uart_tx_buf        44 kernel/uart.c  char uart_tx_buf[UART_TX_BUF_SIZE];
uart_tx_buf       100 kernel/uart.c    uart_tx_buf[uart_tx_w % UART_TX_BUF_SIZE] = c;
uart_tx_buf       150 kernel/uart.c      int c = uart_tx_buf[uart_tx_r % UART_TX_BUF_SIZE];