are looking for desktop performance in a portable system, CardBus slots are highly desirable.
Current Status
- Although the ISA bus may be ten years old, it remains a perfectly reasonable option for devices that do not require highest performance. The consumer-oriented products all come with built-in video and disk adapters. Those are the two components that have the greatest impact on home computers and probably most desktop business machines. There simply isn't any need for a higher speed to support the sound card on a multimedia system, and the CD ROM is the slowest storage device available.
- The higher end of the consumer market and the machines sold directly to corporations typically have a few slots with a PCI interface. It is nice to have, but in the next year or so these slots may remain empty. The pre-installed devices cover most requirements, and storage expansion is simple and inexpensive using EIDE disks and devices.
- A Server comes in a full sized floor standing tower. Some servers have room for 18 disk drives. There will certainly be a PCI bus, but there will also be several standard SCSI adapters to connect all the disks, tapes, and CD-ROM units. Every adapter in a dedicated Server should be a PCI card. Some SCSI controller may be built into the mainboard. An external PCI SCSI adapter can provide additional function, cache memory, or RAID support. LAN adapters should plug into the PCI slot. Because the older ISA slots are much slower, there is a strong bias that any function that is not important enough to warrant a PCI adapter is something you probably shouldn't do on a server.
- Laptop computers are appearing with an internal PCI bus. This provides high speed support for the internal connection to the video and disk controller chips. It may also provide for PCI cards when the unit is connected to a docking station.
Interrupts
An I/O bus is similar to bus between the CPU, mainboard control logic, and memory. Both types of bus structure have address wires, data wires, and a similar set of housekeeping wires. Both bus structures must determine if an operation refers to memory or an I/O address. Both must distinguish between 8-bit, 16-bit, and 32-bit operations. Both must be able to introduce "Wait States" to slow down the CPU when a device needs more time to complete an operation.
The most important difference between the CPU-memory local bus and the I/O bus is the presence of Interrupt Request (IRQ) wires. The I/O bus has 15 separate IRQ wires. The CPU has only one interrupt pin. The chip set on the mainboard has to provide a translation between the two.
Without interrupts, the CPU must start an operation to a device and then spin in a loop asking, "Is it done yet? Is it done yet? Is it done yet?" After a few hundred thousand tests, the device will signal that the operation is complete. Interrupts allow the CPU (particularly on a more advanced operating system like Windows 95, OS/2, or NT) to do some other work until the operation is complete.
When a device generates an interrupt, the CPU hardware stops running an ordinary program and jumps to an interrupt handling routine in the Device Driver. The interrupt may signal that:
- A previous request is complete and the device can now start a new request
- Data has arrived that needs to be read (this happens mostly with the keyboard, mouse, modem, or LAN).
- an error has been detected on an idle device
Any device on the I/O bus can request an interrupt by placing a signal on one of the 15 IRQ wires. If more than one IRQ signal is received at the same time, the chip set on the mainboard has to select the one with highest priority to process first. The CPU is interrupted (by sending a signal on its one wire) and the chip set then transfer the identity of the IRQ level to be processed.
Each IRQ wire goes to every slot in the I/O bus. An adapter card is configured, physically with switches or logically with a utility) to use a specific IRQ value. The I/O bus on the first PC assumed that each device would have its own IRQ line, so the circuit to drive an interrupt request was made very simple. Unfortunately, when two adapter cards are incorrectly configured with the same IRQ value, and if both try to generate IRQ's at the same time, the result is to produce a short in the I/O bus. Usually there is no damage, but the effect can be to burn out either card or to trash the mainboard.
Later bus architectures (MCA, EISA, PCI) use safer circuitry that allows two devices to share the same interrupt. When an interrupt is shared, the system responds to an interrupt by calling the device driver for each device associated with that IRQ. The drivers poll their respective adapter cards to determine if there is any pending activity which requires a response. There is a slight loss of efficiency when interrupts are shared, but not enough to cause worry.
Plug and Play
An adapter card designed for Microchannel, EISA, or PCI can handle the IRQ problem automatically. However, the rest of the interface to advanced bus structures is expensive. The ISA bus interface is simple and cheap, but it requires the user to set the IRQ, either with physical switches on the card or through some kind of setup configuration utility. The solution to this problem is a new arrangement called "Plug and Play."
To use Plug and Play you need three things:
- The PC has to be ready to do Plug and Play. Most new computers have this ability, and most old ones don't.
- The adapter card has to be enabled for Plug and Play. It must minimally be able to report to the computer what I/O address and IRQ it is using or is able to use, and it must accept commands to use the values that the computer selects for it.
