Computer Science - Computer Hardware - Discussion

Today, an AT-class system is any computer with a 16-bit, 32-bit, or 64-bit system. Earlier computers (8-bit) were known as PC/XT-class systems. All new computers used today are considered AT-class computers and you're not likely to encounter earlier computers.

Short for advanced technology, AT is an IBM computer first introduced in 1984. The base model AT computer contained an 80286 processor, 256 k of RAM and a 1.2 MB floppy disk drive. The AT was also available with 512 k of RAM and a 20 Mbyte disk drive.

The DMA controller has four wires which are connected to certain devices on the system. On the other end, the DMA chip is connected to the Memory Controller Chip (MMC) that allows access to RAM. That way, devices that are connected to the DMA chip are able to read and write data to RAM through the DMA chip, without involving the CPU.

Similar to IRQs, we actually use two cascaded DMA controllers. Each chip has 4 DMA channels. Since chips are cascaded, we actually get 7 working DMA channels. One wire on the first controller is used to connect to the second DMA controller. Channel 0 (zero) is used to do just that, so it can't be used to connect other devices. Actually, channels 0 (on the first DMA chip), and 4 (on the second DMA chip) are actually the same channel.

Short for direct memory access, a technique for transferring data from main memory to a device without passing it through the CPU. Computers that have DMA channels can transfer data to and from devices much more quickly than computers without a DMA channel can. This is useful for making quick backups and for real-time applications.

Only one DMA channel can be assigned to one device on the system. If devices share the same DMA channel, memory errors can occur. For example, Floppy disk controller is assigned to DMA channel 2. Channel 1 is assigned to sound card. Channels 3, 4, 5, 6, and 7 are usually available to be assigned by other devices on the system. Since DMA channel 0 is used to cascade two DMA chips, it is not assigned to devices.

There is also a newer type of DMA called Ultra DMA, or sometimes called Bus Mastering. This new type of DMA allows devices to bypass DMA controller all together. This way devices can access system bus directly. Ultra DMA supports burst mode data transfer rates, which provides twice the speed of standard DMA performance. This is usually implemented on hard disks. Ultra ATA hard disks use ultra DMA.

Some expansion boards, such as CD-ROM cards, are capable of accessing the computer's DMA channel. When you install the board, you must specify which DMA channel is to be used, which sometimes involves setting a jumper or DIP switch.