Draw the format of PSW register of 8051 microcontroller and explain the function of each bit.

Draw the format of PSW register of 8051 microcontroller and explain the function of each bit.
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1 Answer

Answer :

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1. CY: Carry flag. This flag is set whenever there is a carry out from the D7 bit after an 8 bit addition or subtraction. It can also be set to 1 or 0 directly by instructions such as “SETB C” and CLR C” where “SETB C” stands for “set bit carry” and “CLR C” for “clear carry”.

2. AC: Auxiliary carry flag If there is a carry from D3 and D4 during an ADD or SUB operation, this bit is set; it is cleared. This flag is used by instructions that perform BCD (binary coded decimal) arithmetic. 

3. F0: Available to the user for general purposes. 

4. RS0, RS1: Register bank selects bits These two bits are used to select one of the four register banks from internal RAM as shown in given table. The user can use only one bank of register at one time. By default , bank 0 gets selected.

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5. OV: Overflow flag

This flag is set whenever the result of a signed number operation is too large, causing the high- order bit to overflow into the sign bit. In general, the carry flag is used to detect errors in unsigned arithmetic operations. The overflow flag is only used to detect errors in signed arithmetic operations.

6. P: Parity flag The parity flag reflects the number of 1s in the A (accumulator) register only. If the A register contains an odd number of 1s, then P=1. P=0 if A has an even number of 1s.

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