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)