25.2. DMA: What it is and How it Works

Copyright © 1995,1997 Frank Durda IV , All Rights Reserved. 10 December 1996. Last Update 8 October 1997.

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 one 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 from two adjacent memory locations in each transfer, with the first byte always coming from an even-numbered address. The two controllers are identical components 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 a given DMA channel requests a transfer by asserting the DRQ line for that channel.

25.2.1. A Sample DMA transfer

Here is an example of the steps that occur to cause and perform 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 (the DRQ line for DMA channel 2) 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 unmasked (enabled). The DMA controller also makes sure that none of the other DMA channels are active or want to be active and 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 does need 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 the DMA knows that 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 in the DMA 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, and since only DMA channel can be active at any instant, the DMA channel that is currently active must be the DMA channel that just completed its task.

If a peripheral wants to generate an interrupt when the transfer of a buffer is complete, it can test for its -DACKn signal and the EOP signal both being asserted at the same time. When that happens, it means the DMA will not 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. In the PC architecture, 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. Subsequently, it is possible to have a peripheral that uses DMA but does not use interrupts.

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.

25.2.2. 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 an external data 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 are written to the address bus and kept there until the DMA operation for the channel ends. IBM called these latches "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. For example, if the DMA accesses memory location 0xffff, after that transfer the DMA will then increment the address register and the DMA will access the next byte at location 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 mathematically adding a segment register with an offset register. Page Registers have no address overlap and are mathematically OR-ed together.

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 standard PC-compatible DMA cannot access memory locations above 16Meg.

To get around this restriction, operating systems will reserve a RAM buffer in an area below 16Meg that also does not span a physical 64K boundary. Then the DMA will be programmed to transfer data from the peripheral and into 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".

Note: A new implementation of the 8237, called the 82374, allows 16 bits of page register to be specified, allows access to the entire 32 bit address space, without the use of bounce buffers.

25.2.3. DMA Operational Modes and Settings

The 8237 DMA can be operated in several modes. The main ones are:


A single byte (or word) is transferred. The DMA must release and re-acquire the bus for each additional byte. This is commonly-used by devices that cannot transfer the entire block of data immediately. The peripheral will request the DMA each time it is ready for another transfer.

The standard PC-compatible floppy disk controller (NEC 765) only has a one-byte buffer, so it uses this mode.


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 to suspend the transfer briefly. 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 that 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, at which point the DMA suspends the transfer and releases the bus back to the CPU. 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 transfer the data using the CPU, particularly if the memory locations used in the transfer were above the 16Meg mark.


This mechanism allows a DMA channel to request the bus, but then the attached peripheral device is responsible for placing the addressing information on the bus instead of the DMA. This is also used to implement a technique 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 the DMA normally does when it is active. Instead, the DMA only asserts the -DACK signal for the active DMA channel.

At this point it is up to the peripheral 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 the bus, it de-asserts the DRQ line, and the DMA controller can then 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 architecture. 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 instead of to the CPU. The primary DMA controller, thinking it has work to do on Channel 4, requests the bus from the CPU using HLDREQ signal. Once the CPU grants the bus to the primary DMA controller, -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 (0, 1, 2 or 3), or the slave DMA may grant the bus to a peripheral that wants to perform its own bus-mastering, such as a SCSI controller.

Because of this wiring arrangement, only DMA channels 0, 1, 2, 3, 5, 6 and 7 are usable with peripherals 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 main memory.

The Dynamic RAM used in all PCs for main memory must be accessed frequently to keep the bits stored in the components "charged". Dynamic RAM essentially consists of millions of capacitors with each one holding one bit of data. These capacitors are charged with power to represent a 1 or drained to represent a 0. Because all capacitors leak, power must be added at regular intervals to keep the 1 values intact. The RAM chips actually handle the task of pumping power back into all of the appropriate locations in RAM, but they must be told when to do it by the rest of the computer so that the refresh activity won't interfere with the computer wanting to access RAM normally. If the computer is unable to refresh memory, the contents of memory will become corrupted in just a few milliseconds.

Since memory read and write cycles "count" as refresh cycles (a dynamic RAM 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 host interfaces and other high-performance peripheral controllers.


