Page 64 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Introduction to Microprocessors

Notes 4.2.4 Address, Data and Status Registers and Stack Pointer

The Address Register contains address of a memory location to be accessed.
The Data Register contains the data coming from or going to memory or an I/O port.

The Status Register contains information about the result of the previous ALU operation.
The Stack Pointer Register contains the address of the block of memory (the stack) where subroutine
return addresses are stored.

4.2.5 Control Unit

The Control Unit contains the circuitry that controls the process of fetching, decoding and executing
program instructions.

The control unit is the circuitry that controls the flow of information through the processor, and
coordinates the activities of the other units within it. In a way, it is the “brain within the brain”, as
it controls what happens inside the processor, which in turn controls the rest of the PC.
The functions performed by the control unit vary greatly by the internal architecture of the CPU, since
the control unit really implements this architecture. On a regular processor that executes x86 instructions
natively, the control unit performs the tasks of fetching, decoding, managing execution and then
storing results. On a processor with a RISC core the control unit has significantly more work to do. It
manages the translation of x86 instructions to RISC micro-instructions, manages scheduling the micro-
instructions between the various execution units, and juggles the output from these units to make
sure they end up where they are supposed to go. On one of these processors the control unit may be
broken into other units (such as a scheduling unit to handle scheduling and a retirement unit to deal
with results coming from the pipeline) due to the complexity of the job it must perform.

4.2.6 CISC and RISC

Historically, the first type of ISA was the complex instruction set computers (CISC), and the
second type was the reduced instruction set computers (RISC). It is a common misunderstanding
that RISC systems typically have a small ISA (fewer instructions) but make up for it with faster
hardware. RISC system actually have “reduced instructions”, in the sense that each instruction
does so little that it takes very little time to execute it. It is a common misunderstanding that CISC
systems have more instructions, but typically pay a steep performance penalty for the added
versatility. CISC systems actually have “complex instructions”, in the sense that at least one
instruction takes a long time to execute for example, the “double indirect” addressing mode
inherently requires two memory cycles to execute, and a few CPUs have a “string copy” instruction
that may require hundreds of memory cycles to execute. MIPS and SPARC are examples of RISC
computers. Intel x86 is an example of a CISC computer.

The two main types of microprocessor architecture are complex instruction set computer (CISC)
and reduced instruction set computer (RISC). CISC architecture is much more complex and thus
can handle more complex commands. RISC uses a simpler architecture, so RISC microprocessors
are smaller and faster.

RISC system actually have “reduced instructions”, in the sense that each instruction
does so little that it takes very little time to execute it.

4.2.7 Manufacturing

Microprocessors are manufactured by etching different features (such as transistors and resistors)
into a tiny wafer of pure silicon and then adding various layers of conductors and insulators. The

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