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operations while the transfer is in progress and it finally receives an interrupt from the direct memory access controller dmac when the operation is done this feature is useful at any time that the cpu cannot keep up with the rate of data transfer or when the cpu needs to perform work while waiting for a relatively slow data transfer many hardware systems use dma including disk drive controllers graphics cards network cards sound cards and dedicated dma controllers acting on behalf of the aforementioned device types dma is also used for intra chip data transfer in some multi core processors computers that have dma capability can transfer data to and from devices with much less cpu overhead than computers without dma dma can also be used for memory to memory transfers dma can offload expensive memory operations such as large copies or scatter gather operations from the cpu to a dma controller an implementation example is the i o acceleration technology dma is of interest in network on chip and in memory computing architectures while hardware acceleration is usually vendor specific direct memory access can be used by the class driver 2 principles edit third party edit motherboard of a nextcube computer 1990 the two large integrated circuits below the middle of the image are the dma controller left and unusual an extra dedicated dma controller right for the magneto optical disc used instead of a hard disk drive in the first series of this computer model standard dma also called third party dma uses a dma controller a dma controller can generate memory addresses and initiate memory read or write cycles it contains several hardware registers that can be written and read by the cpu these include a memory address register a count register and one or more control registers depending on what features the dma controller provides these control registers might specify some combination of the source the destination the direction of the transfer reading from the i o device or writing to the i o device 3 to carry out an input output or memory to memory operation the host processor initializes the dma controller with a count of the number of words to transfer and the memory address to use the cpu then commands the peripheral device to initiate a data transfer the dma controller then provides addresses and read write control lines to the system memory each time a byte of data is ready to be transferred between the peripheral device and memory the dma controller increments its internal address register until the full block of data is transferred bus mastering edit in a bus mastering system also known as a first party dma system the cpu and peripherals can each be granted control of the memory bus where a peripheral can become a bus master it can directly write to system memory without the involvement of the cpu providing memory address and control signals as required some measures must be provided to put the processor into a hold condition so that bus contention does not occur the peripheral itself may have hardware registers such as address and count registers to control the dma operation modes of operation edit burst mode edit in burst mode an entire block of data is transferred in one contiguous sequence once the dma controller is granted access to the system bus by the cpu it transfers all bytes of data in the data block before releasing control of the system bus back to the cpu this renders the cpu inactive for relatively long periods of time the mode is also called block transfer mode cycle stealing mode edit the cycle stealing mode is used in systems in which the cpu should not be disabled for the length of time needed for burst transfer modes in the cycle stealing mode the dma controller obtains access to the system bus the same way as in burst mode using br bus request and bg bus grant signals which are the two signals controlling the interface between the cpu and the dma controller however in cycle stealing mode after one unit of data transfer the control of the system bus is deasserted to the cpu via bg it is then continually requested again via br transferring one unit of data per request until the entire block of data has been transferred 4 by continually obtaining and releasing the control of the system bus the dma controller essentially interleaves data transfers with cpu activity the cpu processes an instruction then the dma controller transfers one data value and so on data is not transferred as quickly but the cpu is not idled for as long as in burst mode transparent mode edit transparent mode also called hidden dma data transfer mode is the most efficient mode in terms of overall system performance in transparent mode the dma controller transfers data only when the cpu is performing operations that do not use the system buses the primary advantage of transparent mode is that the cpu never stops executing its programs while the disadvantages are that dma transfers are not prioritized and the hardware needs to predict when the cpu does not intend to use the system buses cache coherency edit dma can lead to cache coherency problems imagine a cpu equipped with a cache and an external memory that can be accessed directly by devices using dma when the cpu accesses location x in the memory the current value will be stored in the cache subsequent operations on x will update the cached copy of x but not the external memory version of x assuming a write back cache if the cache is not flushed to the memory before the next time a device tries to access x the device will receive a stale value of x similarly if the cached copy of x is not invalidated when a device writes a new value to the memory then the cpu will operate on a stale value of x this issue can be addressed in one of two ways in system design cache coherent systems implement a method in hardware called bus snooping whereby external writes are signaled to the cache controller which then performs a cache invalidation for dma writes or cache flush for dma reads non coherent systems leave this to software where the os must then ensure that the cache lines are flushed before an outgoing dma transfer is started and invalidated before a memory range affected by an incoming dma transfer is accessed the os must make sure that the memory range is not accessed by any running threads in the meantime the latter approach introduces some overhead to the dma operation as most hardware requires a loop to invalidate each cache line individually hybrids also exist where the secondary l2 cache is coherent while the l1 cache typically on cpu is managed by software examples edit isa edit in the original ibm pc and the follow up pc xt there was only one intel 8237 dma controller capable of providing four dma channels numbered 0 3 these dma channels performed 8 bit transfers as the 8237 was an 8 bit device ideally matched to the pc s i8088 cpu bus architecture could only address the first i8086 8088 standard megabyte of ram and were limited to addressing single 64 kb segments within that space although the source and destination channels could address different segments additionally the controller could only be used for transfers to from or between expansion bus i o devices as the 8237 could only perform memory to memory transfers using channels 0 1 of which channel 0 in the pc xt was dedicated to dynamic memory refresh this prevented it from being used as a general purpose blitter and consequently block memory moves in the pc