Early Electronic Calculator Casio AL-1000 Calculator Technical Description: The Arithmetic Unit

The arithmetic unit is a 4-bit parallel adder/subtracter with BCD sum correction. Three inputs control the primary function performed as per table 1. A carry flag is incorporated for carry between digits and additional logic permits a carry to be injected for several reasons: incrementing or decrementing a register's decimal point position, incrementing or decrementing a register digit, and incrementing the program counter. Additional inputs permit disabling of the sum correction so that base-16 data (the decimal point position and program instructions) will not be corrected.

The BCD sum correction is performed by some additional combinatorial logic placed at the outputs of the 4-bit parallel adder/subtracter and which adjust the outputs before they leave the arithmetic unit. Table 2 shows the corrections performed. Blank entries in the table are situations that do not occur for the given function.