QUESTION $17$

Given above is the data$-$path of an arithmetic processor. Two $4-$bit numbers A $=$ a$3$ a$2$ a$1$ a$0$ and B $=$ b$3$ b$2$ b$1$ b$0$ are stored in the shift registers R$1$ and R$2$ respectively. An initial carry c$0$ is also stored in the D flip$-$flop. An arithmetic function is carried out serially in four clock cycles after A$,$ B and c$0$ have been stored. The contents in R$1$ and R$2$ are shifted right one bit in each clock cycle. At the completion of the arithmetic operation, the result is stored in R$1$ and the contents of R$2$ remain unchanged.

If the initial carry c$0$ and control signals are as follows, what operation does the processor perform?

Initial

Carry

c$0$

Control Signals

C$1$ C$2$ C$3$

C$4$ C$5$ C$6$

$0$

$101$

$001$

A$-1$

$-$A$-1$

B$-2$

$-$B$-2$