- The operating system must be able to support Plug and Play. Currently, this limits you to Windows 95. Windows NT 4.0 provides limited support, based presumably on the theory that anyone running NT better be sophisticated enough that IRQ issues are no problem. OS/2 Warp doesn't support it but Merlin may.
The ISA Bus is SLOW!
The CPU uses the OUT instruction to send data or commands to an I/O device, and it uses the IN instruction to read data or status from the device. These instructions cause the address of the I/O device to be placed on the bus, and they flag one of the housekeeping wires in the bus to indicate that this is an I/O address and not a memory address.
Each device is configured to respond to a range of addresses (the "ports"). Generally, a device will respond to a range of eight addresses. For example, the COM1 port responds to addresses 03F8 to 03FF. When the device sees an I/O address on the bus that matches a value in its range, it responds.
A device uses each address to process a different type of command or generate a different type of status. COM1, for example, uses the first address of 03F8 to handle all the data. The remaining four addresses are used to configure the line (speed, parity), to control the phone (hang-up, begin), to check modem status, and perform other housekeeping.
Unfortunately, it is still possible to plug a card built in 1984 into a modern PC. The I/O bus cannot know how fast a device is able to operate, so it handles the worst case and slows everything down to match the speeds used ten years ago. The chip set on the main board generates a long stream of Wait State signals, forcing the CPU to wait for 250 nanoseconds. The device itself can respond to request more Wait States to give itself even longer to respond. All this time, the CPU is stopped dead waiting for the IN or OUT instruction to end.
Direct Memory Access
All PC devices support Programmed I/O (PIO). The operating system executes IN and OUT instructions to read or write data one, two, or four bytes at a time to the device. For simple devices like the keyboard or display this may be perfectly reasonable. DOS and Windows 3.x do not support multitasking, so the CPU isn't doing anything useful while any device is busy.
A smarter device can transfer data directly to memory without the use of the CPU. This is called Direct Memory Access or DMA. Some DMA capability was designed into the first IBM PC. However, DMA dropped out of favor during the 1980's. The problem is that the original DMA design required additional bus cycles, and was therefore slower than Programmed I/O. As long as DOS was the primary operating system, it was better to disable or bypass DMA and let the CPU do the work.
One special case was the Multimedia Sound Card. A DOS or Windows game stores data representing some music or speech in a buffer. It then passes the sound card the address of the data and its length. The card uses DMA to fetch bytes of sound and play them while the CPU proceeds to show video or run the game. When the buffer is used up, the sound card generates and interrupt and the program provides a new buffer of data.
There are a set of DMA controller chips on the Mainboard. Each DMA circuit has a level number and can support one device. When installing new adapters, it is important to avoid conflicts for the DMA number just as one avoids conflicts for the I/O address or interrupt level.
A program stores into the DMA circuit a starting memory buffer address and length. When the device is ready for more data, it uses one bus cycle to send a request to the DMA chip, the chip then substitutes for the CPU in generating the next buffer address to the memory circuits to fetch the next chunk of data for the device. The CPU can be running other programs. However, that first signal from the device to the DMA chip takes one more bus cycle than ordinary Programmed I/O. Thus DMA has not been attractive for disk, LAN, and other performance critical I/O.
Busmaster
On a Microchannel, EISA, or PCI machine, the I/O bus presents a full set of 32 address wires to every adapter card. The adapter can then generate a memory address to reference any location in RAM. A Busmaster adapter card has its own Direct Memory Access control chip on the adapter board. That chip can generate a sequence of memory addresses to read data from memory to the adapter, or to write data from the adapter to memory.
Since DMA and I/O functions are combined on the same card, they can be tightly coordinated. A Busmaster EISA device can transfer data every other cycle, and PCI Busmaster cards can move data in every I/O cycle.
The software on the PC builds a high level request to READ or WRITE an entire buffer of data. It passes the address and size of the buffer to the Busmaster card. Then the PC software does something else.
Internally the Busmaster card moves data between itself and the buffer in memory. When the entire buffer has been processed, it generates an interrupt to the CPU indicating that the request is complete.
In order to make effective use of a Busmaster adapter you need two things. First, you need to have something else to do while the adapter performs the I/O. Secondly, you need an operating system that will allow I/O to run in the background. Generally Busmaster adapters make the most sense on Server machines running a multitasking operating system (Windows NT, OS/2, UNIX, or Netware).
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Back Copyright 1996 PCLT -- Introduction to PC Hardware -- H. Gilbert