Direct Memory Access

Direct Memory Access 1. DMA able to direct access to the memory while CPU is disable, this enable the memory components & DMA controller to transfer data between memory & I/O device. 2. Used for DMA is DRAM refresh, refreshing the video display screen, disk memory system reads & writes, fast memory to memory transfer. 3. DMA will always take over CPU when using the bus because only 1 master can be server at a time ,if CPU is working DMA will idel 8237 DMA Controller 1. It bypasses CPU & direct connects between peripherals & memory for fast transfer of data. 2. It controller has 40-pin chip ,4 channels for transferring data ,& each only for 1 device to transfer data 3. Every channel there are 2 associated signals ,DREQ(DMA Request)for input (such as hard disk controller) & DACK(DMA Acknowledge )for signal send back to device. 4. It rate of transfer 1.6M bytes per second ,each channel is capable of addressing a full 64k bytes section of memory & can transfer up to 64k bytes with programming. 8237 DMA Interface 1. DMA has 4 independent channels from 0 through 3,each is dedicated to a specific device ,such as floppy or hard disk. 8237 Controller Chips 1. DMAC chip is use fo servicing 8 bit device. DMA Operations 1. when DMA transfer data by bus it will sends a signal called HRQ to CPU & CPU will respond by sending signal HLDA to give permission. 2. Fixed priority :channel 0 is always servicing & channel 3 service only no other channel is active.