DMA Controller Chip, Computer Resources Required By Hardware, Resolving Memory Conflicts

DMA Controller Chip

A resource a device might request from the system is a DMA (direct memory access) channel. A chip on the systemboard contains the DMA logic. The DMA controller on a systemboard is part of the chip set and provides faster memory access because it handles the movement of data in and out of RAM without involving the CPU.

During startup, hardware devices are assigned IRQs and I/O addresses, and have blocks of memory assigned to their System BIOS programs or device drivers. The DMA chip provides channels that a device can use for fast access when sending data to memory. The DMA chip does this by bypassing the CPU.

Each DMA channel requires two lines to manage it, one for the DMA controller to request clearance from the CPU and the other used by the CPU to acknowledge that the DMA controller is free to send data over the data lines without interference from the CPU.

DMA channel 4 cascades into the lower DMA channels. DMA channels 0–3 use the 8-bit ISA bus, and DMA channels 5, 6, and 7 use the 16-bit ISA bus. This means that the lower four channels provide slower data transfer than the higher channels because they don’t have as many data paths available. Also, an 8-bit expansion card that is only using the 8-bit ISA bus cannot access DMA channels 5, 6, or 7 because it can’t get to these pins on the extended expansion slot.

Some devices, such as a hard drive, are designed to use DMA channels while other devices, like the mouse, are not. Those that use the channels might be able to use only a certain channel and no other.

The BIOS might have the option of changing a DMA channel number to avoid conflicts with other devices. Conflicts occur when more than one device uses the same channel.

DMA channels are not as popular as they once were because their design makes them slower than newer methods. However, slower devices such as floppy drives, sound cards,

and tape drives may still use DMA channels.

Computer Resources Required By Hardware

Peripheral devices require some of the system resources in order to operate. Most devices require one IRQ, although some require more than one and some require none.

The IRQ points to an entry in the I/O address table that tells the CPU at what address to find BIOS or device drivers. Some devices require a DMA channel. These resources are assigned to the device during the booting process. Think of the process as a dialog that might go something like this: The startup BIOS recognizes that a hardware device is present. The BIOS asks the device, “What resources do you need?”

The device requests, “I need this IRQ and that I/O address and these addresses in upper memory for my BIOS.” More cooperative Plug-and-Play devices simply ask, “I need one IRQ, some I/O addresses, and this many upper memory addresses for my BIOS. Please tell me the resources I can use.” A device must be the sole owner of these resources. Problems occur when more than one device attempts to use the same resource.

Resolving Memory Conflicts

A peripheral device’s on-board BIOS can request certain memory addresses either in the I/O address table for its I/O addresses or in upper memory for its BIOS software addresses. Some cards are built so that only these addresses work. When two controller cards require the same memory addresses, memory conflicts occur. Memory conflicts can often be resolved if the BIOS or I/O addresses for a peripheral device can be assigned an alternative memory address. Setting switches on the controller card or allowing BIOS to make new assignments internally can do this.