Linux with Operating System Concepts

vola-
tile memory
meaning that it retains its contents only while power is being supplied to it. 
Shut down the computer and RAM loses its contents, resulting in RAM being empty. Turn 
on the computer and RAM is initially empty.
To run a program, you need the operating system loaded into memory and running. 
This leads to a paradoxical situation: how do we get the operating system loaded into mem-
ory and running if the operating system is needed to be loaded in memory and running in 
order to load and run programs? If memory were not volatile, we could keep the operating 
system in memory permanently; we could install the operating system the first time and 
leave it there. But this is not the case.
The reason that RAM memory is volatile is because of the technology we use. There are 
two forms of RAM memory: SRAM and DRAM. SRAM, or static RAM, is built out of 
transistors where several transistors make up each cell (storage location). One cell stores 
one bit. DRAM is made up of one transistor and one capacitor per storage location (bit). 
DRAM is smaller and cheaper, which results in a greater amount of DRAM storage avail-
able than SRAM. SRAM is used to build cache memories and registers while DRAM makes 
up what we often refer to as “main memory.” See Table 11.1 that differentiates between 
SRAM, DRAM, and ROM.
For SRAM and DRAM to maintain their storage, a constant supply of electrical current 
is needed from a voltage source. For SRAM, without that current, any charge is imme-
diately lost. For DRAM, the capacitor, once charged, retains that charge for a very short 
amount of time before the charge dissipates. DRAM chips are set up to recharge the cells 
that have a charge. If power is no longer supplying that chip, they cannot recharge the cells 
and so very quickly, any current is lost.
For us using the computer, all of memory’s contents would disappear once we shut the 
power off. Whatever had been stored there is lost. Shutting down the computer (or just 

---

System Initialization and Services
◾
447
unplugging it from the power supply) causes memory to become empty. Upon rebooting, 
memory remains empty and thus we need to first locate and load the operating system.
An alternative form of memory from SRAM and DRAM is called ROM, read-only 
memory. This form of memory is 
nonvolatile
meaning that when the power is shut off, 
ROM does not lose its contents. With the power off, ROM is not accessible but once power 
has been restored, ROM retains the contents that it had prior to losing power. ROM is set 
up with the information permanently stored in it. Thus, it cannot change or be written to. 
This is why we call this form of memory “read-only.” We will use ROM to help us solve 
the paradox of restoring DRAM with the operating system upon booting/rebooting the 
computer.
11.2.2 Boot Process
We need some initialization program that can, upon starting the computer, locate and load 
the operating system into DRAM. This initialization process will be called 
booting
(taken 
from the term “bootstrapping”). The boot process begins whenever a computer is cold 
booted (turned on) or soft booted (rebooted from software). We will store permanently 
some portions of the boot process (a program) in ROM chips.
For any computer, the first step in the boot process is to access the ROM BIOS. BIOS 
stands for basic I/O system. In Linux, the boot program starts at ROM BIOS address 
0xFFFF0. This notation is a hexadecimal address. The first task for the BIOS is to perform a 
Power-On Self-Test (POST), which examines various pieces of hardware connected to the 
computer to ensure that they are working properly.
Specifically, the POST tests the CPU registers, main memory, hardware devices such 
as the interrupt controller, disk controllers, and timer, and then identifies all devices cur-
rently connected via the system bus (namely, the keyboard, mouse, monitor). Additionally, 
it assembles a list of all devices that can be booted from (those devices that could store the 
operating system). These include hard disk(s), floppy disk, optical disk, flash drive, and 
network. The POST step may be skipped during a reboot (warm boot) as these devices are 
already on and functioning.
Loading the operating system may start automatically. Or, if instructed to by the user, 
the BIOS can present the list of bootable devices and await the user’s selection of a boot 
location. Most commonly, the list of bootable locations is preenumerated and prioritized. 
The user can later alter this prioritized list. Usually the network is the last on the list and 
TABLE 11.1 
Differences between Memory Types