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Bank switch code reads port, not latch.

Bill Webster

The sample L51_BANK.A51 module uses the following code to determine the current bank:

?B_BANK&N:
                MOV     A,?B_CURRENTBANK
                ANL     A,#?B_MASK
                CJNE    A,#BANK&N,XLABEL

                ; etc...

For 8051 compatible devices, shouldn't it use a R-M-W instruction, so that it reads the latch, rather than the port? Something like:

?B_BANK&N:
                CLR     A
                ORL     A, ?B_CURRENTBANK
                ANL     A,#?B_MASK
                CJNE    A,#BANK&N,XLABEL

                ; etc...

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  • 0 in reply to J Guy

    I was investigating this as a potential cause of a problem I was having. It turned out to be hardware.

    Thanks for the replies.

    Plus it sounds like it would be one hell of a subtle bug if your bank-switch hardware / port setup meant that it was possible to read the "wrong" value.