Bus and Memory Transfers
A typical digital computer has many register and paths must be provided to transfer information from one register to another . The number of wires will be excessive if seperate lines are used between each register and all other registers in the system.A more efficien scheme for transferring information between registers in a multiple register configuration is a common bus system. A bus structure consists of a set of common lines one for each bit of aregister, through which binary informatio is transferred one at a time .control signals determine which register is selected by the bus during each particular register transfer. one way of constructing a common bus system is with multiplexers.The multiplexers select the source registre whose binary information is then placed on the bus. A shared communication path consist of one and more lines is called bus and data transfer through this bus is called bus transfer.What a data read from memory or stored in memory is called memory transfer.When an information is read from memory word is called read operation and data stored in memory is called write operation. The computer has a "bus" where all information travels. It connects all the hardware together. The processor(cpu) calls for a piece of information and it addresses the bus to find that memory location and retrieve it for the operation that needed it. All of the memory cards are hardware plugged into the bus. Read more: http://wiki.answers.com/Q/What_is_bus_memory_transfer#ixzz1JmqCGlYJ
Introduction to the concept of a bus A bus, in computing, is a set of physical connections (cables, printed circuits, etc.) which can be shared by multiple hardware components in order to communicate with one another. The purpose of buses is to reduce the number of "pathways" needed for communication between the components, by carrying out all communications over a single data channel. This is why the metaphor of a "data highway" is sometimes used.
If only two hardware components communicate over the line, it is called a hardware port (such as aserial port or parallel port).
Characteristics of a bus A bus is characterised by the amount of information that can be transmitted at once. This amount, expressed in bits, corresponds to the number of physical lines over which data is sent simultaneously. A 32-wire ribbon cable can transmit 32 bits in parallel. The term "width" is used to refer to the number of bits that a bus can transmit at once. Additionally, the bus speed is also defined by its frequency (expressed in Hertz), the number of data packets sent or received per second. Each time that data is sent or received is called a cycle. This way, it is possible to find the maximum transfer speed of the bus, the amount of data which it can transport per unit of time, by multiplying its width by its frequency. A bus with a width of 16 bits and a frequency of 133 MHz, therefore, has a transfer speed equal to: 16 * 133.106 = 2128*106 bit/s, or 2128*106/8 = 266*106 bytes/s or 266*106 /1000 = 266*103 KB/s or 259.7*103 /1000 = 266 MB/s
Bus subassembly In reality, each bus is generally constituted of 50 to 100 distinct physical lines, divided into three subassemblies: • The address bus (sometimes called the memory bus) transports memory addresses which the processor wants to access in order to read or write data. It is a unidirectional bus. • The data bus transfers instructions coming from or going to the processor. It is a bidirectional bus. • The control bus (or command bus) transports orders and synchonisation signals coming from the control unit and travelling to all other hardware components. It is a bidirectional bus, as it also transmits response signals from the hardware.
The primary buses There are generally two buses within a computer: • the internal bus (sometimes called the front-side bus, or FSB for short). The internal bus allows the processor to communicate with the system's central memory (the RAM). • the expansion bus (sometimes called the input/output bus) allows various motherboard components (USB, serial, and parallel ports, cards inserted in PCI connectors, hard drives, CD-ROM and CD-RW drives, etc.) to communicate with one another. However, it is mainly used to add new devices using what are called expansion slots connected to the input/outpur bus.
The chipset A chipset is the component which routes data between the computer's buses, so that all the components which make up the computer can communicate with each other. The chipset originally was made up of a large number of electronic chips, hence the name. It generally has two components: • The NorthBridge (also called the memory controller) is in charge of controlling transfers between the processor and the RAM, which is way it is located physically near the processor. It is sometimes called the GMCH, forr Graphic and Memory Controller Hub. • The SouthBridge (also called the input/output controller or expansion controller) handles communications between peripheral devices. It is also called the ICH (I/O Controller Hub). The tembridge is generally used to designate a component which connects two buses.
