Evolution of the ALU

Last Edit July 11, 2001

Adding Flexibility

The scratchpad memory B port is connected to the B port of the ALU, to the main memory, and to the data output bus. The A port is connected to the A port of the ALU and also to the status-generating logic for input to the CCU. Note that either the A port or the B port of the scratchpad memory could have been connected to the data output bus.

Input MUX A

The A port of the ALU receives input from the data input bus or the scratchpad memory A port; therefore these devices must be tristate. Assume that all input devices already are tristate. This leaves the tristate requirement for the scratchpad memory output. By adding an input MUX to the A port of the ALU and controlling this MUX from the CCU, the tristate requirements for the scratchpad A port is removed (Figure 5-6).

Figure 5-6 Adding Status Flags

Output MUX B

The ALU output is connected to the input port of the scratchpad and to the input of the MAR register. Improvement is possible by allowing the MAR to be loaded from either the scratchpad registers or from the ALU output and allowing the main memory and the data output bus to share the connection. A MUX added at the output of the ALU and connected to the data output bus under control of the CCU provides this capability.

The AMR can be loaded from the main memory by passing the data through the ALU. The MAR can also be loaded from any of the scratchpad registers for relative addressing or indexed addressing without passing through the ALU. This has advantages, in addition to the faster execution time, which will be demonstrated below. The execution time to transfer from the scratchpad register to main memory is also reduced by bypassing the ALU. (Note that in Figure 5-6 the scratchpad A port rather then the B port was used to input to the MUX.)

Status Lines

CCU testing of the ALU result should be more powerful then the <ACC> = 0 test. The next addition to the ALU is the provision of a number of other status outputs.

These would be connected to the condition MUX of the CCU constructed earlier.

Shift and Rotate

While the ALU is capable of most operations, the ability to shift right or left or to rotate right or left is a desirable feature. This can be accomplished by the addition shown in Figure 5-7, where a shift register has been added to the scratchpad input. The shift register is under CCU control. External connections determine whether a shift or rotate is being performed and what bit, 0 or 1, is shifted into the high or the low-order bit. A shift MUX will be needed for each side of the ALU, which will also be under CCU control.

Figure 5-7 Adding the Shifter at the RAM input

Control Bits

Each item added which requires CCU control adds a field to the microinstruction format. The width of the field added is a function of the amount of flexibility of the device. For a shift MUX, a 2- or 3-bit field is required. The ALU so far requires a 3-bit function field, a carry-in field (or a carry-in MUX control field), A address and B address fields of 4 bits each for fixed register operations, MUX select bits to allow A and B register addresses to be supplied from either the microinstruction register or the machine-level instruction in the IR, and controls for the A port input MUX, the ALU output MUX, and the shift register. The microinstruction fields required by this version of the ALU are shown in Figure 5-8.

Figure 5-8 ALU portion of the microword (simple system)

Double Precision

The simple system under development has no multiply or divide operations. To provide the capability for these operations, the ALU must have at least one double precision register. FOr the system developed so far, this is provided by adding an extension Q register and its own shift register. The ALU inputs to the Q register directly. The Q shift register is connected to the output of the Q register. The Q register output also connects to the B port of the ALU.

To avoid requiring a tristate register and a tristate scratchpad memory, a MUX is added to the ALU B port input and the MUX is under CCU control. External connections determine the shift or rotate operations on Q alone or Q and a scratchpad register. The addition is shown in Figure 5-9.

Figure 5-9 Am2901 ALU

Additional Modifications

A few additional improvements can be made:

First, adding a zero input to the ALU A and B port input MUXs allows incrementing and decrementing as well as PASS operations on both ports.

Second, adding the A port of the scratchpad to the ALU B port input MUX allows a fast multiply by 2:

Third. adding an output enable control and making the ALU output MUX into a tristate MUX allows the ALU to share a bus.

Finally, adding two additional status outputs, carry-generate G' and carry propagate P' , allows fast addition using a carry-look-ahead if the ALU is assumed to be a 4-bit wide slice.

The result is shown in Figure 5-9 and is a logical block diagram of the original Am2901 bit-slice RALU. [This part is no longer in production - but it is around as a soft IP.]