3.5.3. DMA - Direct Memory Access

The work process is associated with DMA (Direct Memory Access) controller-assembly (DMA Controller) direct memory access, which frees up the processor from the task of control, management communication and data exchange with peripherals. DMA operates as a stand-alone data processor. AT Concept PC uses 2 DMA controllers, each managed by accessing a memory-one of four peripherals. One of the channels is not used for connection to peripherals but serves to refresh dynamic memory (Memory Refresh). Can run only one DMA channel, but it is possible to simultaneously work multiple peripherals at the same DMA channel. Communication with peripherals and memory through the processor performs bus which can be accessed directly or communication leaves the DMA controller, which through one of the channels is realized and refresh memory.

Figure 3.5.8 DMA controller in the PC-AT computer.

Direct method transfer data between peripherals and memory work is done via the I / O controller microprocessor supervised when they are in the image of a connection through the '1' and '2', and severed the connection '3' and '4'. In this way, the microprocessor is busy supervising the transfer of unnecessary data. Using DMA controller communication between working memory and peripherals is done via DMA establishing a connection '3' and '4' and break the connection '1' and '2'. In this way, the processor is freed from control of data transfer, and can perform other useful tasks. Hard drive and optical drive peripherals are commonly used by DMA, or updated version of the Ultra DMA communication - UDMA. How has several modes (modes) how this communication is achieved denoted numerically (UDMA1, UDMA2 ..), and certain modes define ATA/ATAPI standards, so you can find the name of the Ultra ATA. DMA using some performance network card.

DMA Controller operates in two states:

           1.) Passive - waiting control requirements and allows
                         subscribe or read data register their
                         by microprocessor.

           2.) Active - controller shall transfer the data in one
                        one cycle of transmission of one or
                        more peripherals sequentially.

DMA controller, so must know addressable memory space. Performative can not support the entire physical address space of a PC, but only blocks of memory per 64 KB. Therefore, it is associated with the address generator folder that expands its 16-bit (64 kB) DMA address of the 24 bit address bus systems (xx286). Generator via control bus controlled by a microprocessor.

DMA is a faster way of communication in the computer. Slower way is via I / O channels. When a device wants to send data to a computer, access to their I / O channel, the microprocessor sends a signal through the power lines to break your application to communicate, and the moment when the communication microprocessor approves possible connection of I / O channels to the destination, and allows the transmission of data through I / O channel. I / O channels are essentially designed for the communication purpose logic devices with the behavior of the input or output oriented 'buffers', each with a different and just pasted it address set, recorded immediately in the initial address of the memory map, that constitute and recognize and specifies the mode as input or output data.

The whole set of individual traffic between these devices should effectively monitor and manage it, what special care electronic logic outlined in a more integrated circuits - 'ChipSet'. Basic settings 'ChipSet' benefits were entered in the computer's BIOS. The current BIOS and are based on the 'flash' type of memory you can make corrections to any errors in the content, or logic, and they incorporate new or improved features. Therefore, we should choose a motherboard with a chipset known manufacturers 'patches' and advancement opportunities BIOS regularly issues.

Example III

'Chipset' INTEL 945G developed for the PC system with support for CPU type Intel Pentium D. The entire set of logic, as the Figure shows, is divided into two parts:

• North Bridge - GMCH
  Supports communication between the processor, RAM and graphics subsystems (Graphics and Memory Controller Hub) and what their features and functions are essential to the speed of data processing. Displayed version of this support chip in it embedded graphics support or graphics card is installed in PCI-E x16 slot. Therefore, it is shown in Figure 3.5.9c and interface graphics card with DVI and VGA connectors. Associated with the 'South Bridge' hub architecture that takes care of other devices.
  
  
• South Bridge - ICH
  Concerned about working with internal and external computer devices (I/O Controller Hub) and are functionally similar because most standardized access devices. It was designed with the intention to include as many elements of multimedia and the Internet, and hence has integrated support for music (CD-DVD), USB (Universal Serial Bus) devices, and network support for LAN communication.

Figure** 3.5.9 Block Diagram of Intel 945G chipset. ( + / - )

Only an increase in bus bandwidth required a redesign compared to the AGP concept regarding communication and processor and the working memory (DDR2), according to which more channel access with lower supply voltage and signaling. Therefore, in order to utilize the power of computers is better to have two memory modules of say 512 MB but one of 1 GB. Besides the improved supply of electricity (power) to the devices. The included RAID technology with SATA-II disks by serial communication brings a new quality in the speed of access to secondary memory (disk). Care should be taken when choosing a motherboard, because some do not support RAID technology, although they have more SATA-II connectors for drives (for example does not support the ICH7 and ICH7R supports) If you are referring to use multiple disks in a RAID array.

Figure 3.5.9b shows the performance of the motherboard chipset shown. Different variants of motherboards have different options, although the same family of products. Therefore, I / O ports can be less or more than the picture 3.5.9c, and the performance does not have to be the same (for incorporated network card for example). The differences are primarily related to the number of audio ports (Surround), the potential of digital coaxial or optical multi-channel audio output (S/PDIF Digital Audio Out) and IEEE 1394 (FireWire) port. Thus, one should be careful when choosing a version of the motherboard to meet those needs that users need. Graphics subsystem certainly can not provide what you would provide a 'real' video card, but because you use your PC while this temporary solution does not replace with someone more powerful. The choice of the microprocessor is very diverse, all the opportunities and needs; Celeron-D (Dual-Core), Pentium-4, Pentium-4-HT (Hyper-Threading Technology), Pentium-D for this example. Please note the review process, as not all the same. Newer have 'patched' failures predecessors though they carry the same label.

PCI and PCI-Express cards do not work according to the same principles and standards are not mutually compatible, PCI uses parallel data transfer while PCI-E uses a synchronous high speed serial data transmission. PCI standard, together with the PCI-X version was developed as a local extension of the ISA bus PC. PCI Express is a brand new product, and 'southern' chipset is the one who allows his character to still be used PCI slot as an intermediate stage in the transition to new solutions. Technology development With the above example illustrates the best pictures chipsets in Chapters 3.3.8 and 3.5.10 that demonstrate their functional similarity although technologically very different.

These logic circuits designed as two separate chips, but in the future can expect their consolidation into one common unit. Of course, come new technologies and generation chipset but this is similar to the organization of work. Currently dominated by Intel, nVidia and VIA chipset's (although there are other brands) but it is a thankless task for the user microprocessors they do not support the same physical connection to the motherboard, which means that the selection of chipsets and define a type of microprocessor (Intel or AMD) the choice is limited. Instruction set that supports both types of microprocessors is quite compatible, although slightly different in newer instructions concerning the encoding and decoding support for music and video (DVD). Incompatibility is the result of competition for customers, which in turn means the price drop with a very good offer, but also confusion.