[U-Boot-Users] what means this symbol "@"?

[seaman]

In u-boot/cpu/mpc8xx/start.S,I usually find this symbol "@",such as
followed:

_start:
 lis r3, CFG_IMMR at h  /* position IMMR */
 mtspr 638, r3
 li r21, BOOTFLAG_COLD /* Normal Power-On: Boot from FLASH */
 b boot_cold

what does it means? Thank you!