Introduction of 8237

Direct Memory Access (DMA) is a method of allowing data to be moved from one location to another in a computer without intervention from the central processor (CPU).
It is also a fast way of transferring data within (and sometimes between) computer.
The DMA I/O technique provides direct access to the memory while the microprocessor is temporarily disabled.
The DMA controller temporarily borrows the address bus, data bus and control bus from the microprocessor and transfers the data directly from the external devices to a series of memory locations (and vice versa).

Two control signals are used to request and acknowledge a direct memory access (DMA) transfer in the microprocessor-based system.
1. The HOLD signal as an input(to the processor) is used to request a DMA action.
2. The HLDA signal as an output that acknowledges the DMA action.
When the processor recognizes the hold, it stops its execution and enters hold cycles.
HOLD input has higher priority than INTR or NMI.
The only microprocessor pin that has a higher priority than a HOLD is the RESET pin.
HLDA becomes active to indicate that the processor has placed its buses at high-impedance state.

Direct memory accesses normally occur between an I/O device and memory without the use of the microprocessor.
1. A DMA read transfers data from the memoryto the I/O device.
2. A DMA write transfers data from an I/O deviceto memory.
The system contains separate memory and I/O control signals.
Hence the Memory & the I/O are controlled simultaneously
The DMA controller provides memory with its address, and the controller signal selects the I/O device during the transfer.
Data transfer speed is determined by speed of the memory device or a DMA controller.
In many cases, the DMA controller slows the speed of the system when transfers occur.
The serial PCI (Peripheral Component Interface) Express bus transfers data at rates exceeding DMA transfers.
This in modern systems has made DMA is less important.

The 8237 supplies memory & I/O with control signals and memory address information during the DMA transfer.
It is actually a special-purpose microprocessor whose job is high-speed data transfer between memory and I/O

8237 is not a discrete component in modern microprocessor-based systems.

It appears within many system controller chip sets

8237 is a four-channel device compatible with 8086/8088, adequate for small systems.

Expandable to any number of DMA channel inputs

8237 is capable of DMA transfers at rates up to 1.6MB per second.

Each channel is capable of addressing a full64K-byte section of memory.