Decoding/Executing instructions

What if I just said readB<=in1(5 DOWNTO 0) ?

You're right about it being in clock synchronous process. The clock synchronous stuff is supposed to represent the control logic to determine when I'm allowed to do which things in hardware. So I have code like this:

case state is
	
			when A=>--fetch
			writeA<='0';
			writeB<='0';
			writeC<='0';'
			writeReg<='0';
			opcode<=in1(15 DOWNTO 12);
			state <= B;
	
			when B=>--decode
			--set up all my registers
			if(opcode = "0001") then --add
			RS<=in1(11 DOWNTO 9);
			RT<=in1(8 DOWNTO 6);
			RD<=in1(5 DOWNTO 3);
			--etc
			state<=C;
			
			when C=>--execute
			if(opcode = "0001") then --add
			writeReg<='1';
			--PC<=next PC
			state<=A;
			--etc
			when D=>--memory
			if(opcode = "1000") then --lw
			writeReg='0';
			state<=E; 
			--etc
			when E=>--writeback
			if(opcode = "1000") then
				writeReg<='1';
			--PC<=next PC
			state<=A;	
			end case;

Basically my control has to go through the different stages. In case B I've set up all the registers but since all of my portmaps happen at the same time everytime, I'm not sure that saying readB<=etc is what I want to do to get an immediate value into my ALU