There are several components inside a CPU, namely, ALU, control unit,
general purpose register, Instruction registers etc. Now we will see
how these components are organized inside CPU. There are several ways to
place these components and inteconnect them. One such organization is shown in the Figure 5.6.
In this case, the arithmatic and logic unit (ALU), and all CPU registers are connected via a single common bus. This bus is internal to CPU and this internal bus is used to transfer the information between different components
of the CPU. This organization is termed as single bus organization, since only one internal bus is used for transferring of information between different components of CPU. We have external bus or buses to CPU also to
connect the CPU with the memory module and I/O devices. The external memory bus is also shown in the Figure 5.6 connected to the CPU via the memory data and address register MDR and MAR.
The number and function of registers R0 to R(n-1) vary considerably from
one machine to another. They may be given for general-purpose for the use
of the programmer. Alternatively, some of them may be dedicated as
special-purpose registers, such as index register or stack pointers.
In this organization, two registers, namely Y and Z are used which are
transperant to the user. Programmer can not directly access these two
registers. These are used as input and output buffer to the ALU which will
be used in ALU operations. They will be used by CPU as temporary storage
for some instructions.