Direct memory access
- 1. DIRECT MEMORY ACCESS(DMA) NAME:HARISH CHANDRA MAHANTA ROLL NO:18 STREAM:BSC(ITM) 3RD SEM KUSHAGRA INSTITUTE OF INFORMATION AND MANAGEMENT SCIENCE
- 2. WHAT IS DMA ? PRINCIPLE DMA OPERATION DMA IN COMPUTER ARCHITECTURE POTENTIAL IMPROVEMENT OF DMA USES LIMITATION CONCLUSION CONTENTS
- 3. WHAT IS DMA ? DIRECT MEMORY ACCESS(DMA) is a feature of modern computers and microprocessors that allow certain hardware subsystems within the computer to access system memory for reading and writing independently of the central processing unit. Many hardware systems use DMA including disk drive controllers , graphic cards , network cards , and sound cards. DMA is used for transferring data between the local memory and the main memory .
- 4. PRINCIPLE DMA is an essential feature of all modern computers , as it allows devices to transfer data without subjecting the CPU to a heavy overhead. Otherwise , the CPU would have to copy each piece of data from the source to destination , making itself unavailable for other tasks. This situation is aggravated because access to I/O devices over a peripheral bus is generally slower than the normal system RAM.
- 5. DMA OPERATION There are three independent channels for DMA operation : SINGLE TRANSFER BLOCK TRANSFER BRUST BLOCK TRANSFER
- 6. SINGLE TRANSFER SINGLE TRANSFER – each trigger causes a single transfer . The module will disable itself when DMAXSZ number of transfers have occurred (setting it to zero prevents transfer). The DMAXSA and DMAXDA registers set the addresses to be transferred to and from. The DMAXCTL register also allows these addresses to be incremented or decremented by one or two bytes with each transfer.
- 7. BLOCK TRANSFER BLOCK TRANSFER – an entire block is transferred on each trigger . The module disables itself when this block transfer is complete. This transfer halts the CPU , and will transfer each memory location one at a time. This module disables the module when the transfer is complete.
- 8. BURST- BLOCK TRANSFER BURST BLOCK TRANSFER – this is very similar to block transfer mode except that the CPU and the DMA transfer can interleave their operation. This reduces the CPU to 20% while the DMA is going on , but the CPU will not be stopped altogether. The interrupt occurs when the block has completely transfer and this mode disables the module when the transfer is complete.
- 11. USES DMA is essential to high performance embedded systems. It is also essential in providing so called zero copy implementations of peripheral device drivers as well as functionalities such as network packet routing , audio playback and streaming video. Normal transfer of one data byte takes up to 29 clock cycles . The DMA transfer requires only 5 clock cycles. Nowadays , DMA can transfer data as fast as 60 M byte per second.
- 12. LIMITATIONS Writing to flash requires setting the DMA ON FETCH bit . If this is not done , the results of the DMA operation are “unpredictable”. Also the behaviour and settings of the DMA module should only be modified when the module is disabled. Each DMA channel has its own flag but the interrupt vector is shared with the DAC .
- 13. CONCLUSION DMA is essential to high performance embedded systems and also playing an essential feature of all modern computers , as it allows devices to transfer data without subjecting the CPU to a heavy overhead .