DMA operation
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1. Direct Memory Access (DMA) allows input/output devices to directly access main memory, bypassing the CPU to speed up memory transfers. This is managed by a DMA controller. 2. During a DMA transfer, the DMA controller gains control of the address bus, data bus, and control bus from the microprocessor to transfer data directly between an I/O port and memory. 3. The DMA controller has several options for transferring data, including cycle stealing for single byte transfers, burst mode for block transfers using address sequencing, and hidden DMA which occurs transparently when the CPU is not using the bus.
DMA operation
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- 3. DMA Basic DMA operation DMA controller Data transfer with DMA controller DMA Controller options for data transfer Data Transfer modes Advantage & Disadvantage 3
- 4. • Direct Memory Access (DMA) is a method that allows an input/output (I/O) device to send or receive data directly to or from the main memory, bypassing the CPU to speed up memory operations. The process is managed by a chip known as a DMA controller (DMAC). DMA 4
- 5. •DMA transfers are performed by a control circuit that is part of the I/O device interface. • It refer to this circuit as a DMA controller. • The DMA controller performs the function that would normally be carried out by the processer when accessing the main memory. • Device wishing to perform DMA asserts the processors bus request signal • Processor completes the current bus cycle and then asserts the bus grant signal to the device. 5 DMA
- 6. • The direct memory access (DMA) I/O technique provides direct access to the memory while the microprocessor is temporarily disabled. • A DMA controller temporarily borrows the address bus, data bus, and control bus from the microprocessor and transfers the data bytes directly between an I/O port and a series of memory locations. • The DMA transfer is also used to do high-speed memory-to- memory transfers. • Two control signals are used to request and acknowledge a DMA transfer in the microprocessor-based system. 6
- 8. During a block input byte transfer, the following sequence occurs as the data byte is sent from the interface to the memory: • The interface sends the DMA controller a request for DMA service. • A Bus request is made to the HOLD pin (active High) on the 8086 microprocessor and the controller gains control of the bus. • A Bus grant is returned to the DMA controller from the Hold acknowledge (HLDA) pin (active High) on the 8086 microprocessor.
- 9. • The DMA controller places contents of the address register onto the address bus. • The controller sends the interface a DMA acknowledgment, which tells the interface to put data on the data bus. (For an output it signals the interface to latch the next data placed on the bus.) • The data byte is transferred to the memory location indicated by the address bus. • The interface latches the data.
- 10. • The Bus request is dropped, the HOLD pin goes Low, and the controller relinquishes the bus. • The Bus grant from the 8086 microprocessor is dropped and the HLDA pin goes Low. • The address register is incremented by 1. • The byte count is decremented by 1. • If the byte count is non-zero, return to step 1, otherwise stop
- 12. • The DMA Controller has several options available for the transfer of data. • They are:- Cycle steal Burst transfer Hidden DMA
- 13. 1) CYCLE STEAL A read or write signal is generated by the DMAC, and the I/O device either generates or latches the data. The DMAC effectively steals cycles from the processor in order to transfer the byte, so single byte transfer is also known as cycle stealing
- 14. 2). BURST TRANSFER To achieve block transfers, some DMAC's incorporate an automatic sequencing of the value presented on the address bus. A register is used as a byte count, being decremented for each byte transfer, and upon the byte count reaching zero, the DMAC will release the bus. When the DMAC operates in burst mode, the CPU is halted for the duration of the data transfer.
- 15. 3) HIDDEN DMA It is possible to perform hidden DMA, which is transparent to the normal operation of the CPU. In other words, the bus is grabbed by the DMAC when the processor is not using it. The DMAC monitors the execution of the processor, and when it recognises the processor executing an instruction which has sufficient empty clock cycles to perform a byte transfer, it waits till the processor is decoding the op code, then grabs the bus during this time. The processor is not slowed down, but continues processing normally. Naturally, the data transfer by the DMAC must be completed before the processor starts
- 16. DMA Data Transfer Modes 1. Single Transfer Mode 2. Block Transfer Mode 3. Demand Transfer Mode 4. Cascade Mode: 5. Memory to memory transfer Single Transfer Mode: In Single Transfer mode the device is programmed to make one transfer only. In single mode only one byte is transferred per request. Block Transfer Mode: Once the DMA controller is granted access to the system bus by the CPU, it transfers all bytes of data in the data block. This mode is called "Block Transfer Mode". once a Block transfer is started, it runs until the transfer count reaches zero
- 17. DMA Data Transfer Modes Demand Transfer Mode: Demand Mode DMA allows PC hardware to transfer many bytes of data, via a DMA channel, with only a single programming of the DMA controller. Cascade Mode: Used to cascade additional DMA controllers Actual bus signals is executed by cascaded chip. Memory to memory transfer: To perform the transfer of the block of data from one set of memory address to another one ,this mode is used. This means data can be transferred from one memory device to another memory device.
- 18. • DMA allows a peripheral device to read from/write to memory without going through the CPU • DMA allows for faster processing since the processor can be working on something else while the peripheral can be populating memory. • DMA transfer requires a DMA controller to carry out the operation, hence cost of the system increases. • Cache Coherence problems.