Copyright © 1995 &a.uhclem;, All Rights Reserved. 18 October 1995. Direct Memory Access (DMA) is a method of allowing data to be moved from one location to another in a computer without intervention from the central processor (CPU). The way that the DMA function is implemented varies between computer architectures, so this discussion will limit itself to the implementation and workings of the DMA subsystem on the IBM Personal Computer (PC), the IBM PC/AT and all of its successors and clones. The PC DMA subsystem is based on the Intel 8237 DMA controller. The 8237 contains four DMA channels that can be programmed independently and any of the channels may be active at any moment. These channels are numbered 0, 1, 2 and 3. Starting with the PC/AT, IBM added a second 8237 chip, and numbered those channels 4, 5, 6 and 7. The original DMA controller (0, 1, 2 and 3) moves one byte in each transfer. The second DMA controller (4, 5, 6, and 7) moves 16-bits in each transfer. The two controllers are identical and the difference in transfer size is caused by the way the second controller is wired into the system. The 8237 has two electrical signals for each channel, named DRQ and -DACK. There are additional signals with the names HRQ (Hold Request), HLDA (Hold Acknowledge), -EOP (End of Process), and the bus control signals -MEMR (Memory Read), -MEMW (Memory Write), -IOR (I/O Read), and -IOW (I/O Write). The 8237 DMA is known as a ``fly-by'' DMA controller. This means that the data being moved from one location to another does not pass through the DMA chip and is not stored in the DMA chip. Subsequently, the DMA can only transfer data between an I/O port and a memory address, but not between two I/O ports or two memory locations. Note: The 8237 does allow two channels to be connected together to allow memory-to-memory DMA operations in a non-``fly-by'' mode, but nobody in the PC industry uses this scarce resource this way since it is faster to move data between memory locations using the CPU. In the PC architecture, each DMA channel is normally activated only when the hardware that uses that DMA requests a transfer by asserting the DRQ line for that channel. A Sample DMA transfer

