Table of Contents
Peripherals
Peripherals or peripheral devices, also known as Input-Output devices, enable the computer to remain in contact with the external environment or expand the computer's functionality. Peripheral devices enhance the computer's capability by making it possible to enter information into a computer for storage or processing and to deliver the processed data to a user, another computer, or a device controlled by the computer. Internal peripherals are connected directly to the computer's address, data, and control buses. External peripherals can be connected to the computer via USB or a similar connection.
The USB controller is also a peripheral device, so every external peripheral (e.g. mouse) is connected to the processor via an internal peripheral. In this chapter, we consider internal peripherals directly connected to the address, data, and control buses.
Types of peripherals
There is a variety of peripherals which can be connected to the computer. The most commonly used are:
- parallel input/output ports,
- serial communication ports,
- timers/counters,
- analogue to digital converters,
- digital to analogue converters,
- interrupt controllers,
- DMA controllers,
- displays,
- keyboards,
- sensors,
- actuators.
Addressing of I/O devices
From the assembler programmer's perspective, the peripheral device is represented as a set of registers available in the I/O address space. Registers of peripherals are used to control their behaviour, including mode of operation, parameters, configuration, transmission speed, etc. Registers are also data exchange points where the processor can store data to be transmitted to the user or an external computer, or read data from the user or another system.
The size of the I/O address space is usually smaller than the size of the program or data address space. The method of accessing peripherals depends on the processor's design. We can find two methods of I/O addressing implementation: separate or memory-mapped I/O.
Separate I/O address space
A separate I/O address space is accessed independently of the program or data memory. In the processor, it is implemented using separate control bus lines to read or write I/O devices. Separate control lines usually mean that the processor also implements different instructions to access memory and I/O devices. It also means that the chosen peripheral and the byte in memory can have the same address, and only the instruction type distinguishes the final destination of the address. A separate I/O address space is shown schematically in Fig. 1. Reading data from memory is activated by the #MEMRD signal; writing is activated by the #MEMWR signal. If the processor needs to read or write the peripheral device's register, it uses #IORD or #IOWR, respectively.
The “#” symbol before a signal name means that the active signal on the line is a LOW state, while idle is a HIGH state.
Memory-mapped I/O address space
In this approach, the processor doesn't implement separate control signals to access the peripherals. It uses a #RD signal to read the data from and a #WR signal to write the data to a common address space. It also doesn't have separate instructions for accessing registers in the I/O address space. Every such register is visible like any other memory location. It means that this is the sole responsibility of the software to distinguish I/O registers from data in memory. Memory-mapped I/O address space is shown schematically in Fig 2.
