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Homework Help: Need help understanding part of this assembly code

  1. Dec 8, 2014 #1
    CHARI 0xFE,d ; read 1stchar
    CHARI 0xFF,d ; read 2ndchar
    LDBYTEA 0xFE,d ; load 1stchar
    ADDA 0xFE,d ; add 2ndchar to low byte(big endian!)
    ANDA 0x0F,i ; keep low 4 bits
    ORA 0x30,i ; convert to ascii
    STBYTEA 0xFD,d ; store for output
    CHARO 0xFD,d ; write result

    Hello, I mostly understand this assembly code, but am confused with this line: "ANDA 0x0F,i".

    This program inputs two single-digit numbers, adds them and then outputs their single-digit sum. Apparently, we are keeping the low 4 bits. What exactly is meant by this, specifically with what the program is trying to accomplish?
  2. jcsd
  3. Dec 8, 2014 #2


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    For the AND operation, each bit that is zero in the 0x0f is zeroed in the result and each bit that is one in the 0xf is left the same as it was before (or copied if the result register is different than the source register), assuming that ANDA 0xf,i means to AND the accumulator with the immediate value 0x0f and store the result back into the accumulator.
  4. Dec 10, 2014 #3


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    Suppose the result of the addition is
    if you AND that with
    you get

    The four most significant bits have been suppressed to zero leaving just the four least significant bits from the sum.
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