Here is an example of the steps that occur to cause a DMA transfer. In this example, the floppy disk controller (FDC) has just read a byte from a diskette and wants the DMA to place it in memory at location 0x00123456. The process begins by the FDC asserting the DRQ2 signal to alert the DMA controller. The DMA controller will note that the DRQ2 signal is asserted. The DMA controller will then make sure that DMA channel 2 has been programmed and is enabled. The DMA controller also makes sure that none of the other DMA channels are active or have a higher priority. Once these checks are complete, the DMA asks the CPU to release the bus so that the DMA may use the bus. The DMA requests the bus by asserting the HRQ signal which goes to the CPU. The CPU detects the HRQ signal, and will complete executing the current instruction. Once the processor has reached a state where it can release the bus, it will. Now all of the signals normally generated by the CPU (-MEMR, -MEMW, -IOR, -IOW and a few others) are placed in a tri-stated condition (neither high or low) and then the CPU asserts the HLDA signal which tells the DMA controller that it is now in charge of the bus. Depending on the processor, the CPU may be able to execute a few additional instructions now that it no longer has the bus, but the CPU will eventually have to wait when it reaches an instruction that must read something from memory that is not in the internal processor cache or pipeline. Now that the DMA ``is in charge'', the DMA activates its -MEMR, -MEMW, -IOR, -IOW output signals, and the address outputs from the DMA are set to 0x3456, which will be used to direct the byte that is about to transferred to a specific memory location. The DMA will then let the device that requested the DMA transfer know that the transfer is commencing. This is done by asserting the -DACK signal, or in the case of the floppy disk controller, -DACK2 is asserted. The floppy disk controller is now responsible for placing the byte to be transferred on the bus Data lines. Unless the floppy controller needs more time to get the data byte on the bus (and if the peripheral needs more time it alerts the DMA via the READY signal), the DMA will wait one DMA clock, and then de-assert the -MEMW and -IOR signals so that the memory will latch and store the byte that was on the bus, and the FDC will know that the byte has been transferred. Since the DMA cycle only transfers a single byte at a time, the FDC now drops the DRQ2 signal, so that the DMA knows it is no longer needed. The DMA will de-assert the -DACK2 signal, so that the FDC knows it must stop placing data on the bus. The DMA will now check to see if any of the other DMA channels have any work to do. If none of the channels have their DRQ lines asserted, the DMA controller has completed its work and will now tri-state the -MEMR, -MEMW, -IOR, -IOW and address signals. Finally, the DMA will de-assert the HRQ signal. The CPU sees this, and de-asserts the HOLDA signal. Now the CPU activates its -MEMR, -MEMW, -IOR, -IOW and address lines, and it resumes executing instructions and accessing main memory and the peripherals. For a typical floppy disk sector, the above process is repeated 512 times, once for each byte. Each time a byte is transferred, the address register in the DMA is incremented and the counter that shows how many bytes are to be transferred is decremented. When the counter reaches zero, the DMA asserts the EOP signal, which indicates that the counter has reached zero and no more data will be transferred until the DMA controller is reprogrammed by the CPU. This event is also called the Terminal Count (TC). There is only one EOP signal, because only one DMA channel can be active at any instant. If a peripheral wants to generate an interrupt when the transfer of a buffer is complete, it can test for its -DACK signal and the EOP signal both being asserted at the same time. When that happens, it means the DMA won't transfer any more information for that peripheral without intervention by the CPU. The peripheral can then assert one of the interrupt signals to get the processors' attention. The DMA chip itself is not capable of generating an interrupt. The peripheral and its associated hardware is responsible for generating any interrupt that occurs. It is important to understand that although the CPU always releases the bus to the DMA when the DMA makes the request, this action is invisible to both applications and the operating systems, except for slight changes in the amount of time the processor takes to execute instructions when the DMA is active. Subsequently, the processor must poll the peripheral, poll the registers in the DMA chip, or receive an interrupt from the peripheral to know for certain when a DMA transfer has completed. DMA Page Registers and 16Meg address space limitations

You may have noticed earlier that instead of the DMA setting the address lines to 0x00123456 as we said earlier, the DMA only set 0x3456. The reason for this takes a bit of explaining. When the original IBM PC was designed, IBM elected to use both DMA and interrupt controller chips that were designed for use with the 8085, an 8-bit processor with an address space of 16 bits (64K). Since the IBM PC supported more than 64K of memory, something had to be done to allow the DMA to read or write memory locations above the 64K mark. What IBM did to solve this problem was to add a latch for each DMA channel, that holds the upper bits of the address to be read to or written from. Whenever a DMA channel is active, the contents of that latch is written to the address bus and kept there until the DMA operation for the channel ends. These latches are called ``Page Registers''. So for our example above, the DMA would put the 0x3456 part of the address on the bus, and the Page Register for DMA channel 2 would put 0x0012xxxx on the bus. Together, these two values form the complete address in memory that is to be accessed. Because the Page Register latch is independent of the DMA chip, the area of memory to be read or written must not span a 64K physical boundary. If the DMA accesses memory location 0xffff, the DMA will then increment the address register and it will access the next byte at 0x0000, not 0x10000. The results of letting this happen are probably not intended. Note: ``Physical'' 64K boundaries should not be confused with 8086-mode 64K ``Segments'', which are created by adding a segment register with an offset register. Page Registers have no address overlap. To further complicate matters, the external DMA address latches on the PC/AT hold only eight bits, so that gives us 8+16=24 bits, which means that the DMA can only point at memory locations between 0 and 16Meg. For newer computers that allow more than 16Meg of memory, the PC-compatible DMA cannot access locations above 16Meg. To get around this restriction, operating systems will reserve a buffer in an area below 16Meg that also doesn't span a physical 64K boundary. Then the DMA will be programmed to read data to that buffer. Once the DMA has moved the data into this buffer, the operating system will then copy the data from the buffer to the address where the data is really supposed to be stored. When writing data from an address above 16Meg to a DMA-based peripheral, the data must be first copied from where it resides into a buffer located below 16Meg, and then the DMA can copy the data from the buffer to the hardware. In FreeBSD, these reserved buffers are called ``Bounce Buffers''. In the MS-DOS world, they are sometimes called ``Smart Buffers''. DMA Operational Modes and Settings

