Geoff suggested a more succinct description for "top".

 typedef struct bignum_st

        {

        int top;      /* index of last used d (most significant word) + 1 */

        BN_ULONG *d;  /* pointer to an array of 'BITS2' bit chunks */

        int top;      /* number of words used in d */

        BN_ULONG *d;  /* pointer to an array containing the integer value */

        int max;      /* size of the d array */

        int neg;      /* sign */

        } BIGNUM;

The big number is stored in B<d>, a malloc()ed array of B<BN_ULONG>s,

least significant first. A B<BN_ULONG> can be either 16, 32 or 64 bits

in size (B<BITS2>), depending on the 'number of bits' specified in

The integer value is stored in B<d>, a malloc()ed array of words (B<BN_ULONG>),

least significant word first. A B<BN_ULONG> can be either 16, 32 or 64 bits

in size, depending on the 'number of bits' (B<BITS2>) specified in

C<openssl/bn.h>.

B<max> is the size of the B<d> array that has been allocated.  B<top>

is the 'last' entry being used plus one, so for a value of 4, bn.d[0]=4 and

is the number of words being used, so for a value of 4, bn.d[0]=4 and

bn.top=1.  B<neg> is 1 if the number is negative.  When a B<BIGNUM> is

B<0>, the B<d> field can be B<NULL> and B<top> == B<0>.