The 4bit Juno ALU is based around an 74 series 181 ALU arithmetic logic chip which implements mathematical and logical operations. The '181 was used in many early minicomputers like the NOVA and PDP-11.
The mathematical operations include addition, subtration with or without carry and the logical operations include logical ∧ AND, NAND, ∨ OR, NOR, ⊕ XOR and shift.
The inputs to the ALU are the DR register and the data bus. Operations are performed by loading the DR register with the first operand and then placing the second operand on the data bus. The output of the operation is stored in A register.
The Carry input is selected using the carry multiplexer and can be (0) zero, (1) one or (2) the carry bit of the status register.
The Carry and Comparator result of the operation can be saved to the status register using LD_Carry and LD_Comparator.
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