Five small boards for main ALU are completed:

1: Source1 + Source2 16-bit input adaptor for double-board fast adder

2: Output adaptor (16-bit) for double-board fast adder

3,4: Two 16-bit input boards

5: 16-bit output board (leds).

The boards were used to test 16-bit Adder unit and 16-bit Logic Operations unit. While the latter one was tested, small bug was found - there was a short in one place - and fixed.

The output adaptor ties two 8-bit adder boards via routing Carry_out signal from one board to Carry_in to other, so that they act as one 16-bit adder. Also the output and input adaptors serve some mechanical support role making the construction stiffer.

The input boards are just switch banks connected to IDC connector, while output board have 16 red LEDs connected to the same connector.

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Next step will be a board combining function of the Incrementor and Negator units, and some glue logic.

Incrementor is just 16-bit 2:1 multiplexer which switches between 16-bit value from Source 2 and 8-bit constant that would be coming from the instruction word.

As for Negator - this circuit will consist of 5 74HC86 (4 x XOR) ICs, four of which are used to conditionally invert 16-bit value, and the fifth one is for the conditional invert of C_in and C_out signals.

And probably this same board could accommodate the shifter -- I am not sure, will investigate this.

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Adaptor boards:

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Input adaptor tied to adder boards:

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Input and output boards:

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Here are all these boards in action, while testing function of the adder: