trick/trick_source/sim_services/MemoryManager/extract_bitfield.c

/*
   PURPOSE: (Extract a bitfield. And return bitfield value.)

   REFERENCE: ((meow) (woof))

   PROGRAMMERS: (((Jane Falgout) (Lincom)) ((Alex Lin) (NASA) (8/00))) */

#include "trick/bitfield_proto.h"

int extract_bitfield_any(int value,     /* In: Value to extract bits from */
                         int size,      /* In: Declared size of bitfield */
                         int start,     /* In: Starting bit */
                         int bits)
{                                      /* In: Number of bits in bitfield */

    int i = 0, j = 0;
    unsigned int mask;
    unsigned int bf;
    int sbf;
    union {
        long l;
        char c[sizeof(long)];
    } un;
    un.l = 1;                          /* Used for Little/Big Endian Detection */

    bf = 0x00000000;
    sbf = 0x00000000;

    if (un.c[sizeof(long) - 1] == 1) {

        /* 
         * Big endian 
         */

        mask = 0x00000001 << (start + bits - 1);

        if (mask & value) {

            /* 
             * This is a negative value 
             */

            for (i = start, j = 0; j < bits; i++, j++) {
                /* 
                 * First, turn off all bits associated with 
                 * this bit field within the unsigned integer.
                 * Then assign the input value to the 
                 * appropriate bits in the unsigned integer.
                 */
                mask = 0x00000001 << i;
                if ((value & mask) != mask) {
                    sbf |= mask;
                }
            }
            sbf = -((sbf >> start) + 1);
        } else {

            /* 
             * This is a positive value 
             */

            for (i = start, j = 0; j < bits; i++, j++) {
                /* 
                 * First, turn off all bits associated with 
                 * this bit field within the unsigned integer.
                 * Then assign the input value to the appropriate 
                 * bits in the unsigned integer.
                 */
                mask = 0x00000001 << i;
                if ((value & mask) == mask) {
                    bf |= mask;
                }
            }
            sbf = bf >> start;
        }
    } else {

        /* 
         * Little endian 
         */

        /* 
         * Shift the bit field contents of the current value
         * to the lsb of the underlying int.
         */
        if (size == sizeof(short)) {
            value = value << 16;
        } else if (size == sizeof(char)) {
            value = value << 24;
        }
        if ((start + bits) < 32) {
            bf = (value) >> (32 - start - bits);
        } else {
            bf = value;
        }

        /* Now, AND the shifted bits with a mask of the appropriate size to obtain the bitfield value. */
        bf &= (0xFFFFFFFF >> (32 - bits));

        /* If this bitfield is a signed bitfield and the sign bit is set, take the two's complement of the value. */
        mask = 0x00000001 << (bits - 1);

        if (mask & bf) {
            /* This bitfield is signed and has the sign bit set. Set sbf to the reverse bits of the bitfield contents.
               Then right shift and add 1. */

            sbf = 0x00000000;
            for (i = 0; i < bits; i++) {
                mask = 0x00000001 << i;
                if ((bf & mask) != mask) {
                    sbf |= mask;
                }
            }

            sbf = -(sbf + 1);

        } else {
            sbf = bf;
        }
    }

    return (sbf);
}