Decoding/Executing instructions

As an example, you have the readB input signal to your ALU instance. You were talking of register signals in this respect, thus I understand the example code is placed in clock synchronous process. Otherwise the signals wouldn't exist as register but be simple wires.

immed<=in1(5 DOWNTO 0);
readB<=immed

If the code above is placed in clock synchronous process, the action would take two clock cycles. Could be shortened to one cycle by making immed a variable instea of a register. But readB would be a register anyway. If readB is intended as immediate input to ALU instance, the assignment must be done in combinational process. I can't see, if this could show intended results, cause I don't know the timing requirements of further processing.