l Look-through read In look-through read, the
cache is checked first. If a miss occurs, the reference is
sent to main memory to be serviced. This is known as
a serial read policy.
l Look-aside read A look-aside read presents
both cache and main memory with the reference
simultaneously. Since the cache will respond faster, if
a hit occurs, the request can be terminated before main
memory responds. This is known as a parallel read
CACHE REPLACEMENT POLICIES. When
new data is read into the cache, a replacement policy
determines which block of old data should be replaced.
The objective of replacement policies is to retain data
that is likely to be used in the near future and discard
data that wont be used immediately. The replacement
policies include the following:
. FIFO The first block that was read into cache
is the first one to be discarded.
l LRU The block that hasnt been used in the
longest period of time is replaced by the new block.
Blocks are replaced randomly.
. Optimum This cache replacement algorithm is
psychic and has perfect knowledge of the future.
Optimum replacement is what the other three strive for,
with LRU coming the closest.
CACHE WRITE. Since the cache contents area
duplicate copy of information in main memory, writing
(instructions to enter data) to the cache must eventually
be made to the same data in main memory. This is done
in two ways as follows:
l Write-through cache Writing is made to the
corresponding data in both cache and main memory.
. Write-back cache Main memory is not updated
until the cache page is returned to main memory.
READ-ONLY MEMORY (ROM)
Every computer comes with a set of software
instructions supplied by the manufacturer. This enables
the computer to perform its I/O operations. These
permanent instructions (routines) reside in a read-only
memory (ROM). ROM is often referred to as
software permanently contained in
hardware. The instructions are considered permanent
or nonvolatile, since they are not erased each time the
computer loses power or is turned off. The ROM
contains the program that defines its uniqueness
compared with all other types of computers.
The ROM is programmed at the time of
manufacture and cannot be altered. It is tailored to
system requirements. It cannot be altered except by
removing and replacing iteither a module or IC chip
on a board. The contents of the ROM are electrically
unalterable. Other variations of ROMs called PROMS
can be reprogrammed as required. This and other
variations are covered in further detail in chapter 6 on
In connection with the ROM, you will hear the term
boot procedure used.
The ROM initiates the boot
procedurea sequence of steps followed when you
turn on the power to the computer or initiate the boot
procedure. The steps required to successfully boot the
computer depend on the type of computer. Other terms
that have the same meaning as boot include boot up,
booting, or bootstrap. They all refer to the process of
loading the software.
Consult your computers
technical or owners manual for the exact procedures
for your computer system. We use two types of ROMs
to discuss some of the programs associated with the
ROM: nondestructive readout (NDRO) memory and
basic input/output system (BIOS).
Nondestructive Readout (NDRO) Memory
A nondestructive readout (NDRO) memory is
usually associated with a militarized mainframe or
The NDRO is a small module that
occupies two or more slots. For mainframes, it is
located in the CPU module. For minicomputers, it is
located in the chassis that contains the CPUs pcbs.
The functions of an NDRO are controlled from the
computers controlling device: a maintenance console
or equivalent. The sizes of the NDRO addresses vary
with the type of computer and its requirements.
Selection of a particular word in the NDRO is via the
NDRO address select, line selector, and current switch
AN NDRO consists of hardwired circuits to
create the bootstrap programs or a ROM or PROM.
Some of the programs contained on an NDRO include
l Two bootstrap programs Used to load
programs from peripheral equipments into main
. Autostart programs