The 8237 DMA can be operated in several modes. The main ones are: Block/Demand Once the DMA acquires the system bus, an entire block of data is transferred, up to a maximum of 64K. If the peripheral needs additional time, it can assert the READY signal. READY should not be used excessively, and for slow peripheral transfers, the Single Transfer Mode should be used instead. The difference between Block and Demand is the once a Block transfer is started, it runs until the transfer count reaches zero. DRQ only needs to be asserted until -DACK is asserted. Demand Mode will transfer one more bytes until DRQ is de-asserted, then when DRQ is asserted later, the transfer resumes where it was suspended. Older hard disk controllers used Demand Mode until CPU speeds increased to the point that it was more efficient to read the data using the CPU. Cascade This mechanism allows a DMA channel to request the bus, but then the attached peripheral device is responsible for placing addressing information on the bus. This is also known as ``Bus Mastering''. When a DMA channel in Cascade Mode receives control of the bus, the DMA does not place addresses and I/O control signals on the bus like it normally does. Instead, the DMA only asserts the -DACK signal for this channel. Now it is up to the device connected to that DMA channel to provide address and bus control signals. The peripheral has complete control over the system bus, and can do reads and/or writes to any address below 16Meg. When the peripheral is finished with bus, it de-asserts the DRQ line, and the DMA controller can return control to the CPU or to some other DMA channel. Cascade Mode can be used to chain multiple DMA controllers together, and this is exactly what DMA Channel 4 is used for in the PC. When a peripheral requests the bus on DMA channels 0, 1, 2 or 3, the slave DMA controller asserts HLDREQ, but this wire is actually connected to DRQ4 on the primary DMA controller. The primary DMA controller then requests the bus from the CPU using HLDREQ. Once the bus is granted, -DACK4 is asserted, and that wire is actually connected to the HLDA signal on the slave DMA controller. The slave DMA controller then transfers data for the DMA channel that requested it, or the slave DMA may grant the bus to a peripheral that wants to perform its own bus-mastering. Because of this wiring arrangement, only DMA channels 0, 1, 2, 3, 5, 6 and 7 are usable on PC/AT systems. Note: DMA channel 0 was reserved for refresh operations in early IBM PC computers, but is generally available for use by peripherals in modern systems. When a peripheral is performing Bus Mastering, it is important that the peripheral transmit data to or from memory constantly while it holds the system bus. If the peripheral cannot do this, it must release the bus frequently so that the system can perform refresh operations on memory. Since memory read and write cycles ``count'' as refresh cycles (a refresh cycle is actually an incomplete memory read cycle), as long as the peripheral controller continues reading or writing data to sequential memory locations, that action will refresh all of memory. Bus-mastering is found in some SCSI adapters and other high-performance peripheral cards. Autoinitialize This mode causes the DMA to perform Byte, Block or Demand transfers, but when the DMA transfer counter reaches zero, the counter and address is set back to where they were when the DMA channel was originally programmed. This means that as long as the device requests transfers, they will be granted. It is up to the CPU to move new data into the fixed buffer ahead of where the DMA is about to transfer it for output operations, and read new data out of the buffer behind where the DMA is writing on input operations. This technique is frequently used on audio devices that have small or no hardware ``sample'' buffers. There is additional CPU overhead to manage this ``circular'' buffer, but in some cases this may be the only way to eliminate the latency that occurs when the DMA counter reaches zero and the DMA stops until it is reprogrammed. Programming the DMA

