How to set 45th bit in a 64 bit register field?

问题

I was trying to set 46th bit in a register which of 64 bits wide using C.How do i go about setting this bit ?

Currently i am doing this:

uint32_t= address ;
uint64_t data =1ULL << 46;

Printing this is showing that bit 14 is getting set.I am not able to set even bit 32. If i set bit 32 it sets bit 0. 33 will set bit 1. Looks like it is doing circular shifting after 0-31 again it starts over with 0.

Any idea how do i go about setting this bit ?

Eg:

reg_addr.val = FEATURE_REG;

printf(stdout, "Programming enable at address %x=%llx\n",
    reg_addr.val,reg_addr.val);

data.val = (1ULL << 46);

printf("Data value %llx\n",data.val);}

回答1:

If you use types as uint32_t or uint64_t printing correctly is done with:

printf(stdout, "Programming enable at address %" PRIu32 "=%" PRIu64 "\n",reg_addr.addr, reg_addr.val);

assuming reg_addr.addr is of type uint32_t and type reg_addr.val is of uint64_t.

回答2:

Your code is correct.

You should check that 64-bit integer types are correctly supported in your platform:

printf("%zu %zu\n", sizeof 1ULL, sizeof data);

should print

8 8

If this is the case, the error is probably (as mentioned by @pmg in the comments) in how you check if the bit is set.

In the new edit, you mentioned it is a register. IO registers can have special behavior due to their volatile property. I suggest you to first check to set a bit with a normal object.

回答3:

I hope you do not check the output of this line to conclude the shifting is not done correctly!?

printf(stdout, "Programming enable at address %x=%llx\n", reg_addr.val,reg_addr.val);

Passing the 64 bit value two times to printf, but using %x (probably 32 bit) and %llx (64 bit) to output the values will not work. I guess you meant to take the address of reg_addr.val at the first parameter!?

printf(stdout, "Programming enable at address %x=%llx\n", &reg_addr.val,reg_addr.val);

来源：https://stackoverflow.com/questions/9207611/how-to-set-45th-bit-in-a-64-bit-register-field

标签

bit-manipulation