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The ALU/Accumulator partnership

In the Heritage/1 (as in most computers) the Accumulator register (A) holds the result of the last ALU operation. Additionally, it serve as a general purpose register. The ALU has two 16 bits inputs: A and whatever is in the Data Bus.

The figure below illustrates the relationship between A, the ALU and the Data Bus.


Control signal S_ALU (Select ALU) causes the ALU output to be routed to the Accumulator's input. When S_ALU is not active, the Accumulator is fed from the Data Bus instead so it can be used as general purpose register.

Signal S_A opens the Accumulator's output to the bus. We activate this signal when we want to read from A.

Signal WR_A clocks the Accumulator so whatever is at the input, gets latched. We activate this signal to write A from the Data Bus (example: mov a, b); the signal will actually commit the transfer. For performing logical or arithmetic operations, the same signal will commit the operation but in this case, S_ALU must be active.

Finally, the signal f (which takes n bits) defines the ALU operation to be performed.

We can see the ALU and the Accumulator as being part of the same circuit. The following figure illustrate such vision.


The signal S_ALU decides whether the box will behave as ALU or general purpose register A. In the lastest, signal S_A will be used to read from A whereas signal WR_A will be used to write (latch) toward A.

Register B is similar in the sense that the signal WR_B actually clocks its latches whereas S_B opens its three-state buffers to the BUS so others can read it. It is not shown in the figure but one can imagine.

Finally, here is a timing diagram of two operations with the A/ALU unit. We assume that f (not shown in the diagram) contains the code for the SUM operation.


During the first clock cycle, A is latched with the sum of itself and whatever is in the bus (B in this case). In a further clock cycle, B is moved to A.


The A/ALU unit will be most likely implemented with PLD whereas other registers will be built with 74HC374 chips.