CODEWITHSUNDEEP
cachingdma
dtc
dtc

Reputation: 1849

DMA vs Cache difference

Probably a stupid question for most that know DMA and caches... I just know cache stores memory to somewhere closer to where you can access so you don't have to spend as much time for the I/O.

But what about DMA? It lets you access that main memory with less delay?

Could someone explain the differences, both, or why I'm just confused?

Upvotes: 0

Views: 7559

Answers (3)

fazkan
fazkan

Reputation: 352

I know its a bit late but answering this question will help someone like me I guess, Agreeing with the above answers, I think the question was in relation to cache.

So Yes a cache does store information somewhere closer to the memory, this could be the results of earlier computations. Moreover, whenever a data is found in cache (called a cache hit) the value is used directly. when its not found (called a cache-miss), the processor goes on to calculate the required value. Peripheral Devices (SD cards, USBs etc) can also access this data, which is why on startup we usually invalidate cache data so that the cache line is clean. We also flush cache data on startup so that all the cache data is written back to the main memory for cpu to use, after which we proceed to reset or initialize the cache.

DMA (Direct Memory Access), yes it does let you access the main memory. But I think the better definition is, it lets you access the system register, which can only be accessed by the processor. @Ronnie and @Yann Ramin were both correct in that DMA can be a device hardware, so it can be used by your serial peripheral to access system registers, but it can also be used for memory to memory transfers between two cores.

You can read up further on DMA from wikipedia, about the modes in which DMA can access the system memory. I ll explain it simply

Burst mode: DMA takes full control of the bus, CPU is idle during this time. Data is transferred in burst (as a whole) without interruption.

Cycle stealing mode: In this data is transfered one byte at a time, transfer is slow, but CPU is not idle.

Upvotes: 1

Ronnie
Ronnie

Reputation: 146

I agree fully with the first answer, and there are some common additions...

On most DMA hardwares you can also set it up to do memory to memory transfers - there are not always external devices involved. Also depending on the system you may or may not need to sync the CPU-cache in software before (or after the transfer), since the data the DMA transfers into/from memory may be done without the knowledge of the CPU-cache.

The benefit of doing any DMA is that the CPU(s) is/are able to do other things simultaneously. Of course when the CPU also needs to access the memory, only one can gain access and the other must wait.

Mem to mem DMA is often used in embedded systems to increase performance, or may be vital to be able to access some parts of the memory at all.

To answer the question, DMA and CPU-cache are totally different things and not comparable.

Upvotes: 1

Yann Ramin
Yann Ramin

Reputation: 33187

DMA is a hardware device that can move to/from memory without using CPU instructions.

For instance, a hardware device (lets say, your PCI sound device) wants audio to play back. You can either:

  1. Write a word at a time via a CPU mov instructions.
  2. Configure the DMA device. You give it a start address, a destination, and the number of bytes to copy. The transfer now occurs while the CPU does something else instead of spoon feeding the audio device.

DMA can be very complex (scatter gather, etc), and varies by bus type and system.

Upvotes: 4

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