It is interesting to note that, in order to communicate, two buses must have the same width. The explains why RAM modules sometimes have to be installed in pairs (for example, early Pentium chips, whose processor buses were 64-bit, required two memory modules each 32 bits wide). Here is a table which gives the specifications for the most commonly used buses:
Standard
Bus width (bits) Bus speed (MHz) Bandwidth (MB/sec)
ISA 8-bit
8
8.3
7.9
ISA 16-bit
16
8.3
15.9
EISA
32
8.3
31.8
VLB
32
33
127.2
PCI 32-bit
32
33
127.2
PCI 64-bit 2.1
64
66
508.6
AGP
32
66
254.3
AGP (x2 Mode)
32
66x2
528
AGP (x4 Mode)
32
66x4
1056
AGP (x8 Mode)
32
66x8
2112
ATA33
16
33
33
ATA100
16
50
100
ATA133
16
66
133
Serial ATA (S-ATA)
1
180
Serial ATA II (S-ATA2)
2
380
USB
1
1.5
USB 2.0
1
60
FireWire
1
100
FireWire 2
1
200
SCSI-1
8
4.77
5
SCSI-2 - Fast
8
10
10
SCSI-2 - Wide
16
10
20
SCSI-2 - Fast Wide 32 bits 32
10
40
SCSI-3 - Ultra
8
20
20
SCSI-3 - Ultra Wide
16
20
40
SCSI-3 - Ultra 2
8
40
40
SCSI-3 - Ultra 2 Wide
16
40
80
SCSI-3 - Ultra 160 (Ultra 3)
16
80
160
SCSI-3 - Ultra 320 (Ultra 4)
16
80 DDR
320
SCSI-3 - Ultra 640 (Ultra 5)
16
80 QDR
640
The memory bus is used to transfer information between the CPU and main memorythe RAM in your system. This bus is usually connected to the motherboard chipset North Bridge or Memory Controller Hub chip. Depending on the type of memory your chipset (and therefore motherboard) is designed to handle, the North Bridge runs the memory bus at various speeds. The best solution is if the memory bus runs at the same speed as the processor bus. Systems that use PC133 SDRAM have a memory bandwidth of 1066MBps, which is the same as the 133MHz CPU bus. In another example, Athlon systems running a 266MHz processor bus also run PC2100 DDR-SDRAM, which has a bandwidth of 2133MBpsexactly the same as the processor bus in those systems. Systems running a Pentium 4 with its 400MHz processor bus also use dual-channel RDRAM memory, which runs 1600MBps for each channel, or a combined bandwidth (both memory channels run simultaneously) of 3200MBps, which is exactly the same as the Pentium 4 CPU bus. Pentium 4 systems with the 533MHz bus run dual-channel DDR PC2100 or PC2700 modules, which match or exceed the throughput of the 4266MBps processor bus. Running memory at the same speed as the processor bus negates the need for having cache memory on the motherboard. That is why when the L2 cache moved into the processor, nobody added an L3 cache to the motherboard. Some very high-end processors, such as the Itanium and Itanium 2 and the Intel Pentium 4 Extreme Edition, have integrated 2MB4MB of full-core speed L3 cache into the CPU. However, the most recent high-performance chips, such as the new Pentium Extreme Edition, use only L1 and L2 cache. Thus, it appears that L2 cache will continue to be the most common type of secondary cache for the foreseeable future.
