7.2. Representing Objects in Assembly¶

To produce something equivalent in assembly, we need to keep the data for an object organized in a way that makes it possibly to find the correct data for a given field and a way to represent this as functions are called. We will start with how to store and find the data.

Since our Time class consists of three ints, it occupies 3 words or 12 bytes of memory. If we have:

Time time1(12, 45, 00);

We will need to allocate 12 bytes of space in which to store the second, minute, and hour. Then, we need to place the 3 words into those bytes in a consistent way so that if we need to find the hour, we know where to look for it.

Here, we have decided to place the second variable first, then minute then hour. If we keep track of the address where the data for time1 starts, 0xfffffffc in this case, we can find second at that address, minute at that address - 4, and hour at that address - 8:

Address	Contents
0xfffffff4	12	address of time1 - 8 (time1.hour)
0xfffffff8	45	address of time1 - 4 (time1.minute)
0xfffffffc	00	address of time1 (time1.second)
0x00000000	...

Each Time object will be laid out the same way, so any time we want to find the hour of a Time, we just need to know where the Time object is located and then subtract 8 bytes. These two objects:

Time time1(12, 45, 00);
Time time2(8, 30, 00);

Would be laid out like this:

Address	Contents
0xffffffe8	8	address of time2 - 8 (time2.hour)
0xffffffec	30	address of time2 - 4 (time2.minute)
0xfffffff0	00	address of time2 (time2.second)
0xfffffff4	12	address of time1 - 8 (time1.hour)
0xfffffff8	45	address of time1 - 4 (time1.minute)
0xfffffffc	00	address of time1 (time1.second)
0x00000000	...

As long as we keep track that time1 is at 0xfffffffc and time2 is at 0xfffffff0, we can find the members of each object: time1.minute is at 0xfffffffc - 4 = 0xfffffff8. time2.hour is at 0xfffffff0 - 8 = 0xffffffe8.