limited by the general pio speed of the cpu were very slow with the ibm pc at the enhanced at bus more familiarly retronymed as the industry standard architecture isa added a second 8237 dma controller to provide three additional and as highlighted by resource clashes with the xt s additional expandability over the original pc much needed channels 5 7 channel 4 is used as a cascade to the first 8237 isa dma s extended 24 bit address bus width allows it to access up to 16 mb of lower memory 5 the page register was also rewired to address the full 16 mb memory address space of the 80286 cpu this second controller was also integrated in a way capable of performing 16 bit transfers when an i o device is used as the data source and or destination as it actually only processes data itself for memory to memory transfers otherwise simply controlling the data flow between other parts of the 16 bit system making its own data bus width relatively immaterial doubling data throughput when the upper three channels are used for compatibility the lower four dma channels were still limited to 8 bit transfers only and whilst memory to memory transfers were now technically possible due to the freeing up of channel 0 from having to handle dram refresh from a practical standpoint they were of limited value because of the controller s consequent low throughput compared to what the cpu could now achieve i e a 16 bit more optimised 80286 running at a minimum of 6 mhz vs an 8 bit controller locked at 4 77 mhz in both cases the 64 kb segment boundary issue remained with individual transfers unable to cross segments instead wrapping around to the start of the same segment even in 16 bit mode although this was in practice more a problem of programming complexity than performance as the continued need for dram refresh however handled to monopolise the bus approximately every 15 μs prevented use of large and fast but uninterruptible block transfers due to their lagging performance 1 6 mb s maximum 8 bit transfer capability at 5 mhz 6 but no more than 0 9 mb s in the pc xt and 1 6 mb s for 16 bit transfers in the at due to isa bus overheads and other interference such as memory refresh interruptions 1 and unavailability of any speed grades that would allow installation of direct replacements operating at speeds higher than the original pc s standard 4 77 mhz clock these devices have been effectively obsolete since the late 1980s 80386 and 32 bit systems edit particularly the advent of the 80386 processor in 1985 and its capacity for 32 bit transfers although great improvements in the efficiency of address calculation and block memory moves in intel cpus after the 80186 meant that pio transfers even by the 16 bit bus 286 and 386sx could still easily outstrip the 8237 as well as the development of further evolutions to eisa or replacements for mca vlb and pci the isa bus with their own much higher performance dma subsystems up to a maximum of 33 mb s for eisa 40 mb s mca typically 13 mb s vlb pci made the original dma controllers seem more of a performance millstone than a booster they were supported to the extent that they are required to support built in legacy pc hardware on later machines the pieces of legacy hardware that continued to use isa dma after 32 bit expansion buses became common were sound blaster cards that needed to maintain full hardware compatibility with the sound blaster standard and super i o devices on motherboards that often integrated a built in floppy disk controller an irda infrared controller when fir fast infrared mode is selected and an ieee 1284 parallel port controller when ecp mode is selected in cases where an original 8237s or direct compatibles were still used transfer to or from these devices may still be limited to the first 16 mb of main ram regardless of the system s actual address space or amount of installed memory each dma channel has a 16 bit address register and a 16 bit count register associated with it to initiate a data transfer the device driver sets up the dma channel s address and count registers together with the direction of the data transfer read or write it then instructs the dma hardware to begin the transfer when the transfer is complete the device interrupts the cpu scatter gather or vectored i o dma allows the transfer of data to and from multiple memory areas in a single dma transaction it is equivalent to the chaining together of multiple simple dma requests the motivation is to offload multiple input output interrupt and data copy tasks from the cpu drq stands for data request dack for data acknowledge these symbols seen on hardware schematics of computer systems with dma functionality represent electronic signaling lines between the cpu and dma controller each dma channel has one request and one acknowledge line a device that uses dma must be configured to use both lines of the assigned dma channel 16 bit isa permitted bus mastering 7 standard isa dma assignments citation needed dram refresh obsolete user hardware usually isa sound card floppy disk controller wdma for hard disk controller replaced by udma modes parallel port ecp capable port or certain soundblaster clones like the opti 928 8237 dma controller hard disk controller ps 2 only or user hardware usually an isa sound card user hardware user hardware pci edit a pci architecture has no central dma controller unlike isa instead a pci device can request control of the bus become the bus master and request to read from and write to system memory more precisely a pci component requests bus ownership from the pci bus controller usually pci host bridge and pci to pci bridge 8 which will arbitrate if several devices request bus ownership simultaneously since there can only be one bus master at one time when the component is granted ownership it will issue normal read and write commands on the pci bus which will be claimed by the pci bus controller as an example on an intel core based pc the southbridge will forward the transactions to the memory controller which is integrated on the cpu die using dmi which will in turn convert them to ddr operations and send them out on the memory bus as a result there are quite a number of steps involved in a pci dma transfer however that poses little problem since the pci device or pci bus itself are an order of magnitude slower than the rest of the components see list of device bandwidths a modern x86 cpu may use more than 4 gb of memory either utilizing the native 64 bit mode of x86 64 cpu or the physical address extension pae a 36 bit addressing mode in such a case a device using dma with a 32 bit address bus is unable to address memory above the 4 gb line the new double address cycle dac mechanism if implemented on both the pci bus and the device itself 9 enables 64 bit dma addressing otherwise the operating system would need to work around the problem by either using costly double buffers dos windows nomenclature also known as bounce buffers freebsd linux or it could use an iommu to provide address translation services if one is present i oat edit as an example of dma engine incorporated in a general purpose cpu some intel xeon chipsets include a dma engine called i o acceleration technology i oat which can offload memory copying from the main cpu freeing it to do other work 10 in 2006 intel s linux kernel developer andrew grover performed benchmarks using i oat to offload network traffic copies and found no more than 10 improvement in cpu...
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