This mode causes the DMA to perform Byte, Block or Demand transfers, but when the DMA transfer counter reaches zero, the counter and address are set back to where they were when the DMA channel was originally programmed. This means that as long as the peripheral 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 when doing output operations, and read new data out of the buffer behind where the DMA is writing when doing 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 transfers until it is reprogrammed.

25.2.4. 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 unexpectedly assert the DRQ for that channel, 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 of the length and address.

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 the DRQ line for that channel is asserted.

Refer to a hardware data book for precise programming details for the 8237. You will also need to refer to the I/O port map for the PC system, which describes where the DMA and Page Register ports are located. A complete port map table is located below.

25.2.5. 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

0x00writeChannel 0 starting address
0x00readChannel 0 current address
0x01writeChannel 0 starting word count
0x01readChannel 0 remaining word count
0x02writeChannel 1 starting address
0x02readChannel 1 current address
0x03writeChannel 1 starting word count
0x03readChannel 1 remaining word count
0x04writeChannel 2 starting address
0x04readChannel 2 current address
0x05writeChannel 2 starting word count
0x05readChannel 2 remaining word count
0x06writeChannel 3 starting address
0x06readChannel 3 current address
0x07writeChannel 3 starting word count
0x07readChannel 3 remaining word count

DMA Command Registers

0x08writeCommand Register
0x08readStatus Register
0x09writeRequest Register
0x0awriteSingle Mask Register Bit
0x0bwriteMode Register
0x0cwriteClear LSB/MSB Flip-Flop
0x0dwriteMaster Clear/Reset
0x0dreadTemporary Register (not available on newer versions)
0x0ewriteClear Mask Register
0x0fwriteWrite All Mask Register Bits
0x0freadRead All Mask Register Bits (only in Intel 82374) 0xc0-0xdf DMA Controller #2 (Channels 4, 5, 6 and 7)

DMA Address and Count Registers

0xc0writeChannel 4 starting address
0xc0readChannel 4 current address
0xc2writeChannel 4 starting word count
0xc2readChannel 4 remaining word count
0xc4writeChannel 5 starting address
0xc4readChannel 5 current address
0xc6writeChannel 5 starting word count
0xc6readChannel 5 remaining word count
0xc8writeChannel 6 starting address
0xc8readChannel 6 current address
0xcawriteChannel 6 starting word count
0xcareadChannel 6 remaining word count
0xccwriteChannel 7 starting address
0xccreadChannel 7 current address
0xcewriteChannel 7 starting word count
0xcereadChannel 7 remaining word count

DMA Command Registers

0xd0writeCommand Register
0xd0readStatus Register
0xd2writeRequest Register
0xd4writeSingle Mask Register Bit
0xd6writeMode Register
0xd8writeClear LSB/MSB Flip-Flop
0xdawriteMaster Clear/Reset
0xdareadTemporary Register (not present in Intel 82374)
0xdcwriteClear Mask Register
0xdewriteWrite All Mask Register Bits
0xdfreadRead All Mask Register Bits (only in Intel 82374) 0x80-0x9f DMA Page Registers

0x87r/wChannel 0 Low byte (23-16) page Register
0x83r/wChannel 1 Low byte (23-16) page Register
0x81r/wChannel 2 Low byte (23-16) page Register
0x82r/wChannel 3 Low byte (23-16) page Register
0x8br/wChannel 5 Low byte (23-16) page Register
0x89r/wChannel 6 Low byte (23-16) page Register
0x8ar/wChannel 7 Low byte (23-16) page Register
0x8fr/wLow byte page Refresh 0x400-0x4ff 82374 Enhanced DMA Registers

The Intel 82374 EISA System Component (ESC) was introduced in early 1996 and includes a DMA controller that provides a superset of 8237 functionality as well as other PC-compatible core peripheral components in a single package. This chip is targeted at both EISA and PCI platforms, and provides modern DMA features like scatter-gather, ring buffers as well as direct access by the system DMA to all 32 bits of address space.

If these features are used, code should also be included to provide similar functionality in the previous 16 years worth of PC-compatible computers. For compatibility reasons, some of the 82374 registers must be programmed after programming the traditional 8237 registers for each transfer. Writing to a traditional 8237 register forces the contents of some of the 82374 enhanced registers to zero to provide backward software compatibility.

