CS501 Final Term Subjective Notes by Saeed

CS501 Final Term Subjective Notes by Saeed · Memory and I/O subsystems differ in the following ways: · 1. Wider range of data transfer speed:

I/O devices can be very slow such as a keyboard in which case the interval between two successive bytes (or keystrokes) can be in seconds. On the other extreme, I/O devices can be very fast such as a disk drive sending data to the CPU or a stream of packets arriving over a network,

2. Asynchronous activity:

Memory subsystems are almost always synchronous. This means that most memory transfers are governed by the CPU’s clock. Generally this is not the case with I/O subsystems. Additional signals, called handshaking signals, are needed to take care of asynchronous I/O transfers.

3. Larger degradation in data quality:

Data transferred by I/O subsystems can carry more noise. As an example, telephone line noise can become part of the data transferred by a modem.

4. Mechanical nature of many I/O devices:

Many I/O devices or a large portion of I/O devices use mechanical parts which inherently have a high failure rate. In case an I/O device fails, interruptions in data transfer will occur, reducing the throughput. As an example, if a printer runs out of paper, then additional bytes cannot be sent to it. The CPU’s data should be buffered (or kept in a temporary place) till the paper is supplied to the printer, otherwise the CPU will not be able to do anything else during this time. To deal with these differences, special software programs called device drivers are made a part of the operating system.

Major components of an I/O subsystem

I/O subsystems have two major parts:

• The I/O interface, which is the electronic circuitry that connects the CPU to the I/O device.
• Peripherals, which are the devices used to communicate with the CPU, for example, the keyboard, the monitor, etc.

Computer Interface

A Computer Interface is a piece of hardware whose primary purpose is to connect together any of the following types of computer elements in such a way that the signal levels and the timing requirements of the elements are matched by the interface. Those elements are:

• The processor unit · • The memory subsystem(s) · • Peripheral (or I/O) devices · • The buses (also called "links") · I/O ports serve the following three purposes:

• Buffering (i.e., holding temporarily) the data to and from the computer bus.
• Holding control information that dictates how a transfer is to be conducted.
• Holding status information so that the processor can monitor the activity of the interface and its associated I/O element.

In most cases, the word buffer refers to I/O registers in an interface where data, status or control information is temporarily stored. A block of memory locations within the main memory or within the peripheral devices is also called a buffer if it is used for temporary storage. Special circuits used in the interfaces for voltage/current matching, at the input and the output, are also called buffers.

The system bus actually consists of three buses, namely the address bus, the data bus and the control bus.

Memory Mapped I/O versus Isolated I/O

In isolated I/O, a separate address space of the CPU is reserved for I/O operations. This address space is totally different from the address space used for memory devices. In other words, a CPU has two distinct address spaces, one for memory and one for input/output. Unique CPU instructions are associated with the I/O space, which means that if those instructions are executing on the CPU, then the accessed address space will be the I/O space and hence the devices mapped on the I/O space. In some processors, like the SRC, there is no separate I/O space. In this case, some address space out of the memory address space must be used to map I/O devices. The benefit will be that all the instructions which access memory can be used for I/O devices. There is no need for including separate I/O instructions in the ISA of the processor. However, the disadvantage will be that the I/O interface will become complex. If partial decoding is used to reduce the complexity of the I/O interface, then a lot of memory addresses will be consumed.

Considerations during I/O Subsystem Design

Certain things must be taken care of during the design of an I/O subsystem.

Data location:

The designer must identify the device where the data to be accessed is available, the address of this device and how to collect the data from this device.

Data transfer:

This includes the direction of transfer of data; whether it is out of the CPU or into the CPU, whether the data is being sent to the monitor or the hard drive, or whether it is being received from the keyboard or the mouse. It also includes the amount of data to be transferred and the rate at which it should be transferred