LFSR (Random Number Generator)
Inputs
| Pin | Type | Description |
|---|---|---|
| Clock | bit | Shift register on rising edge (0→1) |
| DevAddr | bus8 | Device address — 0x1A for port 0xFA |
| IO_SEL | bit | I/O select — must be 1 to read the LFSR |
Outputs
| Pin | Type | Description |
|---|---|---|
| Data | bus8 | Current 8-bit pseudorandom value (0–255) |
How It Works
The LFSR (Linear Feedback Shift Register) generates an 8-bit pseudorandom sequence using the polynomial:
x⁸ ⊕ x⁶ ⊕ x⁵ ⊕ x⁴ ⊕ 1
On each clock rising edge, the register shifts left by one bit and the new least-significant bit is computed as:
new_bit = state[7] ⊕ state[5] ⊕ state[4] ⊕ state[3]
The seed value is 0x9C (156 decimal). The sequence is deterministic — given the same seed, the same sequence is always produced — but it passes statistical tests for randomness and cycles through all 255 non-zero states before repeating.
To read the current value via the I/O subsystem, set IO_SEL = 1 and DevAddr = 0x1A (port 0xFA, decimal 250). In assembly: LDA 250 reads the LFSR into the accumulator.
Usage
The LFSR is used in the following levels:
- Level 26: generate random values for dice rolling.
- Level 29: Snake game — generate random coordinates for food placement.
To generate a new random number, pulse the Clock input. To read the value, enable IO_SEL and set DevAddr = 0x1A. Typical usage in assembly:
; Read random number for food X coordinate
LDA 250 ; Read LFSR (port 0xFA)
AND 15 ; Mask to 0–15 range
STA 0 ; Store as X coordinate