Embedded systems are computers that are designed to perform a narrow range of functions as part of a larger system. Typically, an embedded system is housed on a single microprocessor chip with the programs stored in ROM. Virtually all appliances that have a digital interface—watches, microwaves, VCRs, cars—utilize embedded systems. In fact, embedded systems are everywhere: From consumer electronics to kitchen appliances, to automobiles, to networking equipment, to industrial control systems, you find embedded systems lurking in the device. Some embedded systems include an operating system, but many are so specialized that the entire logic can be implemented as a single program.⁵

Early embedded systems were stand-alone 8-bit microprocessors with their own homegrown operating system. Today, they range from 8-bit controllers to 32-bit digital signal processors (DSPs) to 64-bit RISC (reduced instruction set computer) chips. More and more embedded systems are based on networks of distributed microprocessors that communicate through wired and wireless buses, remotely monitored and controlled by regular network management communications protocols.

In fact, the term embedded system is nebulous because it encompasses just about everything except desktop PCs. The term originated because the first such computers were physically embedded within a product or device and could not be accessed. Now the term refers to any computer that is preprogrammed to perform a dedicated or narrow range of functions as part of a larger system. The implication is that there is only minimal end-user or operator intervention, if any.

Because the average person encounters an embedded system only in his or her kitchen, entertainment room, or car, we tend to equate these systems with hardware. In reality, programs must be written and burned into the read-only memory that comes with the system to make it accomplish its assigned function. Given that programs cannot be developed and tested on the embedded processor itself, how are they implemented? Programs are written on a PC and compiled for the target system, where the executable code is generated for the processor in the embedded system.

In early embedded systems, the size of the code and the speed at which it executed were very important. Because assembly-language programs provided the best opportunity to streamline and speed up the code, they were used almost exclusively for embedded systems. Even when the C language became popular and cross-compilers for C to embedded systems became available, many programmers continued to use assembly language for this purpose. C programs are approximately 25% larger and slower, but are easier to write than assembly-language programs. Even today, the size of the ROM may dictate that the code be as small as possible, leading to an assembly-language program.⁶