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

[Pantelis Antoniou <panto@intracom.gr>]

Jerry Van Baren wrote:

> @h  = high order 16 bits
> @l  = low order 16 bits
> @ha = high order 16 bits adjusted so that, when you add the 
> sign-extended low order 16 bits, the value comes out to the right number
>
> Example of @ha:
>   lis  r3,     0x0000FFFF at ha    /* r3 = 0x0001oooo */
>   addi r3, r3, 0x0000FFFF at l     /* r3 = 0x0001oooo + 0xffffFFFF = 
> 0x0000FFFF */
>
> will actually load 0x00010000 into r3 in the first instruction because 
> the second instruction (addi) sign-extends the 0x0000FFFF to become 
> 0xFFFFFFFF.  After the addi, r3 will have the proper value 0x0000FFFF.
>
> ori does not sign extend the immediate value, so it does not have this 
> quirk and therefore you would use @h and @l:
>   lis r3,     0x0000FFFF at h      /* r3 = 0x00000000 */
>   ori r3, r3, 0x0000FFFF at l      /* r3 = 0x0000oooo | 0xooooFFFF = 
> 0x0000FFFF */
>
> I've never figured out why someone would want to use addi instead of 
> ori in the second instruction of the load sequence.
>
> gvb 

  It's useful when the second instruction is a load, or a store.

Instead of doing this to store a value to the global variable foo:

   lis  r3,foo at h
   ori  r3,r3,foo at l
   stw  r4,0(r3)       ; store r4 to foo

do this:
  
   lis  r3,foo at ha
   stw  r4,foo at l(r3)   ; store r4 to foo

that is because the store and load immediate offsets are sign extended.

Regards

Pantelis

>
>
> At 11:32 AM 5/8/2003 -0400, okisoftxman at hotmail.com wrote:
>
>> 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!
>
>
>
>
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