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Reding TBR registers P2020 CPU

Question asked by Ilya German on Mar 11, 2019
Latest reply on Mar 11, 2019 by ufedor



I have a quastion regarding reading TBR registers on p2020 e500v2 CPU.

I wrote the following asm instructions:


 mftbu   r5      /* Read upper Time Base */
 mftb    r6      /* Read lower Time Base */
 mftbu   r7      /* Read upper Time Base again */


When i comliped (using GNU 4.3.3) and viewed the asm instruction I saw that mftbu and mftb were replaced with mfspr instruction.


after search in documentation i found in: EREF 2.0: A Programmer’s Reference Manual for Freescale Power Arch Processors (P.504). The following note : mftb was part of the original PowerPc arch, but is being phased out of PowerISA....


How did the GNU compilar knew that it should replace the obsolete instruction ? did NXP added a patch to the GNU release ? should i worry that the compilar can change explicitly written asm instructions ?



Ilya German