How many DMA channels are there in an AT class PC?

ANS:D - 8

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.