the need for expansion slots The I/O bus or expansion slots enable your CPU to communicate with peripheral devices. The bus and its associated expansion slots are needed because basic systems can't possibly satisfy all the needs of all the people who buy them. The I/O bus enables you to add devices to your computer to expand its capabilities. The most basic computer components, such as sound cards and video cards, can be plugged into expansion slots; you also can plug in more specialized devices, such as network interface cards, SCSI host adapters, and others. Note In most modern PC systems, a variety of basic peripheral devices are built in to the motherboard. Most systems today have at least dual (primary and secondary) IDE interfaces, four USB ports, a floppy controller, two serial ports, a parallel port, keyboard, and mouse controller built directly into the motherboard. These devices are usually distributed between the motherboard chipset South Bridge and the Super I/O chip. Many add even more items, such as a built-in sound card, video adapter, SCSI host adapter, network interface or IEEE 1394a port, that also are built in to the motherboard. Those items, however, might not be built in to the motherboard chipset or Super I/O chip; they are sometimes configured as additional chips installed on the board. Nevertheless, these built-in controllers and ports still use the I/O bus to communicate with the CPU. In essence, even though they are built in, they act as if they were cards plugged into the system's bus slots, including using system resources in the same manner.
Direct Memory Access (DMA) Modes and Bus Mastering DMA As described in the page describing programmed I/O, that method of transferring data between the hard disk and the rest of the system has a serious flaw: it requires a fair bit of overhead, as well as the care and attention of the system's CPU. Clearly, a better solution is to take the CPU out of the picture entirely, and have the hard disk and system memory communicate directly. Direct memory access or DMA is the generic term used to refer to a transfer protocol where a peripheral device transfers information directly to or from memory, without the system processor being required to perform the transaction. DMA has been used on the PC for years over the ISA bus, for devices like sound cards and the floppy disk interface. Conventional DMA uses regular DMA channels which are a standard system resource. DMA is discussed in full detail here. Several different DMA modes have been defined for the IDE/ATA interface; they are grouped into two categories. The first set of modes are single word DMA modes. When these modes are used, each transfer moves just a single word of data (a word is the techie term for two bytes, and recall that the IDE/ATA interface is 16 bits wide). There are (or were!) three single word DMA modes, all defined in the original ATA standard: DMA Mode
Cycle Time (nanoseconds)
Maximum Transfer Rate (MB/s)
Defining Standard
Single Word Mode 0
960
2.1
ATA
Single Word Mode 1
480
4.2
ATA
Single Word Mode 2
240
8.3
ATA
(As I discussed in the page on PIO, maximum transfer rate is double the reciprocal of the specific cycle time for each mode.) Obviously, these are not impressive transfer rate numbers by today's standards. Further, performing transfers of a single word at a time is horribly inefficient--each and every transfer requires overhead to set up the transfer. For that reason, single word DMA modes were quickly supplanted by multiword DMA modes. As the name implies, under these modes a "burst" of transfers occurs in rapid succession,
one word after the other, saving the overhead of setting up a separate transfer for each word. Here are the multiword DMA transfer modes: DMA Mode
Cycle Time (nanoseconds)
Maximum Transfer Rate (MB/s)
Defining Standard
Multiword Mode 0
480
4.2
ATA
Multiword Mode 1
150
13.3
ATA-2
Multiword Mode 2
120
16.7
ATA-2
Since multiword DMA transfers are more efficient, and also have higher maximum transfer rates, single word DMA modes were quickly abandoned after ATA-2 was widely adopted--they were actually removed from the ATA standards in ATA-3. So all DMA accesses today (including Ultra DMA) are actually multiword; the term "multiword" is now often assumed and no longer specifically mentioned. Another important issue with DMA is that there are in fact two different ways of doing DMA transfers. Conventional DMA is what is called third-party DMA, which means that the DMA controllers on the motherboard coordinate the DMA transfers. (The "third party" is the DMA controller.) Unfortunately, these DMA controllers are old and very slow--they are basically unchanged since the earliest days of the PC. They are also pretty much tied to the old ISA bus, which was abandoned for hard disk interfaces for performance reasons. When multiword DMA modes 1 and 2 began to become popular, so did the use of the high-speed PCI bus for IDE/ATA controller cards. At that point, the old way of doing DMA transfers had to be changed. Modern IDE/ATA hard disks use first-party DMA transfers. The term "first party" means that the peripheral device itself does the work of transferring data to and from memory, with no external DMA controller involved. This is also called bus mastering, because when such transfers are occurring the device becomes the "master of the bus". Bus mastering allows the hard disk and memory to work without relying on the old DMA controller built into the system, or needing any support from the CPU. It requires the use of the PCI bus--older buses like MCA also supported bus mastering but are no longer in common use. Bus-mastering DMA allows for the efficient transfer of data to and from the hard disk and system memory. Bus mastering DMA keeps CPU utilization low, which is the amount of work the CPU must do during a transfer.
