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There are signals DACK, DRQ, and TC. When a peripheral wants to move a byte or 2 bytes into memory (is dependent on whether 8 bit or 16 bit DMA channel is in use -- 0,1,2,3 are 8-bit, 5,6,7 are 16-bit), it issues DRQ. DMA controller chats with CPU and after some time DMA controller issues DACK. Seeing DACK, the peripheral puts it's byte on data bus, DMA controller takes it and puts it in memory. If it was the last byte/word to move, DMA controller sets up also TC during the DACK. When peripheral sees TC, it is possible it will not want any more movements,
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What is DMA structure?
Using interrupts driven device drivers to transfer data to or from hardware devices works well when the amount of data is reasonably low. For example a 9600 baud modem can transfer approximately one character every millisecond ( 'th second). If the interrupt latency, the amount of time that it takes between the hardware device raising the interrupt and the device driver's interrupt handling routine being called, is low (say 2 milliseconds) then the overall system impact of the data transfer is very low. The 9600 baud modem data transfer would only take 0.002% of the CPU's processing time. For high speed devices, such as hard disk controllers or ethernet devices the data transfer rate is a lot higher. A SCSI device can transfer up to 40 Mbytes of information per second. Direct Memory Access, or DMA, was invented to solve this problem. A DMA controller allows devices to transfer data to or from the system's memory without the intervention of the processor. A PC's ISA DMA controller has 8 DMA channels of which 7 are available for use by the device drivers. Each DMA channel has associated with it a 16 bit address register and a 16 bit count register. 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 tells the device that it may start the DMA when it wishes. When the transfer is complete the device interrupts the PC. Whilst the transfer is taking place the CPU is free to do other things. Device drivers have to be careful when using DMA. First of all the DMA controller knows nothing of virtual memory, it only has access to the physical memory in the system. Therefore the memory that is being DMA'd to or from must be a contiguous block of physical memory. This means that you cannot DMA directly into the virtual address space of a process. You can however lock the processes physical pages into memory, preventing them from being swapped out to the swap device during a DMA operation. Secondly, the DMA controller cannot access the whole of physical memory. The DMA channel's address register represents the first 16 bits of the DMA address, the next 8 bits come from the page register. This means that DMA requests are limited to the bottom 16 Mbytes of memory. DMA channels are scarse resources, there are only 7 of them, and they cannot be shared between device drivers. Just like interrupts the device driver must be able to work out which DMA channel it should use. Like interrupts, some devices have a fixed DMA channel. The floppy device, for example, always uses DMA channel 2. Sometimes the DMA channel for a device can be set by jumpers, a number of ethernet devices use this technique. The more flexible devices can be told (via their CSRs) which DMA channels to use and, in this case, the device driver can simple pick a free DMA channel to use. Linux tracks the usage of the DMA channels using a vector of dma_chan data structures (one per DMA channel). The dma_chan data structure contains just two fields, a pointer to a string describing the owner of the DMA channel and a flag indicating if the DMA channel is allocated or not. It is this vector of dma_chan data structures that is printed when you cat /proc/dma
What are types of DMA?
DMA is a mechanism through which a hardware device can transfer data to or from memory without using the processor. The processor is required to set up the transfer, and the device will signal the processor when it has completed the transfer. The advantage of this system is that the processor can perform other tasks while the DMA transfer is being performed. There are several types of DMA used in Windows 2000 and later: ; Common-buffer DMA : Common-buffer DMA is performed when the system can allocate a single buffer that is accessible by both the hardware and the software. The driver is responsible for synchronizing accesses to the buffer. The memory is not cached, making this synchronization easier for the driver. After setting up a common buffer, both the driver and the hardware can write directly to the addresses in the buffer without any intervention from the HAL. ; Packet DMA : Packet DMA is performed when there is a single existing buffer that must be mapped for use by the hardware. An example of using packet DMA is the transfer of a file from memory to a disk. Using common-buffer DMA in this situation would be wasteful, because the file would have to be transferred to the common buffer before the hardware could transfer it to the disk. Instead, the HAL is consulted; it gives the driver the information it needs to help the hardware find the actual buffer in memory. This operation is complicated by the need for the routines involved to work across different architectures. ; Scatter/gather DMA : Scatter/gather DMA is a shortcut method that sets up several packet DMA transfers at once. If you are transferring a packet over the network, for example, each part of the network stack adds its own header (TCP, IP, Ethernet, and so forth). These headers are all allocated from different places in memory. In this case, the scatter/gather DMA saves time by issuing a batch request to the HAL to map each header plus the data segment for access by the hardware. Instead of having to call the packet DMA routines on each part of the packet, this method calls each routine once, and lets the HAL be responsible for mapping each one individually. Note Scatter/gather capability does not mean that the device can use the scatter/gather routines. Scatter/gather capability refers to a flag in the device description that indicates that the device is able to read or write from any area in memory, instead of just a certain range. ; System DMA : System DMA is performed by programming the system DMA controller on the motherboard to do the transfer directly. Only ISA cards can use system DMA.
Which mode involves the CPU and slower than DMA mode?
PIO
Why are DMA channels not as popular as they once were with high-speed devices?
ATA is much faster.
Name one system resource that a video card most likely will not need?
DMA channel
What types of operations is DMA used to accelerate?
DMA operations
When was DMA Distribuidora created?
DMA Distribuidora was created in 1950.
When was DMA - magazine - created?
DMA - magazine - was created in 1993.
When did DMA - magazine - end?
DMA - magazine - ended in 2003.
Does a DMA channel bypass the CPU?
Yes, DMA does bypass the CPU.
How is DMA different that busmastering?
DMA is the same thing as bus-mastering.
Generally which transfer mode is faster DMA or PIO?
DMA transfers data directly from the drive to memory without involving the CPU. PIO involves the CPU and is slower than DMA mode.
If ECP mode does not work on your parallel port what should you do next?
· Recall the ECP uses a DMA channel. Allow setup to keep DMA3 unless you suspect a conflict with another device trying to use DMA
How many DMA channels are supported by Windows XP?
8 DMA Channels .
How many DMA channels are in AT class?
There are 8 DMA channels in an AT class computer.
How do you get onto anti-dma?
anti-dma code is a code that disables bad egs
What technology has replaced DMA?
If by DMA you mean Direct Memory Access, nothing has replaced it.
