How to construct a seven-segment display using a breadboard connected to an Arduino Uno to display a countdown from 9 to 0 and back to 9 as it repeats using only registers? 

Also, the use of pinMode(), digitalWrite(), or any libraries is NOT ALLOWED. 
Also, make it so that the USART is the one controlling the display, then implement two functions for USART, namely: 

void USART_interrupt_init(void); //To Initialize 
USART ISR(USART_RX_vect) //send one character 

Note: With 8 bits data; 1 stop bit; Disable Parity; 9600 Baud rate 

Also, implement a "Timer Overflow Interrupt" to control the display with approximately 1 second transition delay between each digit. 

Note: Here are the 7 segment pin connections, along with the corresponding arduino pins: 
A - 5 
B - 6 
C - 7 
D - 8 
E - 9 
F - 10 
G - 11 
DP - 4 

Write it as clear as possible and with comments on what each function does. Also note that the 7 segment used is a cathode display. Provide your response in exactly 6 paragraphs (separated nearby paragraphs with 3 aterisk symbols ***).
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