0x401r/wChannel 0 High byte (bits 23-16) word count
0x403r/wChannel 1 High byte (bits 23-16) word count
0x405r/wChannel 2 High byte (bits 23-16) word count
0x407r/wChannel 3 High byte (bits 23-16) word count
0x4c6r/wChannel 5 High byte (bits 23-16) word count
0x4car/wChannel 6 High byte (bits 23-16) word count
0x4cer/wChannel 7 High byte (bits 23-16) word count
0x487r/wChannel 0 High byte (bits 31-24) page Register
0x483r/wChannel 1 High byte (bits 31-24) page Register
0x481r/wChannel 2 High byte (bits 31-24) page Register
0x482r/wChannel 3 High byte (bits 31-24) page Register
0x48br/wChannel 5 High byte (bits 31-24) page Register
0x489r/wChannel 6 High byte (bits 31-24) page Register
0x48ar/wChannel 6 High byte (bits 31-24) page Register
0x48fr/wHigh byte page Refresh
0x4e0r/wChannel 0 Stop Register (bits 7-2)
0x4e1r/wChannel 0 Stop Register (bits 15-8)
0x4e2r/wChannel 0 Stop Register (bits 23-16)
0x4e4r/wChannel 1 Stop Register (bits 7-2)
0x4e5r/wChannel 1 Stop Register (bits 15-8)
0x4e6r/wChannel 1 Stop Register (bits 23-16)
0x4e8r/wChannel 2 Stop Register (bits 7-2)
0x4e9r/wChannel 2 Stop Register (bits 15-8)
0x4ear/wChannel 2 Stop Register (bits 23-16)
0x4ecr/wChannel 3 Stop Register (bits 7-2)
0x4edr/wChannel 3 Stop Register (bits 15-8)
0x4eer/wChannel 3 Stop Register (bits 23-16)
0x4f4r/wChannel 5 Stop Register (bits 7-2)
0x4f5r/wChannel 5 Stop Register (bits 15-8)
0x4f6r/wChannel 5 Stop Register (bits 23-16)
0x4f8r/wChannel 6 Stop Register (bits 7-2)
0x4f9r/wChannel 6 Stop Register (bits 15-8)
0x4far/wChannel 6 Stop Register (bits 23-16)
0x4fcr/wChannel 7 Stop Register (bits 7-2)
0x4fdr/wChannel 7 Stop Register (bits 15-8)
0x4fer/wChannel 7 Stop Register (bits 23-16)
0x40awriteChannels 0-3 Chaining Mode Register
0x40areadChannel Interrupt Status Register
0x4d4writeChannels 4-7 Chaining Mode Register
0x4d4readChaining Mode Status
0x40creadChain Buffer Expiration Control Register
0x410writeChannel 0 Scatter-Gather Command Register
0x411writeChannel 1 Scatter-Gather Command Register
0x412writeChannel 2 Scatter-Gather Command Register
0x413writeChannel 3 Scatter-Gather Command Register
0x415writeChannel 5 Scatter-Gather Command Register
0x416writeChannel 6 Scatter-Gather Command Register
0x417writeChannel 7 Scatter-Gather Command Register
0x418readChannel 0 Scatter-Gather Status Register
0x419readChannel 1 Scatter-Gather Status Register
0x41areadChannel 2 Scatter-Gather Status Register
0x41breadChannel 3 Scatter-Gather Status Register
0x41dreadChannel 5 Scatter-Gather Status Register
0x41ereadChannel 5 Scatter-Gather Status Register
0x41freadChannel 7 Scatter-Gather Status Register
0x420-0x423r/wChannel 0 Scatter-Gather Descriptor Table Pointer Register
0x424-0x427r/wChannel 1 Scatter-Gather Descriptor Table Pointer Register
0x428-0x42br/wChannel 2 Scatter-Gather Descriptor Table Pointer Register
0x42c-0x42fr/wChannel 3 Scatter-Gather Descriptor Table Pointer Register
0x434-0x437r/wChannel 5 Scatter-Gather Descriptor Table Pointer Register
0x438-0x43br/wChannel 6 Scatter-Gather Descriptor Table Pointer Register
0x43c-0x43fr/wChannel 7 Scatter-Gather Descriptor Table Pointer Register