2 cissp ® Official Study Guide Eighth Edition

 Read-Only Memory
Read-only memory (ROM)
works like the name implies—it’s memory the PC can read but 
can’t change (no writing allowed). The contents of a standard ROM chip are burned in at 
the factory, and the end user simply cannot alter it. ROM chips often contain “bootstrap” 
information that computers use to start up prior to loading an operating system from disk. 
This includes the familiar
power-on self-test (POST)
series of diagnostics that run each 
time you boot a PC. 
ROM’s primary advantage is that it can’t be modifi ed. There is no chance that user or 
administrator error will accidentally wipe out or modify the contents of such a chip. This 
attribute makes ROM extremely desirable for orchestrating a computer’s innermost workings. 
There is a type of ROM that may be altered by administrators to some extent. It is known 
as programmable read-only memory (PROM), and its several subtypes are described next: 
Programmable Read-Only Memory (PROM)
A basic
programmable read-only memory 
(PROM)
chip is similar to a ROM chip in functionality, but with one exception. During 
the manufacturing process, a PROM chip’s contents aren’t “burned in” at the factory as 
with standard ROM chips. Instead, a PROM incorporates special functionality that allows 
an end user to burn in the chip’s contents later. However, the burning process has a similar 

---

334
Chapter 9 
■
Security Vulnerabilities, Threats, and Countermeasures
outcome—once data is written to a PROM chip, no further changes are possible. After it’s 
burned in, a PROM chip essentially functions like a ROM chip.
PROM chips provide software developers with an opportunity to store information per-
manently on a high-speed, customized memory chip. PROMs are commonly used for hard-
ware applications where some custom functionality is necessary but seldom changes once 
programmed.