请高手指点：prom.c DDR大小的侦测

jyhhappyjyh

1. void __init
   
2. prom_init(int argc, const char **argv)
   
3. {
   
4.     unsigned long mem;
   
5.     mips_machgroup = MACH_GROUP_BRCM;
   
6.     mips_machtype = MACH_BCM947XX;
   
7. 
   
8. /* Figure out memory size by finding aliases */
   
9.         for (mem = (1 << 20); mem < (128 << 20); mem <<= 1) {
   
10.                 if (*(unsigned long *)((unsigned long)(prom_init) + mem) ==
    
11.                     *(unsigned long *)(prom_init))
    
12.                         break;
    
13.         }
    
14. #if CONFIG_RAM_SIZE
    
15.         {
    
16.                 unsigned long config_mem;       
    
17.                 config_mem = CONFIG_RAM_SIZE * 0x100000;
    
18.                 if (config_mem < mem)
    
19.                         mem = config_mem;
    
20.         }
    
21. #endif
    
22. 
    
23.         /* Ignoring the last page when ddr size is 128M. Cached
    
24.          * accesses to last page is causing the processor to prefetch
    
25.          * using address above 128M stepping out of the ddr address
    
26.          * space.
    
27.          */
    
28.         if (MIPS74K(mips_cpu.processor_id) && (mem == 0x8000000))
    
29.                 mem -= 0x1000;
    
30. 
    
31.         add_memory_region(0, mem, BOOT_MEM_RAM);
    
32. }

复制代码

为什么当mem=DDR大小的时候就有
*(unsigned long *)((unsigned long)(prom_init) + mem) == *(unsigned long *)(prom_init)
感觉像是环回了一样！
这个问题困扰我很久了，请高手指点！

jyhhappyjyh

回复 1# jyhhappyjyh

这个汇编中也是有这样的机制！