Direct Memory Access - Examples - ISA

ISA

In the original IBM PC, there was only one Intel 8237 DMA controller capable of providing four DMA channels (numbered 0-3), as part of the so-called Industry Standard Architecture, or ISA. These DMA channels performed 8-bit transfers and could only address the first megabyte of RAM. With the IBM PC/AT, a second 8237 DMA controller was added (channels 5-7; channel 4 is unusable), and the page register was rewired to address the full 16 MB memory address space of the 80286 CPU. This second controller performed 16-bit transfers.

Due to their lagging performance (2.5 Mbit/s), these devices have been largely obsolete since the advent of the 80386 processor in 1985 and its capacity for 32-bit transfers. They are still supported to the extent they are required to support built-in legacy PC hardware on modern machines. The only pieces of legacy hardware that use ISA DMA and are still fairly common are the built-in Floppy disk controllers of many PC mainboards and those IEEE 1284 parallel ports that support the fast ECP mode.

Each DMA channel has a 16-bit address register and a 16-bit count register associated with it. To initiate a data transfer the device driver sets up the DMA channel's address and count registers together with the direction of the data transfer, read or write. It then instructs the DMA hardware to begin the transfer. When the transfer is complete, the device interrupts the CPU.

Scatter-gather or Vectored I/O DMA allows the transfer of data to and from multiple memory areas in a single DMA transaction. It is equivalent to the chaining together of multiple simple DMA requests. The motivation is to off-load multiple input/output interrupt and data copy tasks from the CPU.

DRQ stands for Data request; DACK for Data acknowledge. These symbols, seen on hardware schematics of computer systems with DMA functionality, represent electronic signaling lines between the CPU and DMA controller. Each DMA channel has one Request and one Acknowledge line. A device that uses DMA must be configured to use both lines of the assigned DMA channel.

Standard ISA DMA assignments:

0. DRAM Refresh (obsolete),
1. User hardware,
2. Floppy disk controller,
3. Hard disk (obsoleted by PIO modes, and replaced by UDMA modes),
4. Cascade from XT DMA controller,
5. Hard Disk (PS/2 only), user hardware for all others,
6. User hardware.