In the serial.c files of lpc2106_gcc demos, there is a litte segment code that I am confused. That is:
////////////////////////////////////////////////

xReturn = xQueueSend( xCharsForTx, &cOutChar, xBlockTime );

if( ( *plTHREEmpty == ( portLONG ) pdTRUE ) && ( xReturn == pdPASS ) )
{
xQueueReceive( xCharsForTx, &cOutChar, serNO_BLOCK );
*plTHREEmpty = pdFALSE;
UART0_THR = cOutChar;
}
///////////////////////////////////////////////

When run this code, interrupt is disabled, so xBlockTime should have no effect or this is a wrong using style? And also because interrupt is disabled, so *plTHREEmpty == pdTRUE is impossible, the if code would have no effect too. I am really confused and may be my logic is wrong.

any reply would be appreciated!

This is because the LPC2000 UART is nasty in its implementation. The code determines that the UART already contains a character so posts the new character to a queue. It then checks to see if in the mean time the UART has completed its operation and if so has to retrieve the character from the queue again to send it directly to the UART. The SAM7 UART is much better.

Thanks for your reply.
But I think the interrupt have been disabled in the beginning, so the uart interrupt service will not run before portEXIT_CRITICAL(). You don't have to fetch the character again.
Also, I don't think xBlockTime have effect.