what the meaning of (a&b)>>c in this systemc code?

Question

when I read SYSTEMC code,I find a function return int like this:

static inline int rp_get_busaccess_response(struct rp_pkt *pkt)
{
    return (pkt->busaccess_ext_base.attributes & RP_BUS_RESP_MASK) >>
                                                            RP_BUS_RESP_SHIFT;
}

pkt->busaccess_ext_base.attributes defined as uint64_t.

RP_BUS_RESP_MASK and RP_BUS_RESP_SHIFT defined as:

enum {
    RP_RESP_OK                  =  0x0,
    RP_RESP_BUS_GENERIC_ERROR   =  0x1,
    RP_RESP_ADDR_ERROR          =  0x2,
    RP_RESP_MAX                 =  0xF,
};

enum {
    RP_BUS_RESP_SHIFT    =  8,
    RP_BUS_RESP_MASK     =  (RP_RESP_MAX << RP_BUS_RESP_SHIFT),
};

What the meaning of this function's return?

Thanks!

Answer 1

a & b is a bitwise operation, this will perform a logical AND to each pair of bits, let's say you have 262 & 261 this will translate to 100000110 & 100000101 the result will be 100000100 (260), the logic behind the result is that each 1 AND 1 will result in 1 whereas 1 AND 0 and 0 AND 0 will result in 0, these are normal logical operations but are performed at bit level:

  100000110
& 100000101
-----------
  100000100

In (a & b) >> c, >> will shift the bits of the resulting value of a & b to the right by c positions. For example for the previous result 100000100 and having a c value of 8, all bits will shift to the right by 8, and the result is 000000001. The left most 1 bit in the original value will become the first most right whereas the third 1 bit from the right in the original value will be shifted away.

With this knowledge in mind and looking at the function, we can see that the RP_BUS_RESP_MASK constant is a mask that protects the field of bits from 9th through 12th position(from the right, i.e. the first four bits of the second byte), setting them to 1 (RP_RESP_MAX << RP_BUS_RESP_SHIFT which translates to 1111 << 8 resulting in 111100000000), this will preserve the bit values in that range. Then it sets the other bits of pkt->busaccess_ext_base.attributes to 0 when it performs the bitwise & against this mask. Finally it shifts this field to the right by RP_BUS_RESP_SHIFT(8).

It basically extracts the the first four bits in the second byte of kt->busaccess_ext_base.attributes and returns the result as an integer.

What it's for specifically? You must consult the documentation if it exists or try to understand its use in the global context, for what I can see this belongs to LibSystemCTLM-SoC (In case you didn't know)