nios II boot loader

Hi.

when I use FPGA desgin a NIOS II system,I meet a program. It described as follow:

my system has a external 256K 16-bit SRAM, one Serial controller, one epcs controller 4K onchip_ram and 1K onchip_rom.

Hardware config data and software program are boot load by EPCS. And SOPC CPU's Reset Vector：epcs_controller offset:0x0.

Exception Vector：onchip_RAM offset:0x20.

when NIOS II IDE System Library set as:

.text ext_ram

.rodata ext_ram

.rodata ext_ram

The system boot success,but the program does not run.

but when NIOS II IDE System Library set as:

.text onchip_ram

.rodata ext_ram

.rodata ext_ram

The system boot success,but the program run.

Please give a suggestion what should I do? is there anything worng with my system?

Sorry for my English. Thank you.

PS: my software verion is Q-II 7.2 SP3.