Chip decryption knowledge: the arithmetic unit of the microcontroller

The arithmetic unit is based on the ALU, which is a binary arithmetic/logic operation unit, and is supplemented by the temporary register TMP, the accumulator ACC, the register B, the program status flag register PSW, and the Boolean processor. The accumulator ACC is an eight-bit register, which is the most frequently used register in the CPU. When performing arithmetic and logical operations, the accumulator ACC often temporarily stores an operand (such as the addend) before the operation, and saves the result (such as the algebraic sum) after the operation. Register B is mainly used for multiplication and division operations. The flag register PSW is also an eight-bit register, which is used to store some characteristics of the operation result, such as whether there is a carry or borrow. The specific meaning of each bit is as follows. PSW CY AC FO RS1 RS0 OV － P For users, the following four bits are of most concern.
1? Carry flag CY (PSW?7). It indicates whether there is a carry (or borrow) in the operation. If the operation result has a carry (addition) or a borrow (subtraction) in the highest bit, this bit is 1, otherwise it is 0.
2? Auxiliary carry flag AC Also known as the half-carry flag, it reflects whether there is a half-carry in the lower four bits of the two eight-digit operation, that is, whether there is a carry (or borrow) when adding (or subtracting) the lower four bits. If so, AC is in the 1 state, otherwise it is 0.
3? Overflow flag OV. MCS-51 reflects whether the operation result of the signed number overflows. If there is an overflow, this bit is 1, otherwise it is 0.
4? Parity flag P. Reflects the parity of the content of the accumulator ACC. If the operation result in ACC has an even number of 1s (such as 11001100B, which has 4 1s), P is 0, otherwise, P=1.
The other bits of PSW will be introduced later. Since PSW stores the status of program execution, it is also called the program status word? There is also a logic processor (also known as a Boolean processor) in the operator that performs logical operations on a bit basis. Its function will be explained when introducing bit instructions.