movlw suspicious asm output

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Jerry Messina
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movlw suspicious asm output

Post by Jerry Messina » Wed Sep 25, 2013 11:05 am

I can't recall if I ever mentioned this or not, but I ran across a situation where the compiler produced some questionable ASM...

Code: Select all

program main

public dim errno as shortint
const ERR_CODE as shortint = -3

public sub dummy()
    // this produces 'movlw 65533' ???
    errno = ERR_CODE
end sub

// this produces 'movlw 253'
errno = ERR_CODE


?I000000_F000_000008_P000001 ; L#MK ERRNO = ERR_CODE
    MOVLW 65533
    MOVWF M0_S08,0
?I000001_F000_000009_P000001 ; L#MK END SUB
    RETURN 0

?I000002_F000_000011_M000000 ; L#MK ERRNO = ERR_CODE
    MOVLW 253
    MOVWF M0_S08,0
?I000003_F000_000012_M000000 ; L#MK DUMMY()
    RCALL PROC_L00000000
?I000004_END; L#MK
MPASM truncates the value, so it's not seen as an error. It does issue a warning, but that's masked out since the MPASM command line switches are set to '/w2'. In the past I've seen other cases where MOVLW has values > 255 as an argument.

It's interesting that the same code produces two different outputs.

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