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.
Arithmetic Logic Unit
The ALU performs the actual numerical and logic operation such as ‘add’, ‘subtract’,
‘AND’, ‘OR’, etc. Uses data from memory and from Accumulator to perform
arithmetic. Always stores result of operation in Accumulator.
The 8085/8080A-programming model includes six registers, one accumulator, and
one flag register, as shown in Figure. In addition, it has two 16-bit registers: the stack
pointer and the program counter. They are described briefly as follows.
The 8085/8080A has six general-purpose registers to store 8-bit data; these are
identified as B,C,D,E,H, and L as shown in the figure. They can be combined as
register pairs - BC, DE, and HL - to perform some 16-bit operations. The
programmer can use these registers to store or copy data into the registers by using
data copy instructions.
The accumulator is an 8-bit register that is a part of arithmetic/logic unit (ALU). This
register is used to store 8-bit data and to perform arithmetic and logical operations.
The result of an operation is stored in the accumulator. The accumulator is also
identified as register A.
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), and Auxiliary Carry (AC) flags; they are
listed in the Table and their bit positions in the flag register are shown in the Figure
below. 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 -- called the Carry flag (CY) -- is
set to one. When an arithmetic operation results in zero, the flip-flop called the
Zero(Z) flag is set to one. The first Figure shows an 8-bit register, called the flag
register, adjacent to the accumulator. However, it is not used as a register; five bit
positions out of eight are used to store the outputs of the five flip-flops. The flags are
stored in the 8-bit register so that the programmer can examine these flags (data
conditions) by accessing the register through an instruction.