separating control bits: HDL requires fan-in of one.

[Vijay99]

Most significant bit of instruction input is supposed to be the selection bit for the first Multiplexer that selects between ALU output and instruction input. Multiplexer's output is then connected to A-register.

To do this, I tried Mux16(a=instruction, b=aluout, sel=instruction[15], out=muxone);  this gives an error because chips should have a fan-in of one.

Then I tried:-
    Mux16(a=instruction, b=aluout, sel=sepc[15], out=muxone); // uses op-code
    Or16(a=false, b=instruction, out=sepc);
    Mux16(a=false, b=sepc, sel=instruction[15], out=control); // separated control bits
    Register(in=muxone, load=control[5], out=aout, out=addressM); // A-register, addressM
 err at first line : "sub bus of an internal node may not be used. "

Cannot think of any other way than to construct another chip that accomplishes the task of renaming the instruction input.

[WBahn]

Go the Hardware forum and look at the Hardware survival guide sticky in the section on sub-busing.

[Vijay99]

sub bus of an internal node may not be used.
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[WBahn]

I'm not sure if you are saying you found the solution, or that you are still having problems.