8237 / 8257 DMA

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Information about 8237 / 8257 DMA

Published on March 5, 2014

Author: ajal4u



8237 / 8257


AJAL.A.J Assistant Professor –Dept of ECE, UNIVERSAL ENGINEERING COLLEGE    Mob: 8907305642   MAIL:

•The DMA I/O technique provides direct access to the memory while the microprocessor is temporarily disabled.

Three ways to synchronize the processor to data rate of peripherals: 1- Polling: which provides a fast response but it the processor recourses are dedicated to one peripheral. 2- Interrupt approach: is much more efficient. the processor only services the peripheral when data is required. requires high software overhead. 3-DMA is a third solution but it increases the complexity of the hardware system.

DMA • Direct Memory Access. • In memory-memory or memory-peripherals communication, the processor is a “middleman” which is not really needed. • Used with HOLD HOLDA signals. • DMA requires another processor - The DMA Controller or DMACto generate the memory and I/O addresses. • 8237 is a DMAC. • In IBM PC, 8237 was used to speed up the read or write operation by the slow 8088 processor. • Nowadays, It is usually used by sound cards and by memory controllers to generate row address for refreshing.

I/O Data Transfer

Basic DMA concept Direct memory access (DMA) is a feature of modern computer systems that allows certain hardware subsystems to read/write data to/from memory without microprocessor intervention, allowing the processor to do other work. Used in disk controllers, video/sound cards etc, or between memory locations . Typically, the CPU initiates DMA transfer, does other operations while the transfer is in progress, and receives an interrupt from the DMA controller once the operation is complete. Can create cache coherency problems (the data in the cache may be different from the data in the external memory after DMA)

B ASIC DM T RM A E INOL OGY DMA channel: system pathway used by a device to transfer information directly to and from memory. There are usually 8 in a computer system DMA controller: dedicated hardware used for controlling the DMA operation Single-cycle mode: DMA data transfer is done one byte at a time Burst-mode: DMA transfer is finished when all data has been moved

DMA pins and timing • x86 Interrupt Pins – HOLD: DMA request. • Sampled in the middle of any clocking cycle – HLDA: DMA acknowledge signal. • The address, data and control buses are set to high-Z, so the I/O devices can control the system bus 8

• • • DMA on the 8086 Microprocessor The CPU may have to wait (hold cycles). The DMA will then let the device that requested the DMA transfer know that the transfer is commencing by asserting the -DACK signal. Once the data has been transferred, 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 HOLD signal. The CPU sees this, and de-asserts the HOLDA signal. Now the CPU resumes control of the buses and address lines, and it resumes executing instructions and accessing main memory and the peripherals. 9

8237 pins • CLK: System clock • CS΄: Chip select (decoder output) 8237 • RESET: Clears registers, sets mask register • READY: 0 for inserting wait states • HLDA: Signals that the μp has relinquished buses • DREQ3 – DREQ0: DMA request input for each channel • DB7-DB0: Data bus pins • IOR΄: Bidirectional pin used during programming and during a DMA write cycle • IOW΄: Bidirectional pin used during programming and during a DMA read cycle • EOP΄: End of process is a bidirectional signal used as input to terminate a DMA process or as output to signal the end of the DMA transfer • A3-A0: Address pins for selecting internal registers • A7-A4: Outputs that provide part of the DMA transfer address • HRQ: DMA request output • DACK3-DACK0: DMA acknowledge for each channel. • AEN: Address enable signal • ADSTB: Address strobe • MEMR΄: Memory read output used in DMA read cycle 10 • MEMW΄: Memory write output used in DMA write cycle

8237/ 8257

Minimal System with a DMA Controller CPU Interrupt Bus Arbiter Memory On-Chip-Bus Request DMA Controller Acknowledge Peripheral Device

8237 DMA controller

Description of pin diagram  D0-D7:  it is a bidirectional ,tri state ,Buffered ,Multiplexed data (D0-D7)and (A8-A15).  In the slave mode it is a bidirectional (Data is moving).  In the Master mode it is a unidirectional (Address is moving).

IOR:  It is active low ,tristate ,buffered ,Bidirectional lines.  In the slave mode it function as a input line. IOR signal is generated by microprocessor to read the contents 8257 registers.  In the master mode it function as a output line. IOR signal is generated by 8257 during write cycle

IOW:  It is active low ,tristate ,buffered ,Bidirectional control lines.  In the slave mode it function as a input line. IOR signal is generated by microprocessor to write the contents 8257 registers.  In the master mode it function as a output line. IOR signal is generated by 8257 during read cycle

CLK:  It is the input line ,connected with TTL clock generator.  This signal is ignored in slave mode. RESET:  Used to clear mode set registers and status registers A0-A3: These are the tristate, buffer, bidirectional address lines. In slave mode ,these lines are used as address inputs lines and internally decoded to access the internal registers. In master mode, these lines are used as address outputs lines,A0-A3 bits of memory address on the lines.

