Processing Unit Design - Fundamentals of Computer Organization and Architecture - page 86

2. A read signal is issued by the CPU.
3. The required word will be loaded by the memory into the MDR ready for use
by the CPU.

5.2.2. Instruction Fetching Registers

Two main registers are involved in fetching an instruction for execution: the pro-
gram counter (PC) and the instruction register (IR). The PC is the register that con-
tains the address of the next instruction to be fetched. The fetched instruction is
loaded in the IR for execution. After a successful instruction fetch, the PC is updated
to point to the next instruction to be executed. In the case of a branch operation, the
PC is updated to point to the branch target instruction after the branch is resolved,
that is, the target address is known.

5.2.3. Condition Registers

Condition registers, or flags, are used to maintain status information. Some architec-
tures contain a special program status word (PSW) register. The PSW contains bits
that are set by the CPU to indicate the current status of an executing program. These
indicators are typically for arithmetic operations, interrupts, memory protection
information, or processor status.

5.2.4. Special-Purpose Address Registers

Index Register As covered in Chapter 2, in index addressing, the address of the

operand is obtained by adding a constant to the content of a register, called the index
register. The index register holds an address displacement. Index addressing is indi-
cated in the instruction by including the name of the index register in parentheses
and using the symbol X to indicate the constant to be added.

Segment Pointers As we will discuss in Chapter 6, in order to support segmen-

tation, the address issued by the processor should consist of a segment number (base)
and a displacement (or an offset) within the segment. A segment register holds the
address of the base of the segment.

Stack Pointer As shown in Chapter 2, a stack is a data organization mechanism

in which the last data item stored is the first data item retrieved. Two specific oper-
ations can be performed on a stack. These are the Push and the Pop operations. A
specific register, called the stack pointer (SP), is used to indicate the stack location
that can be addressed. In the stack push operation, the SP value is used to indicate the
location (called the top of the stack). After storing (pushing) this value, the SP is
incremented (in some architectures, e.g. X86, the SP is decremented as the stack
grows low in memory).