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SAFC161PI Datasheet(PDF) 66 Page - Infineon Technologies AG |
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SAFC161PI Datasheet(HTML) 66 Page - Infineon Technologies AG |
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66 / 400 page  &3, 7KH &HQWUDO 3URFHVVLQJ 8QLW &38 User’s Manual 4-18 1999-08 After execution of an instruction which explicitly updates the PSW register, the condition flags cannot be interpreted as described in the following, because any explicit write to the PSW register supersedes the condition flag values, which are implicitly generated by the CPU. Explicitly reading the PSW register supplies a read value which represents the state of the PSW register after execution of the immediately preceding instruction. Note: After reset, all of the ALU status bits are cleared. • N-Flag: For most of the ALU operations, the N-flag is set to ’1’, if the most significant bit of the result contains a ’1’, otherwise it is cleared. In the case of integer operations the N-flag can be interpreted as the sign bit of the result (negative: N=’1’, positive: N=’0’). Negative numbers are always represented as the 2's complement of the corresponding positive number. The range of signed numbers extends from '–8000 H' to '+7FFFH' for the word data type, or from '–80 H' to '+7FH' for the byte data type.For Boolean bit operations with only one operand the N-flag represents the previous state of the specified bit. For Boolean bit operations with two operands the N-flag represents the logical XORing of the two specified bits. • C-Flag: After an addition the C-flag indicates that a carry from the most significant bit of the specified word or byte data type has been generated. After a subtraction or a comparison the C-flag indicates a borrow, which represents the logical negation of a carry for the addition. This means that the C-flag is set to ’1’, if no carry from the most significant bit of the specified word or byte data type has been generated during a subtraction, which is performed internally by the ALU as a 2’s complement addition, and the C-flag is cleared when this complement addition caused a carry. The C-flag is always cleared for logical, multiply and divide ALU operations, because these operations cannot cause a carry anyhow. For shift and rotate operations the C-flag represents the value of the bit shifted out last. If a shift count of zero is specified, the C-flag will be cleared. The C-flag is also cleared for a prioritize ALU operation, because a ’1’ is never shifted out of the MSB during the normalization of an operand. For Boolean bit operations with only one operand the C-flag is always cleared. For Boolean bit operations with two operands the C-flag represents the logical ANDing of the two specified bits. • V-Flag: For addition, subtraction and 2’s complementation the V-flag is always set to ’1’, if the result overflows the maximum range of signed numbers, which are representable by either 16 bits for word operations ('–8000 H' to '+7FFFH'), or by 8 bits for byte operations ('–80 H' to '+7FH'), otherwise the V-flag is cleared. Note that the result of an integer addition, integer subtraction, or 2's complement is not valid, if the V-flag indicates an arithmetic overflow. |
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