Interestingly, despite the obvious advantages of bus mastering DMA, the use of bus-mastering multiword DMA mode 2 never really caught on. There are several reasons for this. The most important was the poor state of support for the technology for the first couple of years. Using PIO required no work and was very simple; DMA was not supported by the first version of Windows 95, so special drivers had to be used. Problems with implementing bus mastering DMA on systems in the 1996 to 1998 time frame were numerous: issues with buggy drivers, software the didn't work properly, CD-ROM drives that wouldn't work with the drivers, and so on. In the face of these problems, DMA didn't offer much incentive to make the switch. Sure, the lower CPU utilization was good, but since the maximum DMA mode's speed was the same as that of the highest PIO mode (16.7 MB/s) there wasn't a great perception that DMA offered much of an advantage over PIO. Given little upside potential, many people (including this author) stayed away from using DMA, to avoid the compatibility and stability problems that sometimes resulted. Bus mastering DMA finally came into its own when the industry moved on to Ultra DMA. Once Ultra DMA/33 doubled the interface transfer rate, DMA had an obvious speed advantage over PIO in addition to its other efficiency improvements. Support for DMA was also cleaned up and made native in Windows 9x, and most of the problems with the old drivers were eliminated. Today, the use of Ultra DMA is the standard in the industry. See here for details on the Ultra DMA modes.
What Is a Memory Bus? he memory bus connects the memory system and the northbridge area of the chipset. This section of the chipset also connects directly to the central processing unit and the graphics system. While this means the northbridge is the center of many important computer functions, it is actually the computer’s memory that determines the bus’s speed. In essence, the speed of the computer’s memory creates the speed of the memory bus, which determines the speed of the rest of the system. In computing, a bus transfers information from one location to another. Most modern computers have a large number of buses that cross-connect all sorts of different areas. The northbridge area of the chipset has four main buses. The front side busconnects to the central processing unit, the graphics bus connects to the graphic system, the internal bus connects to thesouthbridge section of the chipset, and the memory bus connects to the computer’s memory. Each of these busses act independently of one another in most cases. The biggest exception to this is the front side bus and the memory bus. These control the most essential parts of the computer’s operations, and they are tied directly together. The speed of the computer’s memory dictates the speed at which information flows across the memory bus. This
means that the processor is only able to send and receive information as quickly as the memory bus allows. Using memory that is slower than the processor will have a direct impact on a computer’s speed. Basic calculations will often sit idle and occupy processor space while they wait for follow-up information. This can create periods of latency, even on a fast computer. Oddly, more strenuous operations are rarely affected by a slow memory bus, as the time it takes for the processor to compete its operations is often greater than the transferal time. On a technical level, the memory bus is made of two parts. The data bus transfers information between the memory and the chipset. This portion of the bus is often incorrectly referred to as the memory bus, as it does the job most often associated with that part. The second part of the memory bus is the address bus. The address bus tells the system where information may be stored as it comes into memory and where the information is when it needs to leave memory. The speed of the address bus affects every action on a computer, since all applications need some access to the memory. Regardless of how fast that information comes and goes from the system, it is limited by the speed at which the address bus directs it.
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Apparatus for controlling data transfer between a bus and memory array and method for operating same A structure and method of controlling data transfer between a memory and a bus. For write operations, a write buffer is coupled between the bus and the memory array. Data that has been transferred into the write buffer is transferred from the write buffer to the memory array at a faster rate than data is transferred from the bus to the write buffer. For read operations, a read buffer is coupled between the bus and the memory array. Data is transferred from the memory array to the read buffer at a faster rate than data is transferred from the read buffer to the bus.
