PA3 – Another memory system!

Hey guys! Long time no see, eeeeeh..... not!

Another assignment submitted, and if you have been following this blog for a long time (seriously?) you will know that I have “been there, done that” with this assignment. This one is Assignment #3 (PA3) – Memory System.

The whole idea behind this assignment was to build my own Memory System to replace the standard one that comes with C++, in other words, I had to create my own version of malloc() and free() functions. In order to achieve this I allocated one big block of memory (50KB + alignment padding), which we call the heap, then moved the initial block address so that it was 16-byte aligned, placed the heap header on this address and created a free block with the remaining space of the original block. The 16-byte alignment is not essential to the functionality of this project, but it will be useful later (SIMD).

The heap header is the Memory System part that holds all the pieces together. It has pointers to the first used and free blocks, and has an internal structure that keeps updated stats about the system usage. All blocks in the system are connected by pointers (next and previous) to form doubly-linked lists for easier insertion and removal of new blocks. Now, let me talk briefly about the two functions implemented:

• malloc(): this function first rounds up the requested size to the next multiple of 16 (to preserve the 16-byte alignment), then checks if there is enough space available in the heap, if that’s the case it proceeds to find the first free block that can hold the required amount. When the block is found, if its size is bigger than the requested one it is split into a used block and a free block, then both blocks are added to their respective lists. Finally, the stats are updated and the address of the used block is returned to the user.

•  free(): this functions takes the address of the block to be freed as a parameter, then converts this used block into a free one and adds it to the respective list. When this is done, the function also checks if the blocks above and below the new free block are not used, and if that is the case, it coalesces them together into a bigger free block. As was the case before, the stats are updated at the end.

Beside this, PA3 also had another part related to using the Placement New operator. The big difference between this operator and the regular New is that this one takes a pointer to a previously allocated memory location as a parameter, and then uses it to create the new object there. On the other hand, the traditional new first allocates the blocks (malloc) and then creates the object there.

I was going to add some diagrams to this post, but I am kind of lazy today. Anyway, I hope this information was useful. See you later! Adios!