ALU Design (bits 0 - 3) #1

The ALU is a complex part of the Juno and all the components will not fit on a single PCB board therefore it is broken down into seperate boards as follows

• ALU (bits 0 - 3)
• ALU (bits 4 - 7)
• Shifter and flag logic 

ALU (bits 0 -3)

The following schematic shows the design for the ALU (bits 0 -3).

Control 

There are 8 control inputs

• C0 -  Combined with C1 used to select ALU operation to output; 00 = AND, 01=OR, 10=XOR, 11=ADDER
• C1 - see above
• CIN - Carry input from the Control Register
• LBUSINPUTSELECT - select between the external left bus (L) or hardcoded zero (H) for the ALU's left input
• RBUSINPUTSELECT - select between the external right bus (L) or hardcoded zero (H) for the ALU's right input
• CINPUTSEL0 - combined with CINPUTSEL1to select the carry input into the adder; 00 = 0, 01=1, 10 = CIN, 11 = 0
• CINPUTSEL1 - see above
• INVB - If enabled (H) inverts the ALU's right input. Used for performing subtraction. 

 Input

There are two bus data inputs consisting of 4 bits each, bits 0 to 3. 

1. L_0 - left bus bit 0
2. L_1 - left bus bit 1
3. L_2 - left bus bit 2
4. L_3 - left but bit 3
5. R_0 - right bus bit 0
6. R_1 - right bus bit 1
7. R_2 - right bus bit 2
8. R_3 - right bus bit 3

Ouput

The output consists of 4 output bits plus the carry out from the adder which is fed into the ALU board for bits 4 to 7.  

1. Z0 - output bus bit 0
2. Z1 - output bus bit 1
3. Z2 - output bus bit 2
4. Z3 - output bus bit 3
5. ADDERCOUT - carry out from the adder