Doing multiplication "by hand"

I wrote the following three functions. They're doing addition and multiplication "by hand". However, I can't get the third one to work properly, because I'm not sure how to go about adding the 0's for additional multiplications.

Code:

Code:

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public static int[] ppAdd (int[] a, int[] b) {
    // Store the length of the largest array:
    int iLen = (a.length > b.length) ? a.length : b.length;
    // Create the sum array (largest array length + 1):
    int[] sum = new int [iLen + 1];

    int iRes = 0, iCarry = 0;

    for (int i = 0; i < iLen; i++) {
        int a_curr = (a.length - 1) - i;
        int b_curr = (b.length - 1) - i;

        if (a_curr < 0) {
            iRes = b[b_curr] + iCarry;   
        } else if (b_curr < 0) {
            iRes = a[a_curr] + iCarry;
        } else {
            iRes = a[a_curr] + b[b_curr] + iCarry;
        }

        iCarry = 0;
        if (iRes > 9) {
            iCarry = 1;
            iRes = iRes - 10;
        }

        sum[iLen - i] = iRes;
    }

    return sum;
}

public static int[] ppMult (int[] mltp, int d) {
    int[] prod = new int[mltp.length + 1];
    int iRes = 0, iCarry = 0;

    // Loop the mltp array backwards:
    for (int i = (mltp.length - 1); i >= 0; i--) {
        // Multiply the current index by d:
        iRes = mltp[i] * d + iCarry;

        // Grab the 1's spot:
        prod[i + 1] = iRes % 10;
        // Carry the 10's spot:
        iCarry = iRes / 10;
    }

    // Set the first value as the last carried value:
    prod[0] = iCarry;

    return prod;
}

public static int[] ppMultMM (int[] a, int[] b) {
    // If needed, we're going to switch a and b
    // so that the top is always the largest
    // number. This makes computation below much easier:
    if (b.length > a.length) {
        int[] tmp = a;
        a = b;
        b = tmp;
    }

    int[] prod = new int[a.length + b.length];
    int[][] rows = new int[b.length][a.length + b.length];

    for (int i = (b.length - 1); i >= 0; i--) {
        rows[i] = ppMult(a, b[i]);
    }

    // Loop the rows and add them together:
    for (int i = 0; i < rows.length; i++) {
        prod = ppAdd(rows[i], prod);
    }

    return prod;
}

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Any help would be appreciated.