Direct Memory Entry

From gpu
Revision as of 04:53, 27 September 2025 by BettyDelamothe (talk | contribs) (Created page with "<br>With out DMA, when the CPU is utilizing programmed enter/output, it is typically absolutely occupied for the whole duration of the learn or write operation, and is thus unavailable to carry out different work. With DMA, the CPU first initiates the transfer, then it does different operations while the transfer is in progress, and it lastly receives an interrupt from the DMA controller (DMAC) when the operation is finished. This characteristic is helpful at any time th...")
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to navigation Jump to search


With out DMA, when the CPU is utilizing programmed enter/output, it is typically absolutely occupied for the whole duration of the learn or write operation, and is thus unavailable to carry out different work. With DMA, the CPU first initiates the transfer, then it does different operations while the transfer is in progress, and it lastly receives an interrupt from the DMA controller (DMAC) when the operation is finished. This characteristic is helpful at any time that the CPU cannot keep up with the rate of knowledge switch, or when the CPU must perform work while ready for a relatively slow I/O information transfer. Many hardware programs use DMA, including disk drive controllers, graphics playing cards, network playing cards and sound cards. DMA can be used for intra-chip data switch in some multi-core processors. Computers that have DMA channels can switch data to and from gadgets with much less CPU overhead than computer systems without DMA channels. Equally, a processing circuitry inside a multi-core processor can switch information to and from its native memory with out occupying its processor time, permitting computation and information transfer to proceed in parallel.



DMA can be used for "memory to memory" copying or moving of data within memory. DMA can offload expensive memory operations, similar to large copies or scatter-gather operations, from the CPU to a dedicated DMA engine. An implementation example is the I/O Acceleration Technology. DMA is of interest in network-on-chip and in-memory computing architectures. Customary DMA, also known as third-social gathering DMA, uses a DMA controller. A DMA controller can generate memory addresses and initiate Memory Wave read or write cycles. It contains a number of hardware registers that can be written and skim by the CPU. These embody a memory tackle register, a byte count register, and one or more control registers. Relying on what features the DMA controller offers, these control registers might specify some mixture of the source, the vacation spot, the direction of the transfer (studying from the I/O machine or writing to the I/O device), the dimensions of the transfer unit, and/or the number of bytes to transfer in a single burst.



To carry out an input, output or memory-to-memory operation, the host processor initializes the DMA controller with a rely of the variety of words to transfer, and the Memory Wave clarity support handle to make use of. The CPU then commands the peripheral device to provoke an information transfer. The DMA controller then gives addresses and read/write management traces to the system memory. Every time a byte of knowledge is ready to be transferred between the peripheral system and memory, the DMA controller increments its inner deal with register till the full block of data is transferred. Some examples of buses utilizing third-get together DMA are PATA, USB (earlier than USB4), and SATA; nevertheless, their host controllers use bus mastering. In a bus mastering system, also called a primary-get together DMA system, the CPU and peripherals can every be granted management of the memory bus. The place a peripheral can develop into a bus master, it could actually instantly write to system memory without the involvement of the CPU, providing memory deal with and management alerts as required.



Some measures have to be offered to place the processor right into a hold situation in order that bus contention doesn't occur. In burst mode, a complete block of knowledge is transferred in a single contiguous sequence. Once the DMA controller is granted access to the system bus by the CPU, it transfers all bytes of knowledge in the info block earlier than releasing control of the system buses back to the CPU, however renders the CPU inactive for comparatively lengthy periods of time. The mode can be referred to as "Block Switch Mode". The cycle stealing mode is utilized in methods by which the CPU should not be disabled for the length of time needed for burst switch modes. Within the cycle stealing mode, the DMA controller obtains entry to the system bus the identical manner as in burst mode, utilizing BR (Bus Request) and BG (Bus Grant) indicators, which are the two alerts controlling the interface between the CPU and the DMA controller. Nevertheless, in cycle stealing mode, after one unit of knowledge transfer, the management of the system bus is deasserted to the CPU through BG.