The most common and popular mechanisms of transferring large blocks of data between memory and I/O ports are Direct Memory Access (DMA) and Programmed I/O (PIO). DMA is a method of transferring data without CPU intervention. By contrast, the PIO method uses the CPU for data transfers.

Amphus offers the VG-330 single-chip platform for embedded, handheld, and Internet applications. The VG-330 is based on the NEC V30MX 8086-compatible microprocessor. It is a highly-integrated solution targeted for low power, high performance systems. The VG-330 is targeted specifically towards 3.3 V systems requiring increased processing power but requiring longer battery life than less integrated solutions.

The common techniques used for data transfers in an 8086-based system are DMA and PIO. In systems where DMA is integrated, software developers have the option to employ either technique when developing applications.

This paper discusses the relative merits of DMA and PIO. It suggests that for the class of applications targeted by single-chip systems such as VG-330, and given the class of such devices, DMA offers little or no benefit for data transfer. The way DMA is implemented varies from one computer architecture to another, so this discussion will be limited to the DMA and PIO mechanisms based on the x86 industry standard architecture.

Executive Summary

DMA was originally implemented using the 8237 DMA controller to facilitate floppy and hard disk data transfer in the original IBM PCs. DMA has traditionally dominated the following areas:

1. Short latency requirement

2. Computation and data access simultaneously

3. PC legacy

However, it has been found over the years that DMA has two main drawbacks:

1. Lack of speed

2. Large setup overhead

Use of programmed I/O, especially in single-tasking environments, has proven to outpace DMA in terms of data transfer speed and overhead.

It has also been shown that using Amphus's VG-330 single-chip platform in the PIO mode can achieve comparable or better performance than DMA. It is suggested that PIO is more appropriate in handheld, embedded systems where floppy drives and similar peripherals are not typically found.

DMA Operation

PC DMA operation is based on the 8237 DMA controller. The 8237 transfers either 8 or 16 bits of data between memory and an I/O port. A basic theory of operation of DMA is given in the following paragraph.

The CPU programs the DMA controller registers with several parameters (for example, transfer direction and size of transfer) to initialize the DMA channel prior to the actual DMA transfer. After controller programming is complete, the DMA channel is normally activated only when the hardware that uses DMA asserts the DRQ signal to request a transfer for that channel. When the DMA controller receives the request, it transfers data between the memory and I/O port without CPU intervention. The DMA controller begins by requesting the address and data bus from the CPU. After some latency (a few microseconds, worst case), the CPU gives up the bus and the DMA controller executes one or more DMA transfer cycles on the bus. After the transfer, the DMA controller returns control of the bus to the CPU. The DMA transfer operation delays, but does not require, interruption of the ongoing CPU instruction stream.

Drawbacks

DMA has the following drawbacks:

1. Speed. The 8237 DMA controller is slow. It was originally designed to interface with the 8085 microprocessor, and is not suited for data transfer tasks in the original PC. In fact, the hard disk drives that originally used DMA for data transfers now use PIO for speed. The standard 8237 DMA controller and interface supports a data transfer rate of approximately 1 MB/sec. Another data transfer method is fast DMA. Amphus has chosen not to implement DMA in the VG-330 because of the cost sensitivity of handheld and embedded system devices.

2. DMA Channel Set Up. Initializing the DMA channel can create a large overhead. Several registers have to be programmed before the data transfer can take place. Also, in systems like DOS that reprogram the DMA channel after each block transfer, the overhead is even greater.

Programmed I/O Operation

Programmed or Polled I/O transfers are done using the CPU. In Polled I/O mode, the CPU first checks to see if the I/O port needing a data transfer has the data ready. If so, the data is transferred to the memory. I/O ports that have buffered data use the programmed I/O (PIO) transfer method. If a block of data is ready, the CPU transfers the data using repeat string instructions. It does not have to check to see if the I/O device is ready to receive or send each individual byte of data. PIO can achieve speeds of about 2 MB/sec on the ISA bus.

Using Programmed I/O to Replace DMA

Traditionally, the DMA data transfer method has been used for the following reasons:

1. Short latency requirement

2. Computation and data access simultaneously

3. PC legacy

Programmed I/O data transfer on the VG-330 single-chip platform achieves traditional DMA performance as explained in the following paragraphs.

Short Latency Requirement

When a short latency is required, DMA is normally used. However, maskable interrupts (such as that used for DMA transfers) introduce too much worst case latency, due to the multiplicity of sources and the ability of any program to mask off interrupts temporarily. On the VG-330, however, the latency issue can be reduced with polled I/O or non-maskable interrupt (NMI) techniques. The polled I/O method checks the status of a device with a very tight loop of instructions and transfers data when the device is ready. The VG-330 services NMIs within 10 microseconds worst case. The interrupt routine itself introduces an overhead of 10 to 20 microseconds, for a total latency of 20 to 30 microseconds. Depending on the system configuration, polled I/O can respond within 5 to 10 microseconds.

Either the polled I/O or NMI techniques can meet the needs of modern high-speed buffered peripherals.

Computation and Data Access Simultaneously

Even though a CPU can potentially do some useful work while the DMA controller is busy transferring data, the 8237 consumes excessive bus bandwidth due to its long DMA cycles. The bus bandwidth available to the CPU is substantially reduced because the long 8237 bus cycles crowd out the short CPU bus cycles.

The fact that Amphus has successfully ported a real time kernel and is able to run Internet applications on the VG-330 using PIO supports the fact that PIO competes very favorably with DMA for data transfer.

PC Legacy

DMA was first introduced in the IBM PC to transfer data to and from floppy disks. Later, DMA was used in the IBM PC/XT to accomplish hard disk data transfers. Since then, due to higher-speed hard disk drives, PIO has been used for hard disk data transfers. Most hand held systems still use single-tasking environments in which the CPU stays idle even if data is transferred using DMA. Hence, in single-tasking environments like DOS, programmed or polled I/O provides a higher transfer rate than DMA. The existing software is old and software developers have recognized the benefit of programmed I/O. Therefore it is not necessary to have the overhead of DMA in silicon. In the past, DMA has been used for floppy operation and at times used for bar code readers-these operations can be done effectively now using PIO.

Summary

PIO provides potential performance improvements over DMA. Even in places where DMA has traditionally been used, programmed I/O can be used effectively. The VG-330 runs at 32 MHz, which allows instructions to be executed extremely fast, allowing PIO data transfers to improve the performance of the system. Amphus has chosen not to implement DMA because the VG-330 is intended for low-cost handheld systems where implementation of DMA would be an expensive solution without any benefit. The VG-330 evaluation board supports a 1.44 MB floppy drive, using PIO for data transfers.

Copyright 1996 Amphus. All Rights Reserved
Information furnished by Amphus is believed to be accurate and reliable. However, no responsibility is assumed by Amphus for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Amphus. Amphus reserves the right to change specifications at any time without notice. Trademarks mentioned herein belong to their respective companies. Medical Disclaimer: Amphus's products are not authorized for use in life support devices or systems or in any medical applications and cannot be used in such applications without the written consent of the President of Amphus.

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