Flags

The ALU includes five flip-flops, which are set or reset after an operation according

to data conditions of the result in the accumulator and other registers.

They are called

Zero(Z)

Carry (CY)

Sign (S)

Parity (P)

Auxiliary Carry (AC)

The most commonly used flags are Zero, Carry, and Sign. The microprocessor uses these flags to test data conditions.

For example, after an addition of two numbers, if the sum in the accumulator id larger than eight bits, the flip-flop uses to indicate a carry is called the Carry flag (CY) which is

set to one. When an arithmetic operation results in zero, the flip-flop called the

Zero(Z) flag is set to one.

Program Counter (PC)

This 16-bit register deals with sequencing the execution of instructions. This register

is a memory pointer. Memory locations have 16-bit addresses, and that is why this is a

16-bit register.

The microprocessor uses this register to sequence the execution of the instructions.

The function of the program counter is to point to the memory address from which the

next byte is to be fetched. When a byte (machine code) is being fetched, the program

counter is incremented by one to point to the next memory location

Stack Pointer (SP)

The stack pointer is also a 16-bit register used as a memory pointer. It points to a

memory location in R/W memory, called the stack. The beginning of the stack is

defined by loading 16-bit address in the stack pointer. The stack concept is explained

in the chapter "Stack and Subroutines."

Instruction Register/Decoder

Temporary store for the current instruction of a program. Latest instruction sent here

from memory prior to execution. Decoder then takes instruction and ‘decodes’ or

interprets the instruction. Decoded instruction then passed to next stage.

Memory Address Register

Holds address, received from PC, of next program instruction. Feeds the address bus

with addresses of location of the program under execution.

Control Generator

Generates signals within uP to carry out the instruction which has been decoded. In

reality causes certain connections between blocks of the uP to be opened or closed, so

that data goes where it is required, and so that ALU operations occur.

Register Selector

This block controls the use of the register stack in the example. Just a logic circuit

which switches between different registers in the set will receive instructions from