The original poster has indicated he is using the C8051F040 part from Silicon Laboratories. This part actually has two UARTs which is the reason I belive he has referred to the UART he is trying to program as UART0. The second UART is referred to as UART1. With these parts the UART0 operates with a high degree of sililarity to the old defacto standard Intel 8051 UART. The UART0 serial interrupt enable bit in the IE register is actually named ES0. There is a corresponding ES1 bit to enable the interrupts for the second UART located in an extended interrupt enable register called IEI2 at the bit 6 position.

Michael Karas