Page 32 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Unit 2: Data Processing

2.4 Registers Notes

In computer architecture, a processor register (or general purpose register) is a small amount
of storage available on the CPU whose contents can be accessed more quickly than storage
available elsewhere. Typically, this specialized storage is not considered part of the normal
memory range for the machine. Most, but not all, modern computers adopt the so-called load-
store architecture. Under this paradigm, data is loaded from some larger memory be it cache
or RAM into registers, manipulated or tested in some way (using machine instructions for
arithmetic/logic/comparison) and then stored back into memory, possibly at some different
location. A common property of computer programs is locality of reference: the same values
are often accessed repeatedly; and holding these frequently used values in registers improves
program execution performance.

Processor registers are at the top of the memory hierarchy, and provide
the fastest way for a CPU to access data. The term is often used to refer only
to the group of registers that are directly encoded as part of an instruction, as
defined by the instruction set.

Allocating frequently used variables to registers can be critical to a program’s performance. This
action (register allocation) is performed by a compiler in the code generation phase.

2.4.1 Categories of Registers

Registers are normally measured by the number of bits they can hold, for example, an “8-bit
register” or a “32-bit register”. A processor often contains several kinds of registers, that can be
classified accordingly to their content or instructions that operate on them:
(a) User-accessible registers: The most common division of user-accessible registers is into
data registers and address registers.
(b) Data registers: These are used to hold numeric values such as integer and floating-point
values. In some older and low end CPUs, a special data register, known as the accumulator,
is used implicitly for many operations.
(c) Address registers: Its hold addresses and are used by instructions that indirectly access
memory.
• Some processors contain registers that may only be used to hold an address or only to
hold numeric values (in some cases used as an index register whose value is added as
an offset from some address); others allow registers to hold either kind of quantity. A
wide variety of possible addressing modes, used to specify the effective address of an
operand, exist.
• A stack pointer, sometimes called a stack register, is the name given to a register that
can be used by some instructions to maintain a stack.
(d) Conditional registers: Its hold truth values often used to determine whether some instruction
should or should not be executed.
(e) General purpose registers (GPRs): Its can store both data and addresses, i.e., they are
combined Data/Address registers.
(f) Floating point registers (FPRs): Its store floating point numbers in many architectures.
(g) Constant registers: Its hold read-only values such as zero, one, or pi.

(h) Vector registers: Its hold data for vector processing done by SIMD instructions (Single
Instruction, Multiple Data).

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