4.1: Fundamentals I/O- handshake and buffering

Introduction

This section introduces the learner to the various strategies used in I/O interfaces and other operations possible on an interface

Activity details

The computer is useless without some kind of interface to to the outside world. There are many different devices which we can connect to the computer system; keyboards, VDUs and disk drives are some of the more familiar ones. Irrespective of the details of how such devices are connected we can say that all I/O is governed by three basic strategies.

• Programmed I/O
• Interruptdriven I/O
• Direct Memory Access (DMA)

In programmed I/O all data transfers between the computer system and external devices are completely controlled by the computer program. Part of the program will check to see if any external devices require attention and act accordingly. This process is known as polling. Programmed I/O is probably the most common I/O technique because it is very cheap and easy to implement, and in general does not introduce any unforeseen hazards.

Programmed I/O

Is a method of transferring data between the CPU and a peripheral, such as a network adapter or an ATA storage device. In general, programmed I/O happens when software running on the CPU uses instructions that access I/O address space to perform data transfers to or from an I/O device.

The PIO interface is grouped into different modes that correspond to different transfer rates. The electrical signaling among the different modes is similar — only the cycle time between transactions is reduced in order to achieve a higher transfer rate

The PIO modes require a great deal of CPU overhead to configure a data transaction and transfer the data. Because of this inefficiency, the DMA (and eventually UDMA) interface was created to increase performance. The simple digital logic required to implement a PIO transfer still makes this transfer method useful today, especially if high transfer rates are not required like in embedded systems, or with FPGA chips where PIO mode can be used without significant performance loss.

Interrupt driven I/O

Is a way of controlling input/output activity in which a peripheral or terminal that needs to make or receive a data transfer sends a signal that causes a program interrupt to be set. At a time appropriate to the priority level of the I/O interrupt, relative to the total interrupt system, the processor enters an interrupt service routine (ISR). The function of the routine will depend upon the system of interrupt levels and priorities that is implemented in the processor.

In a single-level single-priority system there is only a single I/O interrupt – the logical OR of all the connected I/O devices. The associated interrupt service routine polls the peripherals to find the one with the interrupt status set.

Handshaking

Handshaking is a I/O control method to synchronize I/O devices with the microprocessor. As many I/O devices accepts or release information at a much slower rate than the microprocessor, this method is used to control the microprocessor to work with a I/O device at the I/O devices data transfer rate.

Handshaking is an automated process of negotiation that dynamically sets parameters of a communications channel established between two entities before normal communication over the channel begins. It follows the physical establishment of the channel and precedes normal information transfer. The handshaking process usually takes place in order to establish rules for communication when a computer sets about communicating with a foreign device. When a computer communicates with another device like a modem, printer, or network server, it needs to handshake with it to establish a connection.

Handshaking can negotiate parameters that are acceptable to equipment and systems
at both ends of the communication channel, including information transfer rate, coding alphabet, parity, interrupt procedure, and other protocol or hardware features. Handshaking is a technique of communication between two entities. However, within TCP/IP RFCs, the term “handshake” is most commonly used to reference the TCP three-way handshake. For example, the term “handshake” is not present in RFCs covering FTP or SMTP. One exception is Transport Layer Security, TLS, setup, FTP RFC 4217. In place of the term “handshake”, FTP RFC 3659 substitutes the term “conversation” for the passing of commands.

A simple handshaking protocol might only involve the receiver sending a message meaning “I received your last message and I am ready for you to send me another one.” A more complex handshaking protocol might allow the sender to ask the receiver if it is ready to receive or for the receiver to reply with a negative acknowledgement meaning “I did not receive your last message correctly, please resend it” (e.g., if the data was corrupted en route).

Handshaking facilitates connecting relatively heterogeneous systems or equipment over a communication channel without the need for human intervention to set parameters.

Example: Supposing that we have a printer connected to a system. The printer can print 100 characters/second, but the microprocessor can send much more information to the printer at the same time. That’s why, just when the printer gets it enough data to print it places a logic
1 signal at its Busy pin, indicating that it is busy in printing. The microprocessor now tests the busy bit to decide if the printer is busy or not. When the printer will become free it will change the busy bit and the microprocessor will again send enough amounts of data to be printed. This process of interrogating the printer is called handshaking.

Buffering

Is the process of transferring data between a program and an external device, The process of optimizing I/O consists primarily of making the best possible use of the slowest part of the path between the program and the device. The slowest part is usually the physical channel, which is often slower than the CPU or a memory-to-memory data transfer. The time spent in I/O processing overhead can reduce the amount of time that a channel can be used, thereby reducing the effective transfer rate. The biggest factor in maximizing this channel speed is often the reduction of I/O processing overhead.

A buffer is a temporary storage location for data while the data is being transferred. A buffer is often used for the following purposes:

• Small I/O requests can be collected into a buffer, and the overhead of making many relatively expensive system calls can be greatly reduced.

• A collection buffer of this type can be sized and handled so that the actual physical I/O requests made to the operating system match the physical characteristics of the device being used.

• Many data file structures, such as the f77 and cos file structures, contain control words. During the write process, a buffer can be used as a work area where control words can be inserted into the data stream (a process called blocking). The blocked data is then written to the device. During the read process, the same buffer work area can be used to examine and remove these control words before passing the data on to the user (deblocking ).

• When data access is random, the same data may be requested many times. A cache is a buffer that keeps old requests in the buffer in case these requests are needed again. A cache that is sufficiently large and/or efficient can avoid a large part of the physical I/O by having the data ready in a buffer. When the data is often found in the cache buffer, it is referred to as having a high hit rate. For example, if the entire file fits in the cache and the file is present in the cache, no more physical requests are required to perform the I/O. In this case, the hit rate is 100%.

• Running the disks and the CPU in parallel often improves performance; therefore, it is useful to keep the CPU busy while data is being moved. To do this when writing, data can be transferred to the buffer at memory-to- memory copy speed and an asynchronous I/O request can be made. The control is then immediately returned to the program, which continues to execute as if the I/O were complete (a process called write-behind). A similar process can be used while reading; in this process, data is read into a buffer before the actual request is issued for it. When it is needed, it is already in the buffer and can be transferred to the user at very high speed. This is another form or use of a cache.

Conclusion

This section introduced the learner to the various ways, interfaces access and pass data. They include polling, interrupt and DMA. In them, speed between the different devices connected to the CPU are synchronized to be able to communicate effectively

Assessment

1. What is the difference between programmed-driven I/O and interrupt-driven I/O?

Programmed-driven I/O means the program is polling or checking some hardware item e.g. mouse within a loop.

For Interrupt driven I/O, the same mouse will trigger a signal to the program to process the mouse event.

2. What is one advantage and one disadvantage of each?

Advantage of Programmed Driven: easy to program and understand

Disadvantages: slow and inefficient

Advantage of Interrupt Driven: fast and efficient

Disadvantage: Can be tricky to write if you are using a low level language.

Can be tough to get the various pieces to work well together. Usually done by the hardware manufacturer or the OS maker e.g. Microsoft.