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670 Part 3½ Computer Classes Section 3 ½Minicomputers

The inputs to the ALU are the A leg and the B leg. The A leg is normally fed from a multiplexer (Aleg MUX), which may select from an operand supplied it from the scratch-pad memory (S PM) and possibly from a small set of constants and/or the processor status register (PS). The B leg also is typically fed from its own MUX (Bleg MUX), its selections being among the B register and certain constants. In addition, the Bleg MUX may be configured so that byte selection, sign extension, and other functions may be performed on the operand which it supplies to the ALU.

Following the ALU is a multiplexer (the AMUX) typically used to select between the output of the ALU, the data lines of the Unibus, and certain constants. The output of the AMUX provides


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