Some terms

INSTRUCTION CYCLE


Instruction cycle is defined as the time required to complete one instruction.The instruction cycle
consists of 1-6 machine cycle.


MACHINE CYCLE


Machine cycle is defined as the time required to complete one operation of accessing memory, I/O operation or
acknowlegding an external request. This cycle may consist of 3-6 T states.


T-STATE


T-state is defined as one subdivision of operation performed in one clock period. These internal subdivision are
in synchronisation with the system clock. And each T-state is precisely equal to one clock period.

SIGN FLAG

The flag is used to differentiate between signed and unsigned numbers. In a geven byte if D7 is 1, the number will be viewed as a negative number; if it is zero it will be viewed as a positive number.

ZERO FLAG

The zero flag is set when ALU operation leads to 0, and flag is reset if result is not zero.

AUXILLARY CARRY

In an arithmetic operation, when a carry is generated by digit D3 and passed to digit D4, AC flag is set. The flag is used internally for BCD operation and not available to programmer.

PARITY FLAG

After an arithmetic or logical operation if number of 1's in Accumulator is even it is set and if its odd it is reset.

CARRY FLAG

If an aritmetic operation results in a carry the flag is set otherwise it is reset. This also serve as a borrow flag for subtraction.