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Overflow Bit in TMS320C1X Family (instructions ADD, ADDH)

Joao Carlos Neves de Oliveira
Prodigy 20 points

My question is about TMS320C15 instruction ADD (ADDH too). The User's Guide (SPRU013C.PDF) says that ADD and ADDH  instructions don't affect OV bit.  Is that really so or there is some mistake over there?  I've just written a TMS320C15 simulator in C and everything works fine until signal level goes higher.  The power level measurement turns wrong (it's ok once you know the filter equation overloaded) but the overflow bit is not set.

Thank you for your attention.

  • 0
    TI__Guru 61235 points

    Joao,

    First, the TMS320C1x family is ancient, and further the TMS320C15 is obsolete, so I'm not sure why you are working with it.  I'll assume you have some reason, and that you realize you cannot order this part anymore.

    Joao Carlos Neves de Oliveira said:

    The User's Guide (SPRU013C.PDF) says that ADD and ADDH  instructions don't affect OV bit.

    ADD and ADDH instructions not affecting the OV bit seems weird to me.  I looked in SPRU013C, but I do not see what you are referring to.  Please site a page number.

    Regards,

    David

  • 0 in reply to David M. Alter
    Prodigy 20 points

    Mr.Alter, thank you for your attention.  The page you refer to is SPRU013C.PDF 4-17 (173/568 - Adobe Reader's page indicator). There we have an explanation on the operation of the ADD instruction.  Next page (4-18) brings ADDH instruction.

    Now the ADDS explanation shows clearly that it affects the OV bit (page 4-19). APAC, SUB, SUBC, SUBH, SUBS instructions also affect the OV bit.

    Well, now about the first part of your e-mail, yes, I know C15 is obsolete (one can buy some 'dusty' pieces though).  I wrote the simulator (it has a 8085 simulator too, the ancient Intel's processor) so I could develop some new code without the equipment itself. It is a telephone card test system (old analog to PCM technology).

  • 0 in reply to Joao Carlos Neves de Oliveira
    TI__Guru 61235 points

    Joao,

    I suspected you were referring to a conclusion by omission.  I had noticed that ADDS specifically said it affects OV, but I hadn't looked at SUB until you just pointed it out.  SUB and ADD are really complementary instructions.  They should both affect the same status bits.  In my opinion, ADD and ADDH should affect the OV bit, and the statements about this were just mistakenly omitted from the user's guide on these instruction detail pages.

    Good luck,

    David