Each clock cycles ALU performs one of the 8 possible operations and stores result in the 4-bit accumulator register.
accumulator [4 bit] = accumulator [4 bit] (operation) operand [4 bit]
Supported operations: lda imm :: imm -> accumulator neg imm :: 0x0F - imm -> accumulator shr :: accumulator / 2 -> accumulator sub imm :: accumulator - imm -> accumulator and imm :: accumulator & imm -> accumulator xor imm :: accumulator ^ imm -> accumulator or imm :: accumulator | imm -> accumulator add imm :: accumulator + imm -> accumulator
Matrix mapping of operation opcode to internal control signals muxA muxB muxC AtoX negX setC outC invC 000 lda - - 1 0 0 - 0 - 001 neg - - 1 0 1 - 0 - 010 shr - - 1 1 0 - 0 - 011 sub 1 1 0 0 1 1 1 1 100 and 0 0 0 0 0 - 0 - 101 xor 0 1 0 0 0 - 0 - 110 or 1 0 0 0 0 - 0 - 111 add 1 1 0 0 0 0 1 0
The following diagram shows a simple test setup that can be used to test ALU
VCC
| __|__ pushbutton
+----. .-------------+
_|_
schmitt \ /
trigger O
inverter |
+--> CLK OUT0--> +-----------+
+--------+---> OP0 OUT1--> + hex to +
+ +---> OP1 OUT2--> + 7 segment +--->> 7 segment display
+ +---> OP2 OUT3--> + decoder +
+ DIP +---> IMM0 +-----------+
+ switch +---> IMM1
+ +---> IMM2
+ +---> IMM3 CARRY--> LED
+--------+--
To reset ALU set all input pins to 0 which corresponds to lda 0 operation
loading Accumulator register with 0.
| # | Input | Output |
|---|---|---|
| 0 | clock | accumulator value 0th bit |
| 1 | opcode 0th bit | accumulator value 1st bit |
| 2 | opcode 1st bit | accumulator value 2nd bit |
| 3 | opcode 2nd bit | accumulator value 3rd bit |
| 4 | operand 0th bit | {‘unused (TODO’: ’negative flag)’} |
| 5 | operand 1st bit | {‘unused (TODO’: ‘overflow flag)’} |
| 6 | operand 2nd bit | {‘unused (TODO’: ‘zero flag)’} |
| 7 | operand 3rd bit | carry flag |