CertiGuide to A+ (Core Hardware)  9  Chapter 0011:  System Memory       9  II  Memory Types

All Memory is not Equal 1 Programmable Read Only Memory (PROM)

The first type of system memory we will discuss is Read Only Memory, or ROM. As its name implies, this memory can only be read from and not written to. This is opposed to the next type of system memory we will discuss, Random Access Memory, or RAM, that can be read from and written to.

ROM chips serve a significant purpose in computers in that they store preset information that is critical to the computer running properly. The best example of this information that is stored in a ROM is the system BIOS. As you learned in Chapter 0010, the BIOS routine contains the information that the computer needs to boot up when the system is powered on, and is available for the OS to access when it needs to control the hardware inside the computer. This BIOS program is stored on a ROM chip called the System BIOS ROM, or ROM BIOS. The term we use for programs (software) that are stored in ROM chips (hardware) is firmware.

The significant difference between ROM and RAM is that the information that is stored in ROM is maintained even after the power to the system is shut off. We call this type of memory non-volatile memory. As a matter of fact, one could remove a ROM chip from a motherboard (very carefully), put it away for a few years, and then put it back in the motherboard (again very carefully), an the information that it contains would still be there, ready to use.

Originally, ROM chips had their instructions hard wired into the IC chip, and could never be altered after they were produced. Because of this inflexibility, this type of ROM chip is used for programs that are unchangeable (static), and are usually mass-produced.

Of course, this type of ROM has its drawbacks, specifically the fact that it can never be altered. Many manufacturers wanted a ROM chip that could be programmed with their own software that didn't rely on the circuitry of the chip itself. This type of ROM is known as Programmable Read Only Memory, or PROM.

All Memory is not Equal 1 Programmable Read Only Memory (PROM)