Memory busI/O busThe I/O bus has historically been the slowest of allbuses, and the main focus when computer designengineers try to improve bus speeds.Processor BusThe processor bus is communications path betweenthe CPU and the main bus. It is also used forcommunications between the CPU and the processorsupport chipset. The processor support chipset
includeschips such as an external memory cache and the buscontroller chip found on some microcomputers. Thesize of the processor bus matches the size of the datawords used by CPU. For example, the 80486DX chipuses 32-bit words; therefore the processor bus has 32data lines, 32 address lines, and the control lines. ThePentium processors have 64-bit words and use 32bitaddresses. Processor buses can have a maximum datatransfer rate of the motherboard clock.Memory BusThe memory bus transfers data between the RAMand the CPU. This bus can be the processor bus or willbe implemented by a dedicated chipset that controls thememory bus.In most computers that have amotherboard clock that is faster then 16MHz, a specialmemory controller chipset will control the memory bus.Address BusThe address bus transfers the next memory or I/Oaddress to be used in the next data transfer. The addressbus in 486 and Pentium systems is 32 bits wide.I/O BusesTo thoroughly understand the I/O buses used inmodem microcomputer systems, an understanding ofthe development and evolution of bus systems isrequired. The microcomputer’s architecture is directlyrelated to the type of buses in the computer. Originally,microcomputers used a bus system called the S-100 bus.Using this system, any board could be plugged into anyopen slot. The S-100 bus has 62 lines, each connect toeach of the 62-pin connectors. This system dedicatedeight lines for the eight data bits used in the Intel 8088microprocessor.Twenty lines are used for memoryaddressing. The same 20 lines are also used to addressI/O devices. A control line determines whether the dataon these 20 lines will be a memory address or an I/Oaddress. There are also several control lines and powerdistribution lines.The S-100 bus also provided four lines to designatechannels for Direct Memory Accessing (DMA). ADMA channel allows a device, such as the hard drive,to transfer data directly into RAM, vice transferringdata to the CPU and then having the CPU transfer it tothe RAM. The DMA channel number identifies whichdevice is requesting and transferring data on the databus.Buses also need to be clocked to properly transferdata. The early microcomputer buses were designed torun at the speed of the microprocessor that was installedon the board. The 4.7 MHZ 8088 microprocessor clockwas also used to clock the bus. The 7.16 MHZmicroprocessor clocked the bus at the same rate. TheISA standard set the bus clock speed at 8 MHZ. Tomaintain compatibility with the older controller boards,this speed is still common in many computers today.This speed is fine when getting input from a mouse ora keyboard, even for most disk drives. The biggestproblem with bus speeds has occurred because of theincrease in video resolution, the development of videocapture boards and some network interfaces.INDUSTRY STANDARD ARCHITECTURE(ISA). —As the microcomputer evolved, the eight datalines and 20 address lines became insufficient to handlethe increased data capacity of the 16-bit processor.
Thisled to the development of the Industry StandardArchitecture (ISA). To be compatible with the boardsused in eight-bit computers, an additional 36wireconnector was added to the circuit boards and the bus.This added eight more data lines, four more addresslines, four more DMA channels, and five more IRQchannels.LOCAL BUSES. —A local bus is a bus that is adedicated path between the processor and a specificboard. There are several local buses built into varioustypes of computers to increase the speed of datatransfers. Local buses for expanded memory and videoboards are the most common.Some high-endcomputers also provide a local bus for the hard drive.The VESA Local Bus is one of the more popularbuses and was developed to increase the speed of datatransfer between memory and the video processingboard (video graphics adapter). VESA stands for VideoElectronics Standards Association. The VESA LocalBus is a direct bus that connects the video processor
