Virtual memory using bitset algorithms

Question

I’m seeing the following macros that take as parameters how many frames (virtual memory) ram memory may have, but I can’t understand its meaning:

define INDEX_FROM_BIT(a) (a/(8*4))
define OFFSET_FROM_BIT(a) (a%(8*4))

Complete code:

// A bitset of frames - used or free.
u32int *frames;
u32int nframes;

// Defined in kheap.c
extern u32int placement_address;

// Macros used in the bitset algorithms.
#define INDEX_FROM_BIT(a) (a/(8*4))
#define OFFSET_FROM_BIT(a) (a%(8*4))

// Static function to set a bit in the frames bitset
static void set_frame(u32int frame_addr)
{
   u32int frame = frame_addr/0x1000;
   u32int idx = INDEX_FROM_BIT(frame);
   u32int off = OFFSET_FROM_BIT(frame);
   frames[idx] |= (0x1 << off);
}

// Static function to clear a bit in the frames bitset
static void clear_frame(u32int frame_addr)
{
   u32int frame = frame_addr/0x1000;
   u32int idx = INDEX_FROM_BIT(frame);
   u32int off = OFFSET_FROM_BIT(frame);
   frames[idx] &= ~(0x1 << off);
}

// Static function to test if a bit is set.
static u32int test_frame(u32int frame_addr)
{
   u32int frame = frame_addr/0x1000;
   u32int idx = INDEX_FROM_BIT(frame);
   u32int off = OFFSET_FROM_BIT(frame);
   return (frames[idx] & (0x1 << off));
}

// Static function to find the first free frame.
static u32int first_frame()
{
   u32int i, j;
   for (i = 0; i < INDEX_FROM_BIT(nframes); i++)
   {
       if (frames[i] != 0xFFFFFFFF) // nothing free, exit early.
       {
           // at least one bit is free here.
           for (j = 0; j < 32; j++)
           {
               u32int toTest = 0x1 << j;
               if ( !(frames[i]&toTest) )
               {
                   return i*4*8+j;
               }
           }
       }
   }
} 

Code link : http://www.jamesmolloy.co.uk/tutorial_html/6.-Paging.html