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The multitude of complex tasks performed by computers can be broken down
into sequences of simple operations that manipulate individual numbers and then
make decisions based on those calcula- tions. Certain types of basic
instructions are common across nearly every microprocessor in existence and
can be classi?ed as follows for purposes of discussion:

Arithmetic: add or subtract two values
Logical: Boolean (e.g., AND, OR, XOR, NOT, etc.) manipulation of one or
two values
Transfer: retrieve a value from memory or store a value to memory
Branch: jump ahead or back to a particular instruction if a speci?ed condition
is satis?ed

Arithmetic and logical instructions enable the microprocessor to modify and
manipulate speci?c pieces of data. Transfer instructions enable these data to be
saved for later use and recalled when necessary from memory. Branch operations
enable instructions to execute in different sequences, de- pending on the results
of arithmetic and logical operations. For example, a microprocessor can compare
two numbers and take one of two different actions if the numbers are equal or
unequal. Each unique instruction is represented as a binary value called an opcode.
A microprocessor fetches and executes opcodes one at a time from program
memory. Figure 3.4 shows a hypothetical microprocessor to serve as an example
for discussing how a microprocessor actually advances through and executes
the opcodes that form programs.

A microprocessor is a synchronous logic element that advances through opcodes
on each clock cycle. Some opcodes may be simple enough to execute in a single
clock cycle, and others may take multiple cycles to complete. Clock speed is often
used as an indicator of a microprocessors perfor- mance. It is a valid indicator
but certainly not the only one, because each microprocessor requires a different
number of cycles for each instruction, and each instruction represents a different
quantity of useful work.

FIGURE 3.4 Simple microprocessor.

When an opcode is fetched from memory, it must be brie?y examined to determine
what needs to be done, after which the appropriate actions are carried out.
This process is called instruction decod- ing. A central logic block coordinates
the operation of the entire microprocessor by fetching instructions from memory,
decoding them, and loading or storing any data as required. The accumulator is
a register that temporarily holds data while it is being processed. Execution of
an instruction to load the accumulator with a byte from memory would begin with
a fetch of the opcode that represents this action. The instruction decoder would
then recognize the opcode and initiate a memory read via the same microprocessor
bus that was used to fetch the opcode. When the data returns from memory,
it would be loaded into the accumulator. While there may be multiple distinct
logical steps in decod- ing an instruction, the steps may occur simultaneously or
sequentially, depending on the architecture of the microprocessor and its
decoding logic.

The accumulator is sized to hold the largest data value that the microprocessor
can handle in a single arithmetic or logical instruction. When engineers talk of an
8-bit or 32-bit microprocessor, they are usually referring to the internal data-path
width the size of the accumulator and the arith- metic logic unit (ALU). The ALU
is sometimes the most complex single logic element in a micro- processor.
It is responsible for performing arithmetic and logical operations as directed by the
instruction decode logic. Not only does the ALU add or subtract data from the
accumulator, it also keeps track of status ?ags that tell subsequent branch
instructions whether the result was positive, negative, or zero, and whether an
addition or subtraction operation created a carry or borrow bit. These status
bits are also updated for logical operations such as AND or OR so that software
can take different action if a logical comparison is true or false.

For ease of presentation, the microprocessor in Fig. 3.4 is shown having a single
general-purpose accumulator register. Most real microprocessors contain more
than one internal register that can be used for general manipulation operations.
Some microprocessors have as few as one or two such registers, and some have
dozens or more than a hundred. It is the concept of an accumulator that is discussed
here, but there is no conceptual limitation on how many accumulators or registers
a microprocessor can have.

A microprocessor needs a mechanism to keep track of its place in the instruction
sequence. Like a bookmark that saves your place as you read through a book,
the program counter (PC) maintains the address of the next instruction to be
fetched from program memory. The PC is a counter that can be reloaded with
a new value from the instruction decoder. Under normal operation, the
microprocessor moves through instructions sequentially. After executing each
instruction, the PC is incremented, and a new instruction is fetched from the
address indicated by the PC. The major exception to this linear behavior is
when branch instructions are encountered. Branch instructions exist speci?cally
to override the sequential execution of instructions. When the instruction decoder
fetches a branch in- struction, it must determine the condition for the branch.
If the condition is met (e.g., the ALU zero ?ag is asserted), the branch target
address is loaded into the PC. Now, when the instruction decoder goes to
fetch the next instruction, the PC will point to a new part of the instruction
sequence instead of simply the next program memory location.

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