The DMA channel that is to be programmed should always be ``masked'' before loading any settings. This is because the hardware might assert DRQ, and the DMA might respond, even though not all of the parameters have been loaded or updated. Once masked, the host must specify the direction of the transfer (memory-to-I/O or I/O-to-memory), what mode of DMA operation is to be used for the transfer (Single, Block, Demand, Cascade, etc), and finally the address and length of the transfer are loaded. The length that is loaded is one less than the amount you expect the DMA to transfer. The LSB and MSB of the address and length are written to the same 8-bit I/O port, so another port must be written to first to guarantee that the DMA accepts the first byte as the LSB and the second byte as the MSB. Then, be sure to update the Page Register, which is external to the DMA and is accessed through a different set of I/O ports. Once all the settings are ready, the DMA channel can be un-masked. That DMA channel is now considered to be ``armed'', and will respond when DRQ is asserted. Refer to a hardware databook for precise programming details for the 8237. You will also need to refer to the I/O port map for the PC system. This map describes where the DMA and Page Register ports are located. A complete table is located below. DMA Port Map

All systems based on the IBM-PC and PC/AT have the DMA hardware located at the same I/O ports. The complete list is provided below. Ports assigned to DMA Controller #2 are undefined on non-AT designs. 0x00 - 0x1f DMA Controller #1 (Channels 0, 1, 2 and 3)

DMA Address and Count Registers 0x00 write Channel 0 starting address 0x00 read Channel 0 current address 0x02 write Channel 0 starting word count 0x02 read Channel 0 remaining word count 0x04 write Channel 1 starting address 0x04 read Channel 1 current address 0x06 write Channel 1 starting word count 0x06 read Channel 1 remaining word count 0x08 write Channel 2 starting address 0x08 read Channel 2 current address 0x0a write Channel 2 starting word count 0x0a read Channel 2 remaining word count 0x0c write Channel 3 starting address 0x0c read Channel 3 current address 0x0e write Channel 3 starting word count 0x0e read Channel 3 remaining word count DMA Command Registers 0x10 write Command Register 0x10 read Status Register 0x12 write Request Register 0x12 read - 0x14 write Single Mask Register Bit 0x14 read - 0x16 write Mode Register 0x16 read - 0x18 write Clear LSB/MSB Flip-Flop 0x18 read - 0x1a write Master Clear/Reset 0x1a read Temporary Register 0x1c write Clear Mask Register 0x1c read - 0x1e write Write All Mask Register Bits 0x1e read - 0xc0 - 0xdf DMA Controller #2 (Channels 4, 5, 6 and 7)

DMA Address and Count Registers 0xc0 write Channel 4 starting address 0xc0 read Channel 4 current address 0xc2 write Channel 4 starting word count 0xc2 read Channel 4 remaining word count 0xc4 write Channel 5 starting address 0xc4 read Channel 5 current address 0xc6 write Channel 5 starting word count 0xc6 read Channel 5 remaining word count 0xc8 write Channel 6 starting address 0xc8 read Channel 6 current address 0xca write Channel 6 starting word count 0xca read Channel 6 remaining word count 0xcc write Channel 7 starting address 0xcc read Channel 7 current address 0xce write Channel 7 starting word count 0xce read Channel 7 remaining word count DMA Command Registers 0xd0 write Command Register 0xd0 read Status Register 0xd2 write Request Register 0xd2 read - 0xd4 write Single Mask Register Bit 0xd4 read - 0xd6 write Mode Register 0xd6 read - 0xd8 write Clear LSB/MSB Flip-Flop 0xd8 read - 0xda write Master Clear/Reset 0xda read Temporary Register 0xdc write Clear Mask Register 0xdc read - 0xde write Write All Mask Register Bits 0xde read - 0x80 - 0x9f DMA Page Registers

0x87 r/w DMA Channel 0 0x83 r/w DMA Channel 1 0x81 r/w DMA Channel 2 0x82 r/w DMA Channel 3 0x8b r/w DMA Channel 5 0x89 r/w DMA Channel 6 0x8a r/w DMA Channel 7 0x8f Refresh