From http://www.digitalprosound.com/Htm/TechStuff/June/SCSI-Part3_3.htm
What is the difference between regular DMA and bus mastering?
Plenty! First, let's look at bus mastering again but from a DMA point of view. A bus is a data transport. Bus mastering is a very advanced means of transporting data to and from devices and/or memory using the PCI bus as a conduit. A device that issues read and write operations to memory and/or I/O slave devices is considered the master, although a master device can have slave memory and/or I/O ports available to be accessed by other masters. For example, an Ethernet controller must convey data it receives from over the LAN and must also access data to send over the LAN as a bus master, but acts as a slave when the CPU, acting as a master, programs it to initialize and to specify where it must get and put data.
Only one bus master can own, or "drive" the bus at a given instant, and the bus is responsible for arbitrating bus master requests from the various bus master devices. A bus master device will request access to the bus, which is granted immediately providing no other master has it at the moment. If another master device has been granted access, the new one must wait until the first one completes its single or burst transfer, or the bus arbiter times out and yanks the access away in favor of the new requesting master, which ever happens first. If an operation is interrupted by a timeout, it is resumed when that issuing master receives its turn again. The CPU is a bus master device, and is always present.
The Intel PIIX family of IDE controllers found in all modern Intel chipsets for the x86 family are bus master devices. The SoundBlaster Live! is a bus master device that accesses main memory through the bus to read samples. There are many peripherals which use bus mastering on the PCI bus to free the CPU from actually doing every transfer, for example, video cards, network cards, SCSI controllers, other storage devices, and so on. Note that bus mastering transfers do not require and therefore do not tie up the DMA channels like normal DMA devices do.
Normal DMA is controlled by a chip. The DMA chip itself is a bus master device. It can be programmed by the CPU to perform transfers from memory to I/O, or I/O to memory (some also allow memory to memory, but is not in the case with the PC, although two DMA channels can be used to do that given some fancy driver footwork). Therefore, the DMA system acts as a bus master to perform the programmed operation while the CPU can be doing something else. The DMA controller sends a signal to the CPU when the transfer is complete. DMA is used to perform transfers without CPU intervention to or from peripherals that don't have bus master capabilities. DMA issues accesses similar to standard bus I/O accesses, but with the addition of handshaking lines DMA_Request and DMA_Acknowledge. These signals are present on the bus for each DMA channel. A slave device must handle these handshaking lines to be able to be operated through DMA. Obviously, this is a much simpler system than having to support all the complex and necessary logic in a bus master device. The main limitations of a DMA capable slave compared with a bus master peripheral, are:
1) The DMA slave is passive. It is the CPU which must specify the transfers to be done. The bus master device can perform transfers by its own initiative without restrictions.
2) DMA can only transfer blocks of contiguous memory content, and only one block for each programmed transaction. The bus masters can access memory or I/O following any pattern without restriction.
3) In the case of the PC, the DMA device can only transfer blocks of up to 64 KBytes, and always on 64 KByte boundaries, which limits its utility. In older PCs, the DMA system could only access the first megabyte of memory. Later it was extended to the first 16 megabytes and currently the DMA device can more often access all memory, but always within 64 KByte boundaries for each operation.
4) DMA is generally slower, although there are new faster modes and burst timing modes achieving considerable throughputs. These modes must be specifically supported by the slaves in order to used them. The original Intel 8237 DMA controller was extremely slow. So slow that disk transfers were more efficiently done by the CPU using PIO mode 4 because DMA would become the bottleneck. In the best theoretical case (that was never meet) it could only transfer 4 MB/s. The reality was more like 1 MB/s.
அதாவது என்ன வித்தியாசம் ன, A PCI Device , BUS Master Mode ல PCI BUS ah பயன்படுத்தி , எப்பெல்லாம் கெடைக்குதோ(PCI arbitration ல ஜெய்ச்சதுக்கு அப்புறம் ) அப்ப transfer பன்னிக்கும் ..
ஆனா , DMA என்பது தனி CHIP இதை பயன்படுத்தி CPU எங்கெல்லாம் படிக்க / எழுத நினைக்குதோ அங்க Transfer ah initiate செய்யும் ..
