Question

I have a contiguous area of memory containing 320 bits divided into 10 independent “registers”.

Once I create the memory area, I get back the address of the first register element.

At this point, I want to logically map an array of 32 bit structs containing a high and low part of the 32 bit number, then iterate through the array and write the upper and lower segments separately.

I already have a function that will split and inputted 32 bit number into two 16 bit parts, using a bit rotation scheme.

My question is, what is the best way to overlay the array of structs over the contiguous area.

The reason why I need to do this is that I am using an external library to talk to an embedded device, and the library has no concept of 32 bits, even the underlying device stores data in 32 bit registers.

Answer 1

First, make sure your struct is 32 bits as represented by the compiler. Check that sizeof(struct xxx) == 4 and play with the compiler packing options until it is (technically this ensures struct is 4 characters but it should be good enough)..

Second is to just create a pointer to object of the struct type and then assign it the address of the memory block. You will need an old fashioned type-cast operation to do that.

Something like (note the order of which is high and low and which bytes are which inside these will vary by hardware).

typedef struct {
   short high;
   short low;
} hilo;

hilo *registers = (hilo *) memPtr;