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I have just started into RTSP and was looking at the assembly code in Microchip's RTSP examples. There are two files rtsp_api.h and rtsp_api.s. I wanted to know how are the return values fetched and how does that work.

For example, there is a function in rtsp_api.h which is declared as follows

/******************************************************************************
     * Function:        int16_t FlashPageRead(uint16_t nvmAdru, uint16_t nvmAdr, int16_t *pageBufPtr);
     *
     * PreCondition:    None
     *
     * Input:            nvmAdru - Selects the upper 8bits of the location to program or erase
                                    in program flash memory

                         nvmAdr  - Selects the location to program or erase in program flash
                                   memory. It must be aligned to 512 instruction boundary,
                                   LSB 10bits of address must be zero
                         pageBufPtr- Pointer to the data array in which read data will be stored

     *
     * Output:           Function returns ERROREE (or -1), if it is not successful,
                         Function return ZERO, if successful
     *
     * Side Effects:    None
     *
     * Overview:        This function provides the interface to read the flash.
     *****************************************************************************/
    int16_t         FlashPageRead( uint16_t nvmAdru, uint16_t nvmAdr, int16_t *pageBufPtr );

and its definition in rtsp_api.s is as follows 

/******************************************************************************
  Flash Page Read 
  Read EIGHT rows (PAGE) of memory, upper PM byte is discarded
*******************************************************************************/
_FlashPageRead:
        push    TBLPAG
        mov     w0, TBLPAG
        mov     #1024, w3

readNext:     
  tblrdl  [w1],[w2++]
  tblrdh [w1++],w6  ; Discard PM upper byte
        dec     w3, w3
        bra     nz, readNext

  clr  w0
   pop     TBLPAG
        return

The code documentation says that the function returns -1 on failure.

Refering to the X16 Compiler User Guide section 13.8.2, I saw that the return valus are kept in W0 Register, but the code above clears the W0 register before returning.

Then how would the code return -1 ? Could this be a bug or is it my extremely limited knowledge of assembly ?

RishabhHardas
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    Maybe the statement 'returns -1 on error' is generic for many functions. And there is no chance FlashPageRead() could fail, so it returns always SUCCESS. – user5329483 Jun 21 '19 at 11:17
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    Maybe, even the datasheet, flash programming manual and other documents do not mention anything about the read failing. – RishabhHardas Jun 21 '19 at 11:20

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