UART_TX_BUF_SIZE   44 kernel/uart.c  char uart_tx_buf[UART_TX_BUF_SIZE];
UART_TX_BUF_SIZE   95 kernel/uart.c    while(uart_tx_w == uart_tx_r + UART_TX_BUF_SIZE){
UART_TX_BUF_SIZE  100 kernel/uart.c    uart_tx_buf[uart_tx_w % UART_TX_BUF_SIZE] = c;
UART_TX_BUF_SIZE  150 kernel/uart.c      int c = uart_tx_buf[uart_tx_r % UART_TX_BUF_SIZE];