step15中的中断向量跳转指令是怎么计算的？

dragon51

01 static void install_irq_handler( void (*isr)(void) )
02 {
/* ARM irq exception vector addr is 0x00000018  */
03  unsigned int * irq_vec_addr = ( unsigned int * ) 0x18;
/* this is isr entry address, could be another address like 0x3c, 0x58... */
04  unsigned int * isr_entry_addr = ( unsigned int * ) 0x38;
05  unsigned int instruction;

/* set the ISR entry at 0x38  */
06  *isr_entry_addr = (unsigned int)isr;

/* make an instruction: it is machine-code for "ldr  pc, [pc, #(38-18-]"  */
07  instruction = ((unsigned int) isr_entry_addr  - (unsigned int)irq_vec_addr - 0x0 | 0xe59ff000;

/* set this instruction at 0x18  */
08  *irq_vec_addr = instruction;

09  return;
10 }
第7行中的指令是根据什么来构建的？别的资料上的中断向量表的初始化和这种方法有什么区别吗？谢谢指教。