README

**Copyright Maria Pană 315CA 2022-2023 (maria.pana4@gmail.com)**

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# SD Homework 1: Virtual Memory Allocator
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## Description:

The aim of the program is to emulate the functionality of a virtual memory
allocator using two nested and generic doubly linked lists.

Considering an imaginary arena, the virtual memory is represented by a primary
doubly linked list that contains memory blocks. Each block has a set of
miniblocks stored in a secondary doubly linked list. The data being stored in
the virtual memory will be held in miniblocks starting from the desired address.

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## Commands and their parameters (How to use):

`ALLOC_ARENA <arena_size>`: create the VMA arena

`DEALLOC_ARENA`: free arena along with all allocated resources (blocks and
miniblocks) and close the program

`ALLOC_BLOCK <address> <block_size>`: allocate a new block with a specific
size to the arena, starting from a specific address

`FREE_BLOCK <adress>`: free the block at the given address along with its
list of miniblocks

`READ <address> <size>`: print <size> bytes, starting from the given address

`WRITE <adress> <size> <data>`: store <size> bytes of data, starting from the
given address (in allocated memory area)
  
`PMAP`: your everyday pmap function
  
`MPROTECT <adress> <new_permission>`: change permissions at the given address
(default: RW-)

For detailed functionality, check the comments in the source code.

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## Implementation specifics. Main challenges:
* **Genericity**: to ensure it, the program uses just one data structure for
both block and miniblock doubly linked lists.

* **Block allocation**: by far the most difficult aspect of the program, block
allocation implies a handful of special cases that had to be treated separately.

* **Memory leaks**: the program ensures that by the end of program execution,
all allocated resources will be freed.

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## Comments:

* Realising how many block allocation scenarios there are was initially 
intimidating. However, once I started drawing and visualising cases that came
to mind, the whole idea got shape and I could then structure it into code. It
also made things easier to imagine down the road for the other functions,
since I already had a picture in mind of how things worked.
  
* Memory leaks were also quite "fun". At some point, I was forgetting to free
the block and miniblock structures themselves and kept running into memory leaks
without knowing why. Needless to say, valgrind and gdb became my best friends :)

* Overall, I really enjoyed working on the VMA. The topic was extremely
interesting to me and I had the opportunity to work with linked lists in an
applied manner, thus gaining a better understanding of their functionality.

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## Resources. Further reading:

1. [Homework statement](https://ocw.cs.pub.ro/courses/sd-ca/teme/tema1-2023)
2. [Definition: Virtual Memory](https://www.techtarget.com/searchstorage/definition/virtual-memory)