8237 modes :

Intel 8237 can be set to four different type of style of transfer:

1) Single - One transfer at a time,  it allow processor access to the bus between transfers.

2) Block - Transfer all data, it do not allow processor access to the bus (may cause problems with memory refresh).

3) Demand - it keep transferring as long as target keeps DRQ asserted.

4) Cascade - it allow a slave controller use of the DMAC (used for DRQ4).

(A) In addition, the DMA controller can be set to make continuous transfers

o   It known as auto-initialized DMA

o   normally DMA is known as "single-cycle"

(B) 8237 is clocked at 1/2 of ISA Bus (0.5 *BLCK)

o   up to 4.166MHz (8.33 Mhz ISA)

o   Maximum transfer rate: 4.166MB/s (16-bit DMA)

o   Maximum Programmed I/O transfer rate: 2.77 MB/s

(C) Size of transfer

o   Master can only produce word-sized transfers

o   Slave can produce byte-sized transfers

o   Minimum transfer size: 1 byte

o   Maximum transfer size: 64KB (8-bit),128KB (16-bit)