SPI transmit problem

I have a similar question to ask you about SPI.

I use f2802x in project,and it works in non-FIFO mode.When I write some bytes to SpiaRegs.SPIDAT(like SpiaRegs.SPIDAT=a),I find that it is always a wrong value(not equal to a,it changes,most of time is 0xFFFF).If it works in loop back mode,the value of SpiaRegs.SPIDAT is right.

void spi_init()
{
 SpiaRegs.SPICCR.all =0x000F;              // Reset on, rising edge, 16-bit char bits
 SpiaRegs.SPICTL.all =0x0006;           // Enable master mode, normal phase
                                                 // enable talk, and SPI int disabled.
 SpiaRegs.SPIBRR =0x007F;
    SpiaRegs.SPICCR.all =0x008F;           // Relinquish SPI from Reset
    SpiaRegs.SPIPRI.bit.FREE = 1;                // Set so breakpoints don't disturb xmission
}

void spi_xmit(Uint16 a)
{
    SpiaRegs.SPIDAT=a;
    //SpiaRegs.SPICCR.all =0x009F;
    delay_loop(); 
}

void spi_fifo_init()
{
// Initialize SPI FIFO registers
    SpiaRegs.SPIFFTX.all=0xA040; //E040
    SpiaRegs.SPIFFRX.all=0x2044;
    SpiaRegs.SPIFFCT.all=0x0;
}

Thanks for your reply,I really need your help!