It consists of various functional blocks:
Control Unit:
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.
Registers: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.
Accumalator:
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.
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), 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.
These flags have critical importance in the decision-making process of the microprocessor.
The conditions (set or reset) of the flags are tested through the software instructions.
The conditions (set or reset) of the flags are tested through the software instructions.
For example, the instruction JC (Jump on Carry) is
implemented to change the sequence of a program when CY flag is set. The
thorough understanding of flag is essential in writing assembly
language programs.
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 Control Unit.
General Purpose Register:
uP requires extra registers for versatility. Can be
used to store additional data during a program. More complex processors
may have a variety of differently named registers.
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