Programming Embedded Systems in C and C++

Types of Memory 59

Types of ROM

Memories in the ROM family are distinguished by the methods used to write new

data to them (usually called programming) and the number of times they can be

rewritten. This classification reflects the evolution of ROM devices from hardwired

to one-time programmable to erasable-and-programmable. A common feature

across all these devices is their ability to retain data and programs forever, even

during a power failure.

The very first ROMs were hardwired devices that contained a preprogrammed set

of data or instructions. The contents of the ROM had to be specified before chip

production, so the actual data could be used to arrange the transistors inside the

chip! Hardwired memories are still used, though they are now called “masked

ROMs” to distinguish them from other types of ROM. The main advantage of a

masked ROM is a low production cost. Unfortunately, the cost is low only when

hundreds of thousands of copies of the same ROM are required.

One step up from the masked ROM is the PROM (programmable ROM), which is

purchased in an unprogrammed state. If you were to look at the contents of an

unprogrammed PROM, you would see that the data is made up entirely of 1’s. The

process of writing your data to the PROM involves a special piece of equipment

called a device programmer. The device programmer writes data to the device one

DRAM Controllers

If your embedded system includes ...