Processing Unit Design - Fundamentals of Computer Organization and Architecture - page 85

The remainder of this chapter is organized as follows. Section 5.2 presents the
register set and explains the different types of registers and their functions. In Sec-
tion 5.3, we will understand what is meant by datapath and control. CPU instruction
cycle and the control unit will be covered in Sections 5.4 and 5.5, respectively.
5.2. REGISTER SET
Registers are essentially extremely fast memory locations within the CPU that are
used to create and store the results of CPU operations and other calculations. Differ-
ent computers have different register sets. They differ in the number of registers, reg-
ister types, and the length of each register. They also differ in the usage of each
register. General-purpose registers can be used for multiple purposes and assigned
to a variety of functions by the programmer. Special-purpose registers are restricted
to only specific functions. In some cases, some registers are used only to hold data
and cannot be used in the calculations of operand addresses. The length of a data
register must be long enough to hold values of most data types. Some machines
allow two contiguous registers to hold double-length values. Address registers
may be dedicated to a particular addressing mode or may be used as address general
purpose. Address registers must be long enough to hold the largest address. The
number of registers in a particular architecture affects the instruction set design.
A very small number of registers may result in an increase in memory references.
Another type of registers is used to hold processor status bits, or flags. These bits
are set by the CPU as the result of the execution of an operation. The status bits
can be tested at a later time as part of another operation.
5.2.1. Memory Access Registers
Two registers are essential in memory write and read operations: the memory data
register (MDR) and memory address register (MAR). The MDR and MAR are used
exclusively by the CPU and are not directly accessible to programmers.
In order to perform a write operation into a specified memory location, the MDR
and MAR are used as follows:
1. The word to be stored into the memory location is first loaded by the CPU into
MDR.
2. The address of the location into which the word is to be stored is loaded by the
CPU into a MAR.
3. A write signal is issued by the CPU.
Similarly, to perform a memory read operation, the MDR and MAR are used as
follows:
1. The address of the location from which the word is to be read is loaded into
the MAR.