Add-Shift Right算法 - 它是如何工作的？

So I have to build a basic 4 Bit multiplicator using only Add and Shift Right. I basically know how the Shift Add Algorithm works but I do not know how one would get it to work with right shifting instead of left shifting. From a technical standpoint, let us say we have one accumulator, two 4 Bit Shift Right Register and one ALU, it would basically look like this, considering that at the start the multiplicand is in the accumulator and the multiplicator in ShiftReg2, so that we can use the accumulator (Bit 7-4) and ShiftReg1 (Bit 3-0) as the Output for our 8 Bit product. Acc=abcd ShiftReg1=nothing important so far and ShiftReg2=wxyzSo I have to build a basic 4 Bit multiplicator