DMA transfer inside ISR: Problem

Another question:

I try also to transmit data from a variable (placed in the SRAM) to the UART with a DMA. I write a easy test software, but the result is strange. This is the code:

printf("Hello from DMA?!\n");

IOWR_ALTERA_AVALON_UART_DIVISOR(SERIAL_UART_BASE,108); // Baud Rate: 921600 @ 100Mhz

alt_u8 *tx_buffer = alt_uncached_malloc(BUFF_SIZE * sizeof (alt_u8));

tx_buffer = "Hello World\0";

printf("tx buffer position: 0x%X \n",tx_buffer);

IOWR_ALTERA_AVALON_DMA_STATUS(DMA_BASE, 0);

IOWR_ALTERA_AVALON_DMA_RADDRESS(DMA_BASE, tx_buffer);

IOWR_ALTERA_AVALON_DMA_WADDRESS(DMA_BASE, SERIAL_UART_BASE+1); // TxData address is @ SERIAL_UART_BASE + 1(offset)

IOWR_ALTERA_AVALON_DMA_LENGTH(DMA_BASE, 12);

IOWR_ALTERA_AVALON_DMA_CONTROL(DMA_BASE, 0x289);

usleep (500000);

printf("status: 0x%X \n",IORD_ALTERA_AVALON_DMA_STATUS(DMA_BASE));

printf("read add: 0x%X \n",IORD_ALTERA_AVALON_DMA_RADDRESS(DMA_BASE));

printf("write add: 0x%X \n",IORD_ALTERA_AVALON_DMA_WADDRESS(DMA_BASE));

printf("length: 0x%X \n",IORD_ALTERA_AVALON_DMA_LENGTH(DMA_BASE));

printf("control: 0x%X \n",IORD_ALTERA_AVALON_DMA_CONTROL(DMA_BASE));

The DMA write 2* '\0' chars and 3* 'H' chars (look the attachment). Why?

The console output is right:

Hello from DMA?!

tx buffer position: 0x288F638

status: 0x11 // Operation Done - len = 0

read add: 0x288F644 // 0x288F638 + C(12)

write add: 0x2903461

length: 0x0

control: 0x289

Regards