2 cissp ® Official Study Guide Eighth Edition

Real Memory Real memory
(also known as 
main memory
or 
primary memory
) is typi-
cally the largest RAM storage resource available to a computer. It is normally composed 
of a number of dynamic RAM chips and, therefore, must be refreshed by the CPU on a 
periodic basis (see the sidebar “Dynamic vs. Static RAM” for more information on this 
subject).
Cache RAM
Computer systems contain a number of caches that improve performance 
by taking data from slower devices and temporarily storing it in faster devices when 
repeated use is likely; this is 
cache RAM
. The processor normally contains an onboard 
cache of extremely fast memory used to hold data on which it will operate. This can be 

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Assess and Mitigate Security Vulnerabilities 
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referred to as L1, L2, L3, and even L4 cache (with the L being short for level). Many mod-
ern CPUs include up to three levels of on-chip cache, with some caches (usually L1 and/or 
L2) dedicated to a single processor core, while L3 may be a shared cache between cores. 
Some CPUs can involve L4 cache which may be located on the mainboard/motherboard 
or on the GPU (graphics processing unit). Likewise, real memory often contains a cache 
of information stored on magnetic media or SSD. This chain continues down through the 
memory/storage hierarchy to enable computers to improve performance by keeping data 
that’s likely to be used next closer at hand (be it for CPU instructions, data fetches, file 
access, or what have you).
Many peripherals also include onboard caches to reduce the storage burden they place on the 
CPU and operating system. For example, many higher-end printers include large RAM caches 
so that the operating system can quickly spool an entire job to the printer. After that, the 
processor can forget about the print job; it won’t be forced to wait for the printer to actually 
produce the requested output, spoon-feeding it chunks of data one at a time. The printer can 
preprocess information from its onboard cache, thereby freeing the CPU and operating sys-
tem to work on other tasks. Many storage devices, such as hard disc drive (HDD), solid-state 
drive (SSD), and some thumb drives contain caches to assist with improving read and write 
speed. However, these caches must be flushed to the permanent or secondary storage area 
before disconnection or power loss in order to avoid data loss of cache resident data.
dynamic vs. Static ram
There are two main types of RAM: dynamic RAM and static RAM. Most computers con-
tain a combination of both types and use them for different purposes.
To store data, dynamic RAM uses a series of capacitors, tiny electrical devices that hold 
a charge. These capacitors either hold a charge (representing a 1 bit in memory) or do 
not hold a charge (representing a 0 bit). However, because capacitors naturally lose their 
charges over time, the CPU must spend time refreshing the contents of dynamic RAM to 
ensure that 1 bits don’t unintentionally change to 0 bits, thereby altering memory contents.
Static RAM uses more sophisticated technology—a logical device known as a 
flip-flop
, 
which to all intents and purposes is simply an on/off switch that must be moved from one 
position to another to change a 0 to 1 or vice versa. More important, static memory main-
tains its contents unaltered as long as power is supplied and imposes no CPU overhead 
for periodic refresh operations.
Dynamic RAM is cheaper than static RAM because capacitors are cheaper than flip-flops. 
However, static RAM runs much faster than dynamic RAM. This creates a trade-off for 
system designers, who combine static and dynamic RAM modules to strike the right bal-
ance of cost versus performance.

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