The bit position of the flip flop in flag register is: All of the three flip flop set and reset according to the stored result in the accumulator. Sign- If D7 of the result is 1 then sign flag is set otherwise reset. As we know that a number on the D7 always desides the sign of the number. Zeros Z -If the result stored in an accumulator is zero then this flip flop is set otherwise it is reset. Auxiliary carry AC -If any carry goes from D3 to D4 in the output then it is set otherwise it is reset. Carry C -If the result stored in an accumulator generates a carry in its final output then it is set otherwise it is reset.
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Interrupt unit Serial IO unit These five units consist of other internal parts. These units of microprocessor architecture are helpful in understanding the microprocessor. The following section gives a detail view of microprocessor. Two bit address registers PC and SP. Resigter unit of Microprocessor General purpose data register consists of six general-purpose data registers to store 8-bit data.
It is very important section in microprocessor Architecture. During the execution of a program the user can use these registers to store a data or copy a data temporarily by using data transfer instructions.
Though these registers are of 8 bits but whenever the microprocessor has to handle bit data. The microprocessors architecture also have two 8-bit internal data registers. These registers are W and Z. These registers are for internal operation of the microprocessor and not available to the user. Microprocessor uses these registers internally. Program counter PC It is a bit register which deals with sequencing the execution of instructions.
This register is a memory pointer. Memory locations have bit addresses which is why this is a bit register. The microprocessor uses program counter to sequence the execution of the instructions. The function of this register is to point to the memory address from which the next byte is to be fetch.
When a data byte machine code is being fetched, and the program counter is incremented by one to point to the next memory location. Stack pointer SP This register is of bit. Microprocessor uses this register as a memory pointer. The beginning of the stack is defined by loading bit address in the stack pointer. For instance, every time microprocessor accesses a memory, its PC register increments by one.
This unit works as a MUX when data is going from the register to the internal data bus. It works as a DEMUX when data is coming to a register from the internal data bus of the microprocessor. These registers actually work as the buffer stage between the microprocessor and external system buses. Control Unit This is also important unit in microprocessor Architecture analysis. This unit generates signals within microprocessor to carry out the instruction, which has been decoded.
Practically it causes certain connections between blocks of the microprocessor to be opened or closed. The control unit itself consists of three parts; the instructions register IR. Instruction decoder and machine cycle encoder and control and timing unit. Instruction registers The Instruction register holds the machine code of the instruction. When microprocessor executes a program, it reads the machine code or opcode from the memory.
This opcode is stored in the instruction register. Instruction decoder and machine cycle encoder The Instruction register sends the machine code to this unit.
It decodes the opcode and finds out what is to be done in response of the coming opcode and how many machine cycles are required to execute this instruction. Control and timing unit The control unit generates signals internally in the microprocessor to carry out the instruction, which has been decoded. Practically, it causes certain connections between blocks of the microprocessor to be opened or closed.
So that the data goes where it is required to be fetch and the ALU operations occur. ALU uses data from memory and from accumulator to perform the arithmetic operation.
It always stores the result of the operation in accumulator. The ALU has accumulator, flag register and temporary register.
Accumulator The accumulator is an 8-bit register. The microprocessor uses this register to store 8-bit data. It performs arithmetical and logical operations. The microprocessor stores result of every ALU operation in the accumulator. You can identify an accumulator as register A. Flags The ALU also has five flip-flops. They becomes set or reset after an operation. The value is according to the data conditions of the result in the accumulator and other registers.
The bit positions in the flag register are in Figure below. The microprocessor uses these flags to set and to test data conditions. For example, after an addition of two numbers. If the sum in the accumulator is larger than 8-bit. Then the flip-flop indicates a carry called the carry flag CY to set to one. When an arithmetic operation like add or subtract results in zero.
Then the flip-flop called the zero Z flag is set to one. Figure shows this 8-bit flag register, adjacent to the accumulator. Format of the flag register Format of the flag register The flags are stored in the 8-bit register so that the programmer can examine these flags by accessing the register through some instructions. These flags are important in the decision-making process of the microprocessor. The conditions set or reset of the flags are check by the software instructions.
For instance, the instruction JC jump on carry runs to change the sequence of a program when CY flag is set. Proper understanding of flag is essential in writing assembly language programs. Z Zero flag: This flag indicates whether the result of a mathematical or logical operation is zero or not.
If the result of the current operation is zero, then this flag will be set or reset. CY Carry flag: The carry flag indicates whether, during an addition or subtraction operation a carry or a borrow is there or not. If carry is generated then this flag bit will be set. This flag can also be set before a mathematical operation as an extra operand to certain instruction. Auxiliary carry flag Here, a carry generates from D3, bit position and propagates to the D4 position.
This carry is called auxiliary carry. This flag is never used for setting or testing a condition. S Sign flag: This flag indicates whether the result of a mathematical operation is negative or positive. If the result is negative then sign flag will be set and if the result is positive, then this flag will reset. Actually, this bit is a replica of the D7 bit. This flag shows or indicates whether current result is of even parity 1 or of odd parity 0.
8085 Microprocessor Architecture & Working
This microprocessor exhibits some unique characteristics and this is the reason it still holds popularity among the microprocessors. Basically, was the first commercially successful microprocessor by Intel. As some of the architectural drawbacks associated with was also eliminated by The size of the data bus of is 8 bits while that of the address bus is
ADD B? Add the content of register B to the content of the accumulator. All the above two examples are only one byte long. All one-byte instructions contain information regarding operands in the opcode itself. Two-byte instruction: In a two byte instruction the first byte of the instruction is its opcode and the second byte is either data or address. The first byte 06 is the opcode for MVI B and second byte 05 is the data which is to be moved to register B.
Microprocessor - 8085 Architecture