Programming Embedded Systems in C and C++

Examine the Landscape 43

The first thing to notice is that there are two basic types: memories and peripher-

als. Obviously, memories are for data and code storage and retrieval. But you

might be wondering what the peripherals are. These are specialized hardware

devices that either coordinate interaction with the outside world (I/O) or perform

a specific hardware function. For example, two of the most common peripherals in

embedded systems are serial ports and timers. The former is an I/O device, and

the latter is basically just a counter.

Members of Intel’s 80x86 and some other processor families have two distinct

address spaces through which they can communicate with these memories and

peripherals. The first address space is called the memory space and is intended

mainly for memory devices; the second is reserved exclusively for peripherals and

is called the I/O space. However, peripherals can also be located within the mem-

ory space, at the discretion of the hardware designer. When that happens, we say

that those peripherals are memory-mapped.

From the processor’s point of view, memory-mapped peripherals look and act

very much like memory devices. However, the function of a peripheral is obvi-

ously quite different from that of a memory. Instead of simply storing the data that

is provided to it, a peripheral might instead interpret it as a command or as data to

be processed in some way. If