MODE OF OPERATION OF 8237 

SINGLE TRANSFER MODE

In Single Transfer mode the device is programmed to make one transfer only.The word count will be decremented and the address decremented or incremented following each transfer. When the word count ``rolls over'' from zero to FFFFH, a Terminal Count (TC) will cause an Auto initialize if the channel has been programmed to do so. DREQ must be held active until DACK becomes active in order to be recognized. If DREQ is held active through out the single transfer, HRQ will go in active and release the bus to the system. It will again go active and, upon receipt of a new HLDA, an other single transfer will be performed.

In 8080A, 8085AH, 8088, or 8086 system, this will ensure one full machine cycle execution between DMA transfers. Details of timing between the 8237A and other bus control protocols will depend upon the characteristics of the microprocessor involved. 

BLOCK TRANSFER MODE 

In Block Transfer mode the device is activated by DREQ to continue making transfers during the service until a TC, caused byword count going to FFFFH, or an external End of Process (EOP) is encountered. DREQ need only be held active until DACK becomes active. 

Again, an Auto initialization will occur at the end of the service if the channel has been programmed for it.

DEMAND TRANSFER MODE

In Demand Transfer mode the device is programmed to continue making transfers until a TC or external EOP is encountered or until DREQ goes inactive. Thus transfers may continue until the I/O device has exhausted its data capacity. After the I/O device has had a chance to catch up, the DMA service is re-established by means of a DREQ. During the time between services when the microprocessor is allowed to operate, the intermediate values of address and word count are stored in the 8237 A Current Address and Current Word Count registers. Only an EOP can cause an Auto initialize at the end of the service. EOP is generated either by TC or by an external signal. DREQ has to be low before S4 to prevent another Transfer. 

CASCADE MODE 

This mode is used to cascade more than one 8237A together for simple system expansion. The HRQ and HLDA signals from the additional 8237A are connected to the DREQ and DACK signals of a channel of the initial 8237A. This allows the DMA requests of the additional device to propagate through the priority network circuitry of the preceding device. The priority chain is preserved and the new device must wait for its turn to acknowledge requests. Since the cascade channel of the initial 8237A is used only for prioritizing the additional device, it does not output any address or control.