Download Physical Memory

Physical Memory
October 2 (Day); September 26 (Night)

All computer memory is divided into two categories.

Read Only Memory (ROM):
o
ROM consists of chips that contain programs that are acid-etched into chips at the
factory.
o
These programs in ROM are often called firmware because they share characteristics
of both software and hardware.
o
Some ROM is socketed on the motherboard. ROM on the motherboard is
responsible for routine operations such as the POST, BIOS, booting, etc.
o
Expansion cards also have ROM ships that provide programming to instruct the
devices.

Types of ROM chips:
o
EEPROM:

EEPROM = electrically erasable programmable ROM

A.k.a. “Flash ROM”

Allow programs to be changed by applying a higher voltage to erase its
previous memory before new instructions are written.

Characterized by their capacity to be erased and reprogrammed directly
on their motherboard or expansion card they are installed on, with no
special equipment required.

o
Virtually all motherboards today use EEPROM.
EPROM:

EPROM = erasable programmable memory

Have a special window that allows the current memory contents to be
erased with an intense ultraviolet light, so the chip can be reprogrammed.
o
No matter which type of ROM you use, the data stored in a ROM chip is non-volatile
and remains permanently unless intentionally erased or overwritten.

Random Access Memory (RAM):
o Remember that programs and data stored on the hard drive needs to be loaded into
RAM to enable the CPU to swap data in and out quickly (p. 57).
o Remember that the Northbridge acts as the intermediary for all data transfers between
RAM and the CPU. For example, the CPU tells the Northbridge to update the data in
RAM (p. 57).
o RAM is a volatile medium, meaning that it requires continuous electricity to hold
data. Losing power for even a split second clears completely (p. 58).
o
Why Add RAM?

Adding or upgrading the RAM on a PC can make a dramatic difference
in performance (p. 71).

Not only does additional RAM allows you to run more programs at once,
but also make your OS more stable and less likely to crash (p. 71).

When you run out of RAM, your computer will begin to use the hard
drive. The process of computer switching from RAM to the hard drive is
known as virtual memory (p. 72).
o
Two Major Categories of RAM (p. 57):

Dynamic RAM (DRAM):

Memory that must be refreshed or recharged every few
milliseconds.


o
In fact, it can only hold a charge for about 4 milliseconds.
Static RAM (SRAM):

Memory that need not be refreshed.

Generally used only by the L1 cache and L2 cache.
Synchronous vs. Asynchronous:

Synchronous uses a clock signal to manage or synchronize its signals and
run in step with the processor.

Asynchronous means that the CPU waits while the Northbridge fetches
data, usually a period of 10-30 clock cycles.
o
RAM Sticks:

RAM comes on a small circuit board called a stick (p. 58).

RAM sticks are covered by small chips. These chips are the actual
locations of the RAM (p. 58).
Memory Slots
RAM Stick
o
RAM Packages:

When you pick up a stick of RAM, you need to be able to identify it by
its package. A package is basically another word for its form factor (p.
64).

There are four types of RAM package: SIMM, DIMM, SO DIMM, and
RIMM (p. 64).

SIMM (p. 64):

SIMM = Single Inline Memory Module

SIMMs come in two sizes: 30-pin, 72-pin

Occasionally you’ll see sockets for both packages.

30-pin size is 3.5 in. X .75 in.; 72-pin size is 4.25 in. X 1 in.
30-pin SIMM (top); 72-pin SIMM (below)

DIMM (p. 67):

DIMM = Dual Inline Memory Module

DIMMs come in two sizes: 168-pin, 184-pin

168-pin DIMM is 5.4 in. X 1 in.; 184-pin DIMM size is XX x
XX

DIMMs come in different speeds
168-pin DIMM (top); 184-pin DIMM (bottom)
Note the different notches along the bottom


SO-DIMM (p. 68):

SO-DIMM = Small Outline DIMM

Used for laptops
RIMM (p. 69):

RIMM is a package made by the Rambus company.

RIMMs are similar size to 184-pin DIMMs, but uses a faster
memory bus.
RIMM
o
Accessing RAM:

The CPU turns address wires on and off in certain patterns that represent
specific memory addresses. The Northbridge interprets the signal, access
the area addressed, and sends the data back (p. 59).

For example, that your computer only had 3 wires on the address bus.
Each wire has to be a 1 or 0. That leaves us with eight (23) possible
addresses (p. 59).

000
100
001
101
010
110
011
111
Today’s address bar has 32 wires meaning that it can address
4294967296 (232) areas of RAM.