The Digital Computer
A digital computer is a collection of logic elements that can execute arbitrary
algorithms to perform data calculation and manipulation functions. A computer is
composed of a microprocessor, memory, and some input/output (I/O) elements as
shown in Fig. 3.1. The microprocessor, often called a microprocessor unit (MPU)
or central processing unit (CPU), contains logic to step through an algorithm,
called a program, that has been stored in the computers program memory.
The data used and manipulated by that program is held in the computers data
memory. Memory is a repository for data that is usually organized as a linear
array of individually accessible locations. The microprocessor can access a
particular location in memory by presenting a memory address (the index of the
desired location) to the memory element. I/O elements enable the microprocessor
to communicate with the outside world to acquire new data and present the
results of its programmed computations. Such ele- ments can include a
keyboard or display controller.
Programs are composed of many very simple individual operations, called
instructions, that specify in exact detail how the microprocessor should carry
out an algorithm. A simple program may have dozens of instructions, whereas
a complex program can have tens of millions of instructions. Collectively, the
programs that run on microprocessors are called software, in contrast to the
hard- ware on which they run. Each type of microprocessor has its own
instruction set that de?nes the full set of unique, discrete operations that it is
capable of executing. These instructions perform very nar- row tasks that,
on their own, may seem insigni?cant. However, when thousands or millions
of these tiny instructions are strung together, they may create a video game
or a word processor.
A microprocessor possesses no inherent intelligence or capability to spontaneously
begin per- forming useful work. Each microprocessor is constructed with an
instruction set that can be invoked in arbitrary sequences. Therefore, a
microprocessor has the potential to perform useful work but will do nothing of
the sort on its own. To make the microprocessor perform useful work, it
requires ex- plicit guidance in the form of software programming. A task of
even moderate complexity must be broken down into many tiny steps to be
implemented on a microprocessor. These steps include basic arithmetic,
Boolean operations, loading data from memory or an input element such as
a keyboard, and storing data back to memory or an output element such as
a printer.
Memory structure is one of a computers key characteristics, because the
microprocessor is al- most constantly accessing it to retrieve a new instruction,
load new data to operate on, or store a cal- culated result. While program and
data memory are logically distinct classi?cations, they may share the same
physical memory resource. Random access memory (RAM) is the term used
to describe a generic memory resource whose locations can be accessed,
or addressed, in an arbitrary order and either read or written. A read is the
process of retrieving data from a memory address and loading it into the
microprocessor. A write is the process of storing data to a memory address
from the micro- processor. Both programs and data can occupy RAM.
Consider your desktop computer. When you execute a program that is located
on the disk drive, that program is ?rst loaded into the computers RAM and
then executed from a region set aside for program memory. As on a desktop
computer, RAM is most often volatile meaning that it loses its contents when
the power is turned off.
FIGURE 3.1 Generic computer block diagram.
Some software cannot be stored in volatile memory, because basic initialization
instructions, or boot code, must be present when the computer is turned on.
Remember that a microprocessor can do nothing useful without software being
readily available. When power is ?rst applied to a computer, the microprocessor
must be able to quickly locate boot code so that it can get itself ready to accept
input from a user or load a program from an input device. This startup sequence
is called booting, hence the term boot code. When you turn your computer on,
the ?rst messages that it displays on the monitor are a product of its boot code.
Eventually, the computer is able to access its disk drive and begins loading
software into RAM as part of its normal operation. To ensure that boot code
is ready at power-up, nonvolatile memory called read only memory (ROM)
exists. ROM can be used to store both programs as well as any data that must
be present at power-up and immediately accessible. Software contained in
ROM is also known as ?rmware. As its name implies, ROM can only be read
but not written. More complex computers contain a relatively small quantity
of ROM to hold basic boot code that then loads main operating software from
another device into RAM. Small computers may contain all of their software in
ROM. Figure 3.2 shows how ROM and RAM complement each other in
a typical computer architecture.
A microprocessor connects to devices such as memory and I/O via data and
address buses. Col- lectively, these two buses can be referred to as the
microprocessor bus. A bus is a collection of wires that serve a common purpose.
The data bus is a bit array of suf?cient size to communicate one complete data
unit at a time. Most often, the data bus is one or more bytes in width. An
eight-bit micro- processor, operating on one byte at time, almost always has
an eight-bit data bus. A 32-bit microprocessor, capable of operating on up to
4 bytes at a time, can have a data bus that is 32, 16, or 8 bits wide. The exact
data bus width is implementation speci?c and varies according to the intended
application of the microprocessor. A narrower bus width means that it will take
more time to com- municate a quantity of data as compared to a wider bus.
Common notation for a data bus is D[7:0] for an 8-bit bus and D[31:0] for
a 32-bit bus, where 0 is the least-signi?cant bit.
The address bus is a bit array of suf?cient size to fully express the microprocessors
address space. Address space refers to the maximum amount of memory and I/O
that a microprocessor can directly address. If a microprocessor has a 16-bit address
bus, it can address up to 216 = 65,536 bytes. Therefore, it has a 64 kB address
space. The entire address space does not have to be used; it simply establishes
a maximum limit on memory size. Common notation for a 16-bit address bus is
A[15:0], where 0 is the least-signi?cant bit. Figure 3.3 shows a typical
microprocessor bus con?gu- ration in a computer. Note that the address bus is
unidirectional (the microprocessor asserts re- quested addresses to the various
devices), and the data bus is bidirectional (the microprocessor
asserts data on a write and the devices assert data on reads).
FIGURE 3.2 Basic ROM/RAM memory complement.
FIGURE 3.3 Microprocessor buses.
A microprocessors entire address space is never occupied by a single function;
rather, it is shared by ROM, RAM, and various I/Os. Each device is mapped
into its own region of the address space and is enabled only when the
microprocessor asserts an address within a devices mapped region. The
process of recognizing that an address is within a desired region is called
decoding. Ad- dress decoding logic is used to divide the overall address space
into smaller sections in which memory and I/O devices can reside. This logic
generates individual signals that enable the appropri- ate device based on the
state of the address bus so that the devices themselves do not need any
knowledge of the speci?c computers unique address decoding.
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