CS:  It is active low, Chip select input line.  In the slave mode, it is used to select the chip.  In the master mode, it is ignored. A4-A7: These are the tristate, buffer, output address lines. In slave mode ,these lines are used as address input lines. In master mode, these lines are used as address outputs lines,A0-A3 bits of memory address on the lines.

READY:  It is a asynchronous input line.  In master mode, When ready is high it receives the signal. When ready is low, it adds wait state between S1 and S3  In slave mode , this signal is ignored. HRQ:  It is used to receiving the hold request signal from the output device.

HLDA:  It is acknowledgment signal from microprocessor. MEMR:  It is active low ,tristate ,Buffered control output line.  In slave mode, it is tristated.  In master mode ,it activated during DMA read cycle. MEMW:  It is active low ,tristate ,Buffered control input line.  In slave mode, it is tristated.  In master mode ,it activated during DMA write cycle.

AEN (Address enable):     It is a control output line. In master mode ,it is high In slave mode ,it is low Used it isolate the system address ,data ,and control lines. ADSTB: (Address Strobe)     It is a control output line. Used to split data and address line. It is working in master mode only. In slave mode it is ignore.

TC (Terminal Count):  It is a status of output line.  It is activated in master mode only.  It is high ,it selected the peripheral.  It is low ,it free and looking for a new peripheral. MARK:  It is a modulo 128 MARK output line.  It is activated in master mode only.  It goes high ,after transferring every 128 bytes of data block.

DRQ0-DRQ3 (DMA Request):  These are the asynchronous peripheral request input signal.  The request signals is generated by external peripheral device. DACK0-DACK3:  These are the active low DMA acknowledge output lines.  Low level indicate that ,peripheral is selected for giving the information (DMA cycle).  In master mode it is used for chip select.

 HLDA becomes active to indicate the processor has placed its buses at high-impedance state.  as can be seen in the timing diagram, there are a few clock cycles between the time that HOLD changes and until HLDA changes  HLDA output is a signal to the requesting device that the processor has relinquished control of its memory and I/O space.  one could call HOLD input a DMA request input and HLDA output a DMA grant signal

8237 block diagram 25

Description  It containing Five main Blocks. 1. Data bus buffer 2. Read/Control logic 3. Control logic block 4. Priority resolver 5. DMA channels.

DATA BUS BUFFER:  It contain tristate ,8 bit bi-directional buffer.  Slave mode ,it transfer data between microprocessor and internal data bus.  Master mode ,the outputs A8-A15 bits of memory address on data lines (Unidirectional). READ/CONTROL LOGIC:  It control all internal Read/Write operation.  Slave mode ,it accepts address bits and control signal from microprocessor.  Master mode ,it generate address bits and control signal.

Control logic block: It contains , 1. Control logic 2. Mode set register and 3. Status Register.  CONTROL LOGIC:      Master mode ,It control the sequence of DMA operation during all DMA cycles. It generates address and control signals. It increments 16 bit address and decrement 14 bit counter registers. It activate a HRQ signal on DMA channel Request. Slave ,mode it is disabled.

FIGURE A DMA controller allows the peripheral to interface directly with memory without processor intervention. This allows the data transfer rate to approach the access time of memory. Types: Sequential DMA Simultaneous DMA

FIGURE Three methods (MODES) of DMA operation: (a) byte; (b) burst; (c) block.

DMA is implemented using a DMA controller – DMA controller • Acts as slave to processor • Receives instructions from processor • Example: Reading from an I/O device – Processor gives details to the DMA controller » I/O device number » Main memory buffer address » Number of bytes to transfer » Direction of transfer (memory → I/O device, or vice versa)

Steps in a DMA operation  Processor initiates the DMA controller  Gives device number, memory buffer pointer, …  Called channel initialization  Once initialized, it is ready for data transfer  When ready, I/O device informs the DMA controller  DMA controller starts the data transfer process      Obtains bus by going through bus arbitration Places memory address and appropriate control signals Completes transfer and releases the bus Updates memory address and count value If more to read, loops back to repeat the process  Notify the processor when done  Typically uses an interrupt

DMA controller details

Modes of Operation  Rotating priority Mode:  The priority of the channels has a circular sequence.  Fixed Priority Rotating Mode:     The priority is fixed. TC Stop Mode Auto Load mode Extended Write mode

DMA Cycles 1. DMA read 2. DMA write 3. DMA Verify

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