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7.5 I/O Speed Hierarchy

Different devices have different data transfer rates. Some devices, like keyboards, are extremely slow (comparing their speed to CPU speeds). Other devices like disk drives can actually transfer data faster than the CPU can read it. The mechanisms for data transfer differ greatly based on the transfer speed of the device. Therefore, it makes sense to create some terminology to describe the different transfer rates of peripheral devices.

Low-speed devices are those that produce or consume data at a rate much slower than the CPU is capable of processing. For the purposes of discussion, we'll claim that low-speed devices operate at speeds that are two to three orders of magnitude (or more) slower than the CPU. Medium-speed devices are those that transfer data at approximately the same rate (within an order of magnitude slower, but never faster) than the CPU. High-speed devices are those that transfer data faster than the CPU is capable of moving data between the device and the CPU. Clearly, high-speed devices must use DMA since the CPU is incapable of transferring the data between the CPU and memory.

With typical bus architectures, modern day PCs are capable of one transfer per microsecond or better. Therefore, high-speed devices are those that transfer data more rapidly than once per microsecond. Medium-speed transfers are those that involve a data transfer every one to 100 microseconds. Low-speed devices usually transfer data less often than once every 100 microseconds. The difference between these speeds will decide the mechanism we use for the I/O operation (e.g., high-speed transfers require the use of DMA or other techniques).

Note that one transfer per microsecond is not the same thing as a one megabyte per second data transfer rate. A peripheral device can actually transfer more than one byte per data transfer operation. For example, when using the "in( dx, eax );" instruction, the peripheral device can transfer four bytes in one transfer. Therefore, if the device is reaching one transfer per microsecond, then the device can transfer four megabytes per second. Likewise, a DMA device on a Pentium processor can transfer 64 bits at a time, so if the device completes one transfer per microsecond it will achieve an eight megabyte per second data transfer rate.

7.6 System Busses and Data Transfer Rates

Earlier in this text (see "The System Bus" on page122) you saw that the CPU communicates to memory and I/O devices using the system bus. In that chapter you saw that a typical Von Neumann Architecture machine has three different busses: the address bus, the data bus, and the control bus. If you've ever opened up a computer and looked inside or read the specifications for a system, you've probably heard terms like PCI, ISA, EISA, or even NuBus mentioned when discussing the computer's bus. If you're familiar with these terms, you may wonder what their relationship is with the CPU's bus. In this section we'll discuss this relationship and describe how these different busses affect the performance of a system.

Computer system busses like PCI (Peripheral Connection Interface) and ISA (Industry Standard Architecture) are definitions for physical connectors inside a computer system. These definitions describe a set of signals, physical dimensions (i.e., connector layouts and distances from one another), and a data transfer protocol for connecting different electronic devices. These busses are related to the CPU's bus only insofar as many of the signals on one of the peripheral busses also appear on the CPU's bus. For example, all of the aforementioned busses provide lines for address, data, and control functions.

Peripheral interconnection busses do not necessarily mirror the CPU's bus. All of these busses contain several additional lines that are not present on the CPU's bus. These additional lines let peripheral devices communicate with one other directly (without having to go through the CPU or memory). For example, most busses provide a common set of interrupt control signals that let various I/O devices communicate directly with the system's interrupt controller (which is also a peripheral device). Nor does the peripheral bus always include all the signals found on the CPU's bus. For example, the ISA bus only supports 24 address lines whereas the Pentium IV supports 36 address lines. Therefore, peripherals on the ISA bus only have access to 16 megabytes of the Pentium IV's 64 gigabyte address range.

A typical modern-day PC supports the PCI bus (although some older systems also provide ISA connectors). The organization of the PCI and ISA busses in a typical computer system appears in Figure 7.6.

Figure 7.6 Connection of the PCI and ISA Busses in a Typical PC

Notice how the CPU's address and data busses connect to a PCI Bus Controller device (which is, itself, a peripheral of sorts). The actual PCI bus is connected to this chip. Note that the CPU does not connect directly to the PCI bus. Instead, the PCI Bus Controller acts as an intermediary, rerouting all data transfer requests between the CPU and the PCI bus.

Another interesting thing to note is that the ISA Bus Controller is not directly connected to the CPU. Instead, it is connected to the PCI Bus Controller. There is no logical reason why the ISA Controller couldn't be connected directly to the CPU's bus, however, in most modern PCs the ISA and PCI controllers appear on the same chip and the manufacturer of this chip has chosen to interface the ISA bus through the PCI controller for cost or performance reasons.

The CPU's bus (often called the local bus) usually runs at some submultiple of the CPU's frequency. Typical local bus frequencies include 66 MHz, 100 MHz, 133 MHz, 400 MHz, and, possibly, beyond1. Usually, only memory and a few selected peripherals (e.g., the PCI Bus Controller) sit on the CPU's bus and operate at this high frequency. Since the CPU's bus is typically 64 bits wide (for Pentium and later processors) and it is theoretically possible to achieve one data transfer per cycle, the CPU's bus has a maximum possible data transfer rate (or maximum bandwidth) of eight times the clock frequency (e.g., 800 megabytes/second for a 100 Mhz bus). In practice, CPU's rarely achieve the maximum data transfer rate, but they do achieve some percentage of this, so the faster the bus, the more data can move in and out of the CPU (and caches) in a given amount of time.

