Page 183 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

164 Chapter Fifteen

One of the major disadvantages of currently available bulk storage units is
their relatively slow speed.3 The CPU can process data at a much higher rate
than the bulk storage can supply or store it. Semiconductor memories are
significantly more expensive than bulk storage, but they are also a great deal
faster.
ROM devices are said to be musk-programmable because the data they
contain is hard-coded into them during their construction (using photo-masks
as was discussed in the previous chapter). ROMs are also classed as being
nonvolatile, because their data remains when power is removed from the system.
By comparison, RAM devices initialize containing random logic 0 or logic 1
values when power is first applied to a system. Thus, any meaningful data stored
inside a RAM must be written into it by other components in the system after
it has been powered-up. Additionally, RAMS are said to be volatile, because any
data they contain is lost when power is removed from the system.
When a computer system is first powered up, it doesn’t know much about
anything. The CPU is hard-wired so that the first thing it does is read an
instruction from a specific memory address: for example, address zero. The
components forming the system are connected together in such a way that
this hard-wired address points to the first location in a block of ROM?
The ROM contains a sequence of instructions that are used by the CPU to
initialize both itself and other parts of the system. This initialization is known
as boot-strapping, which is derived from the phrase “pulling yourself up by your
boot-straps . ” At an appropriate point in the initialization sequence, instructions
in the ROM cause the CPU to copy a set of master programs, known collectively
as the operating system (OS), from the bulk storage into the RAM. Finally,
the instructions in the ROM direct the CPU to transfer its attention to the
operating system instructions in the RAM, at which point the computer is
ready for the user to enter the game.

3 Note the use of the qualifier “relatively.” Modern bulk storage is actually amazingly fast, but not as
fast as the rest of the system.
4 Actually, if the truth be told, these days the block of memory pointed to by the CPU on power-up is
typically formed from another form of memory like FLASH, which is non-volatile like ROM, but
which can be re-programmed (if necessary) like RAM (FLASH is introduced later in this chapter).