~Suresh
What is the difference between regular DMA and bus mastering?
Plenty! First, let's look at bus mastering again but from a DMA point of view. A bus is a data transport. Bus mastering is a very advanced means of transporting data to and from devices and/or memory using the PCI bus as a conduit. A device that issues read and write operations to memory and/or I/O slave devices is considered the master, although a master device can have slave memory and/or I/O ports available to be accessed by other masters. For example, an Ethernet controller must convey data it receives from over the LAN and must also access data to send over the LAN as a bus master, but acts as a slave when the CPU, acting as a master, programs it to initialize and to specify where it must get and put data.
Only one bus master can own, or "drive" the bus at a given instant, and the bus is responsible for arbitrating bus master requests from the various bus master devices. A bus master device will request access to the bus, which is granted immediately providing no other master has it at the moment. If another master device has been granted access, the new one must wait until the first one completes its single or burst transfer, or the bus arbiter times out and yanks the access away in favor of the new requesting master, which ever happens first. If an operation is interrupted by a timeout, it is resumed when that issuing master receives its turn again. The CPU is a bus master device, and is always present.
The Intel PIIX family of IDE controllers found in all modern Intel chipsets for the x86 family are bus master devices. The SoundBlaster Live! is a bus master device that accesses main memory through the bus to read samples. There are many peripherals which use bus mastering on the PCI bus to free the CPU from actually doing every transfer, for example, video cards, network cards, SCSI controllers, other storage devices, and so on. Note that bus mastering transfers do not require and therefore do not tie up the DMA channels like normal DMA devices do.
Normal DMA is controlled by a chip. The DMA chip itself is a bus master device. It can be programmed by the CPU to perform transfers from memory to I/O, or I/O to memory (some also allow memory to memory, but is not in the case with the PC, although two DMA channels can be used to do that given some fancy driver footwork). Therefore, the DMA system acts as a bus master to perform the programmed operation while the CPU can be doing something else. The DMA controller sends a signal to the CPU when the transfer is complete. DMA is used to perform transfers without CPU intervention to or from peripherals that don't have bus master capabilities. DMA issues accesses similar to standard bus I/O accesses, but with the addition of handshaking lines DMA_Request and DMA_Acknowledge. These signals are present on the bus for each DMA channel. A slave device must handle these handshaking lines to be able to be operated through DMA. Obviously, this is a much simpler system than having to support all the complex and necessary logic in a bus master device. The main limitations of a DMA capable slave compared with a bus master peripheral, are:
1) The DMA slave is passive. It is the CPU which must specify the transfers to be done. The bus master device can perform transfers by its own initiative without restrictions.
2) DMA can only transfer blocks of contiguous memory content, and only one block for each programmed transaction. The bus masters can access memory or I/O following any pattern without restriction.
3) In the case of the PC, the DMA device can only transfer blocks of up to 64 KBytes, and always on 64 KByte boundaries, which limits its utility. In older PCs, the DMA system could only access the first megabyte of memory. Later it was extended to the first 16 megabytes and currently the DMA device can more often access all memory, but always within 64 KByte boundaries for each operation.
4) DMA is generally slower, although there are new faster modes and burst timing modes achieving considerable throughputs. These modes must be specifically supported by the slaves in order to used them. The original Intel 8237 DMA controller was extremely slow. So slow that disk transfers were more efficiently done by the CPU using PIO mode 4 because DMA would become the bottleneck. In the best theoretical case (that was never meet) it could only transfer 4 MB/s. The reality was more like 1 MB/s.
அதாவது என்ன வித்தியாசம் ன, A PCI Device , BUS Master Mode ல PCI BUS ah பயன்படுத்தி , எப்பெல்லாம் கெடைக்குதோ(PCI arbitration ல ஜெய்ச்சதுக்கு அப்புறம் ) அப்ப transfer பன்னிக்கும் ..
ஆனா , DMA என்பது தனி CHIP இதை பயன்படுத்தி CPU எங்கெல்லாம் படிக்க / எழுத நினைக்குதோ அங்க Transfer ah initiate செய்யும் ..
~Suresh
