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With the information saved in the above format in a byte called FLAGS, the following two instructions will restore all the saved flag values:

This instruction sequence loads the saved flags into the accumulator and then doubles the value, thereby moving each bit one position to the left. This causes each flag to be set to its original value, for the following reasons:

• The original value of the CARRY flag, being in the leftmost bit, will be moved out of the accumulator and wind up in the CARRY flag.
• The original value of the SIGN flag, being in bit 6, will wind up in bit 7 and will become the sign of the result. The new value of the SIGN flag will reflect this sign.
• The complement of the original value of the PARITY flag will wind up in bit 1, and it alone will determine the parity of the result (all other bits in the result are paired up and have no net effect on parity). The new setting of the PARITY flag will be the complement of this bit (the flag denotes even parity) and therefore will take on the original value of the PARITY flag.
• Whenever the ZERO flag is 1, the SIGN flag must be 0 (zero is a positive two's-complement number) and the PARITY flag must be 1 (zero has even parity). Thus an original ZERO flag value of 1 will cause all bits of FLAGS, with the possible exception of bit 7, to be 0. After the ADD instruction is executed, all bits of the result will be 0 and the new value of the ZERO flag will therefore be 1.
• An original ZERO flag value of 0 will cause two bits in FLAGS to be 1 and will wind up in the result as well. The new value of the ZERO flag will therefore be 0.

The above algorithm relies on the fact that flag values are always consistent, i.e., that the SIGN flag cannot be a 1 when the ZERO flag is a 1. This is always true in the 8008, since the flags come up in a consistent state whenever the processor is reset and flags can only be modified by instructions which always leave the flags in a consistent state. The 8080 and its derivatives allow the programmer to modify the flags in an arbitrary manner by popping a value of his choice off the stack and into the flags. Thus the above algorithm will not work on those processors.

A code sequence for saving the flags in the required format is as follows:

1. Addition. Numbers can be represented as a sequence of decimal digits by using a 4-bit binary encoding of the digits and packing these encodings two to a byte. Such a representation is called packed BCD (unpacked BCD would contain only one digit per byte). In order to preserve this decimal interpretation in performing binary addition on packed BCD numbers, the value 6 must be added to each digit of the sum whenever (1) the resulting digit is greater than 9 or (2) a carry occurs out of this digit as a result of the addition. This is because the 4-bit encoding contains six more combinations than there are decimal digits. Consider the following examples (numbers are written in hexadecimal instead of binary for convenience).

Example 1: 81+52

d2	d1	d0	names of digit positions packed BCD augend packed BCD addend   adjustment because d1 > 9 packed BCD sum
+	8 5	1 2
+	D 6	3
1	3	3

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