ADDER8
Inputs
| Pin | Type | Description |
|---|---|---|
| A | bus8 | First addend (8-bit) |
| B | bus8 | Second addend (8-bit) |
Outputs
| Pin | Type | Description |
|---|---|---|
| Sum | bus8 | 8-bit sum (A + B) mod 256 |
How It Works
ADDER8 adds two bytes the same way you add numbers on paper, column by column. Only instead of decimal digits there are bits, and instead of the mental "carry the one" there is a carry signal. Inside the chip hides a chain: one half adder handles the lowest bit, and seven full adders line up behind it. Each stage's CarryOut feeds the next stage's CarryIn, so the carry travels from bit to bit.
The key property of this chain: the carry can ripple across the whole chip. Add 00001111 (15) and 00000001 (1). Bit 0 gives 1+1 = 0 with a carry, the carry zeroes bit 1 — and the wave keeps running until it stops at bit 4. The result is 00010000 (16). Just like on paper: changing one low bit can reshape half of the higher ones.
One nuance to remember: the carry wave is not instant. Every full adder answers with a small delay, and in the worst case the signal passes through all eight stages in a row — engineers call this a ripple-carry adder. That is exactly why addition is the slowest basic processor operation, and "how do we speed up the carry" is a classic engineering problem. See the article Propagation Delay for details.
Examples (8-bit)
| A (bin) | A (dec) | B (bin) | B (dec) | Sum (bin) | Sum (dec) |
|---|---|---|---|---|---|
| 00001111 | 15 | 00000001 | 1 | 00010000 | 16 |
| 00000101 | 5 | 00000011 | 3 | 00001000 | 8 |
| 11111111 | 255 | 00000001 | 1 | 00000000 | 0 (overflow) |
| 10000000 | 128 | 10000000 | 128 | 00000000 | 0 (carry out of range) |
In the last two rows the sum does not fit into 8 bits: the real ninth bit is lost, and Sum equals the sum modulo 256.
Usage
ADDER8 is the reward for clearing level 8, where you build it by hand: a Splitter breaks buses A and B into individual bits, a chain of Full Adders sums the digits, and a Maker reassembles the result bits back into a bus. From level 13 the chip is given as a ready component and comes pre-placed in levels 13–16 — building the ALU and the final processor.
Inside the project computer the adder has two main jobs. First, it is the core of the ALU: the ADD operation runs on it, and subtraction is built next to it via two's complement. Second, the Program Counter: after each instruction the PC adds one to the address, and it uses the same adder to do it. Without ADDER8 the processor could neither compute nor step to the next instruction.
Related components
Related articles
Frequently Asked Questions
What happens on overflow, like 255 + 1?
The sum wraps to 0 and the carry flag shows that the result did not fit into 8 bits.
How is ADDER8 built internally?
It is a chain of one half adder (the lowest bit) and seven full adders. Each stage's carry feeds the next one, which is why the design is called a ripple-carry adder.
Why is addition slower than AND or OR?
Logic gates handle bits independently, while in an adder the digits are linked by the carry: in the worst case it ripples through all eight stages, and every stage adds its own delay.