The PCI bus comes in several configurations. The base configuration has a 32-bit wide data bus operating at 33 MHz. Like the CPU's local bus, the PCI is theoretically capable of transferring data on each clock cycle. This provides a theoretical maximum of 132 MBytes/second data transfer rate (33 MHz times four bytes). In practice, the PCI bus doesn't come anywhere near this level of performance except in short bursts. Whenever the CPU wishes to access a peripheral on the PCI bus, it must negotiate with other peripheral devices for the right to use the bus. This negotiation can take several clock cycles before the PCI controller grants the CPU the bus. If a CPU writes a sequence of values to a peripheral a double word per bus request, then the negotiation takes the majority of the time and the data transfer rate drops dramatically. The only way to achieve anywhere near the maximum theoretical bandwidth on the bus is to use a DMA controller and move blocks of data. In this block mode the DMA controller can negotiate just once for the bus and transfer a fair sized block of data without giving up the bus between each transfer. This "burst mode" allows the device to move lots of data quickly.

There are a couple of enhancements to the PCI bus that improve performance. Some PCI busses support a 64-bit wide data path. This, obviously, doubles the maximum theoretical data transfer rate. Another enhancement is to run the bus at 66 MHz, which also doubles the throughput. In theory, you could have a 64-bit wide 66 MHz bus that quadruples the data transfer rate (over the performance of the baseline configuration). Few systems or peripherals currently support anything other than the base configuration, but these optional enhancements to the PCI bus allow it to grow with the CPU as CPUs increase their performance.

The ISA bus is a carry over from the original PC/AT computer system. This bus is 16 bits wide and operates at 8 MHz. It requires four clock cycles for each bus cycle. For this and other reasons, the ISA bus is capable of about only one data transmission per microsecond. With a 16-bit wide bus, data transfer is limited to about two megabytes per second. This is much slower than the CPU's local bus and the PCI bus . Generally, you would only attach low-speed devices like an RS-232 communications device, a modem, or a parallel printer to the ISA bus. Most other devices (disks, scanners, network cards, etc.) are too fast for the ISA bus. The ISA bus is really only capable of supporting low-speed and medium speed devices.

Note that accessing the ISA bus on most systems involves first negotiating for the PCI bus. The PCI bus is so much faster than the ISA bus that this has very little impact on the performance of peripherals on the ISA bus. Therefore, there is very little difference to be gained by connecting the ISA controller directly to the CPU's local bus.

7.7 The AGP Bus

Video display cards are a very special peripheral that need the maximum possible amount of bus bandwidth to ensure quick screen updates and fast graphic operations. Unfortunately, if the CPU has to constantly negotiate with other peripherals for the use of the PCI bus, graphics performance can suffer. To overcome this problem, video card designers created the AGP (Advanced Graphics Port) interface between the CPU and the video display card.

The AGP is a secondary bus interface that a video card uses in addition to the PCI bus. The AGP connection lets the CPU quickly move data to and from the video display RAM. The PCI bus provides a connection to the other I/O ports on the video display card (see Figure 7.7). Since there is only one AGP port per system, only one card can use the AGP and the system never has to negotiate for access to the AGP bus.

Figure 7.7 AGP Bus Interface


If a particular I/O device produces or consumes data faster than the system is capable of transferring data to that device, the system designer has two choices: provide a faster connection between the CPU and the device or slow down the rate of transfer between the two.

Creating a faster connection is possible if the peripheral device is already connected to a slow bus like ISA. Another possibility is going to a wider bus (e.g., to the 64-bit PCI bus) to increase bandwidth, or to use a bus with a higher frequency (e.g., a 66 MHz bus rather than a 33 MHz bus). Systems designers can sometimes create a faster interface to the bus; the AGP connection is a good example. However, once you're using the fastest bus available on the system, improving system performance by selecting a faster connection to the computer can be very expensive.

The other alternative is to slow down the transfer rate between the peripheral and the computer system. This isn't always as bad as it seems. Most high-speed devices don't transfer data at a constant rate to the system. Instead, devices typically transfer a block of data rapidly and then sit idle for some period of time. Although the burst rate is high (and faster than the CPU or system can handle), the average data transfer rate is usually lower than what the CPU/system can handle. If you could average out the peaks and transfer some of the data when the peripheral is inactive, you could easily move data between the peripheral and the computer system without resorting to an expensive, high-bandwidth, solution.

The trick is to use memory to buffer the data on the peripheral side. The peripheral can rapidly fill this buffer with data (or extract data from the buffer). Once the buffer is empty (or full) and the peripheral device is inactive, the system can refill (or empty) the buffer at a sustainable rate. As long as the average data rate of the peripheral device is below the maximum bandwidth the system will support, and the buffer is large enough to hold bursts of data to/from the peripheral, this scheme lets the peripheral communicate with the system at a lower data transfer rate than the device requires during burst operation.

1400 MHz was the maximum CPU bus frequency as this was being written.

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