在setup_arch函数的末尾，有这样的语句：
/* Copy atomic sequences to their fixed location, and sanity check that
these locations are the ones that we advertise to userspace. */
memcpy((void *)FIXED_CODE_START, &fixed_code_start,
FIXED_CODE_END - FIXED_CODE_START);
BUG_ON((char *)&sigreturn_stub - (char *)&fixed_code_start
!= SIGRETURN_STUB - FIXED_CODE_START);
BUG_ON((char *)&atomic_xchg32 - (char *)&fixed_code_start
!= ATOMIC_XCHG32 - FIXED_CODE_START);
BUG_ON((char *)&atomic_cas32 - (char *)&fixed_code_start
!= ATOMIC_CAS32 - FIXED_CODE_START);
BUG_ON((char *)&atomic_add32 - (char *)&fixed_code_start
!= ATOMIC_ADD32 - FIXED_CODE_START);
BUG_ON((char *)&atomic_sub32 - (char *)&fixed_code_start
!= ATOMIC_SUB32 - FIXED_CODE_START);
BUG_ON((char *)&atomic_ior32 - (char *)&fixed_code_start
!= ATOMIC_IOR32 - FIXED_CODE_START);
BUG_ON((char *)&atomic_and32 - (char *)&fixed_code_start
!= ATOMIC_AND32 - FIXED_CODE_START);
BUG_ON((char *)&atomic_xor32 - (char *)&fixed_code_start
!= ATOMIC_XOR32 - FIXED_CODE_START);
BUG_ON((char *)&safe_user_instruction - (char *)&fixed_code_start
!= SAFE_USER_INSTRUCTION - FIXED_CODE_START);
在这里，FIXED_CODE_START等几个定义都在include/asm/fixed_code.h中：
///* This file defines the fixed addresses where userspace programs can find
// atomic code sequences. */
//
#define FIXED_CODE_START 0x400
//
#define SIGRETURN_STUB 0x400
//
#define ATOMIC_SEQS_START 0x410
//
#define ATOMIC_XCHG32 0x410
#define ATOMIC_CAS32 0x420
#define ATOMIC_ADD32 0x430
#define ATOMIC_SUB32 0x440
#define ATOMIC_IOR32 0x450
#define ATOMIC_AND32 0x460
#define ATOMIC_XOR32 0x470
//
#define ATOMIC_SEQS_END 0x480
//
#define SAFE_USER_INSTRUCTION 0x480
//
#define FIXED_CODE_END 0x490
//
而fixed_code_start则是arch/ blackfin/ kernel/fixed_code.s中定义的一个符号。在fixed_code.s中有一段说明：
/*
* This file contains sequences of code that will be copied to a
* fixed location, defined in <asm/atomic_seq.h>. The interrupt
* handlers ensure that these sequences appear to be atomic when
* executed from userspace.
* These are aligned to 16 bytes, so that we have some space to replace
* these sequences with something else (e.g. kernel traps if we ever do
* BF561 SMP).
*/