DMA Problem for msp430f2618

HI

 I am ran a Example code , but the RxString is 0x00 , why?

the Example code is changed the MSP430F550x_dma_03.c

//                MSP430F2618
//             -----------------
//         /|\|              XIN|-
//          | |                 | 32768Hz
//          --|RST          XOUT|-
//            |                 |
//            |             P1.0|-> LED
//            |                 |
//            |             P3.3|-> Data Out (UCA0SIMO)
//            |                 |
//            |             P3.4|<- Data In (UCA0SOMI)
//            |                 |
//            |             P2.7|-> Serial Clock Out (UCA0CLK)

#include "msp430x26x.h"
char TxString;
char RxString;

void main(void)
{
  // Set Watchdog interval to ~30ms
  WDTCTL = WDT_ADLY_1000;                   // WDT 1000ms, ACLK, interval timer
  IE1 |= WDTIE;                          // Enable WDT interrupt
  P1OUT &= ~0x01;                           // Clear P1.0 LED
  P1DIR |= BIT0;
  P3SEL |= BIT0+BIT4+BIT5+BIT6+BIT7;                       // P3.3,4 = USCI_A0 SPI Option
 

  UCA0CTL1 |= UCSWRST;                      // **Put state machine in reset**
  UCA0CTL0 = UCMST+UCSYNC+UCCKPL+UCMSB;     // 3-pin, 8-bit SPI master
                                            // Clock polarity high, MSB
  UCA0CTL1 = UCSSEL_2;                      // SMCLK
  UCA0BR0 = 0x02;                           // /2
  UCA0BR1 = 0x00;                           //
  UCA0MCTL = 0x00;                          // No modulation
  UCA0CTL1 &= ~UCSWRST;                     // **Initialize USCI state machine**

  DMACTL0 |= DMA1TSEL_9+DMA0TSEL_10;        // DMA0 - UCA0TXIFG
 
  DMA0SA = (int)&TxString;                           // Src = RAM memory
  DMA0DA = (int)&UCA0TXBUF;   
 
                                            // Destination single address
  DMA0SZ = 1;                               // Block size
  DMA0CTL = DMASRCINCR_3+DMASBDB+DMALEVEL;  // inc src

  // Setup DMA1
  DMA1SA = (int)&UCA0RXBUF;                           // Src = RAM memory
  DMA1DA = 0x2000;
 
  DMA1SZ = 1;                               // Block size
  DMA1CTL = DMADSTINCR_3+DMASBDB+DMALEVEL;  // inc dst

  TxString = RxString = 0;                  // Clear TxString
   __bis_SR_register(LPM3_bits + GIE);       // Enter LPM3 w/ interrupts
  __no_operation();                         // Required only for debugger
}

// Watchdog Timer interrupt service routine
#pragma vector=WDT_VECTOR
__interrupt void watchdog_timer(void)
{
  if(TxString-1 == RxString)
    P1OUT |= 0x01;                          // Set P1.0 if True
  else
    P1OUT &= ~0x01;                         // Clear P1.0 if False

  TxString++;                               // Increment TxString counter
  DMA0CTL |= DMAEN;                         // DMA0 Enable
  DMA1CTL |= DMAEN;                         // DMA1 Enable
 //DMA2CTL |= DMAEN;                         // DMA2 Enable}
}