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In the serial ISR (`rt_hw_serial_isr`), the previous logic for handling a full RX FIFO was flawed. When the buffer was filled, it would increment `get_index` (`get_index += 1`).
This had two negative consequences:
1. It effectively discarded the oldest byte of data prematurely.
2. It reduced the usable capacity of a buffer of size N to N-1. For example, a 64-byte buffer could only ever hold 63 readable bytes after becoming full.
This patch corrects the behavior by implementing a standard overwriting ring buffer strategy. When the buffer is full, the logic is changed to `get_index = put_index`.
This ensures that:
- When new data arrives, it correctly overwrites the oldest data.
- The `get_index` is advanced along with the `put_index`, correctly marking the new start of the buffer.
- The full N-byte capacity of the buffer is utilized, always storing the N most recent bytes.
This change resolves the unexpected data loss and makes the buffer behavior correct and predictable.
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