Design of Accumulator Unit in Computer Design

Design of Accumulator Unit in Computer Design – The accumulator is a register for short-term storage in which intermediate and logic results are stored in a computer’s CPU (central processing unit). The term “accumulator” is rarely used in reference to contemporary CPUs, having been replaced around the turn of the millennium by the term “register”. In modern computers, any register can function as an accumulator.

The most elementary use for an accumulator is adding a sequence of numbers. The numerical value in the accumulator increases as each number is added, exactly as it happens in a simple desktop calculator (but much faster, of course). Once the sum has been determined, it is written to the main memory or to another register.

The circuits associated with the Accumulator Unit register are shown below.

The adder and logic circuit has three sets of inputs.

1. One set of 16 inputs comes from the output to AC.
2. Another set of 16 inputs comes from the data register DR.
3. The third set of eight inputs comes from the input register INPR.

The output of the adder and logic circuit provides the data inputs for the register. In addition, it is necessary to include logic gates for controlling the LD, INR, and CLR in the register and for controlling the operation of the adder and logic circuits.

In order to design the logic associated with AC, it is necessary to go over the register transfer statements in the table below and extract all the statements that change the content of AC. Using this table, we can derive the control logic gates and the adder logic circuit.

D0T5:	AC ← AC ^ DR	AND with DR
D1T5:	AC ← AC + DR	Add with DR
D2T5:	AC ← DR	Transfer from DR
pB11:	AC (0-7) ← INPR	Transfer from INPR
rB9:	AC ← AC	Complement
rB7:	AC ← shr AC, AC (15) ← E	Shift right
rB6:	AC ← shr AC, AC (0) ← E	Shift left
rB11:	AC ← 0	Clear
rB5:	AC ← AC + 1	Increment