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Hw20 Memory Management (Allocate and Free) Solution
Learning Goals
==============
* Understand how to manage resources
* Design data structures and algorithms to keep track of usage
Resource Management
===================
Resource management is a common problem practically everywhere. There
are two types problems: (1) How much resource to acquire? and (2) How
to allocate resources?
For the first question: How much resource to acquire. Imagine these scenarios:
- You are building a shipping company, how many boats and how many
trucks should you buy?
- You are building a hotel, how many rooms do you need?
- You are building a restaurant, how many tables do you want?
For the second question: How to allocate resource? Imagine these scenarios:
- A customer needs six trucks for one week transporting something from
New York to Seattle. Another customer needs two trucks for one day
from Atlanta to Tampa. The third customers needs four trucks for
four days from Los Angeles to Phoenix.
- One conference needs 20 rooms from August 25 to 28. Another
conference needs 15 rooms from August 27 to September 1. Yet another
conference needs 22 rooms from August 28 to September 2.
Memory Management
=================
This assignment is a *simplified* version of memory management by
operating systems. This assignment specifies the requirements and you
need to
- Design the algorithms and data structures to meet the requirements
- Implement your design by a C program
Inside a computer, the operating system has to keep track of which
piece of memory is used or not. The smallest unit of memory
allocation is called a *block*. The number of *virtual* blocks is
determined by the operating system and the processor. The number of
*physical* blocks is determined by the number of memory chips
installed on the machine. This assignment does not distinguish virtual
and physical blocks.
Consider the following example when a computer has 20 blocks. If a
block is not used, it is marked by `-`. If a block is used, it is
marked by `*`.
At the very beginning, no memory is used and every block is marked by
`-`. Please notice that the blocks start with 0. If there are n
blocks, the last block's number is n - 1.
|block | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|------|---|---|---|---|---|---|---|---|---|---|----|----|----|----|----|----|----|----|----|----|
| | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - |
When several blocks are needed for allocation, your program needs to
find *consecutive* blocks. If you cannot find large enough consecutive
blocks, you have to reject this allocation request.
For example, imagine that after some allocations and releases, this is how the memory is occupied:
|block | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|------|---|---|---|---|---|---|---|---|---|---|----|----|----|----|----|----|----|----|----|----|
| | * | - | * | - | * | - | * | - | * | - | * | - | * | - | * | * | - | * | * | - |
As you can see, 11 blocks are occupied and 9 blocks are
unused. However, these unused blocks are scattered (called
*fragmentation* in operating systems). If two blocks are needed, then
allocation fails.
Consider another example:
|block | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|------|---|---|---|---|---|---|---|---|---|---|----|----|----|----|----|----|----|----|----|----|
| | - | - | - | - | * | - | - | - | - | - | * | - | * | - | - | - | - | - | * | - |
Suppose three blocks are needed. There are multiple options:
- Blocks 0, 1, and 2
- Blocks 1, 2, and 3
- Blocks 5, 6, and 7
- Blocks 6, 7, and 8
- ... some more options
- Blocks 15, 16, and 17
This assignment asks you to use **first fit** to allocate. Among these options, blocks 0, 1, and 2 will be used.
Let's run through several examples:
Starting from all blocks unoccupied.
If allocation succeeds, the program returns the ID of the first block.
Allocate two blocks:
|block | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|------|---|---|---|---|---|---|---|---|---|---|----|----|----|----|----|----|----|----|----|----|
| | * | * | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - |
Return 0.
The first two blocks are occupied now.
Allocate five blocks:
|block | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|------|---|---|---|---|---|---|---|---|---|---|----|----|----|----|----|----|----|----|----|----|
| | * | * | * | * | * | * | * | - | - | - | - | - | - | - | - | - | - | - | - | - |
Return 2.
Allocate three blocks:
|block | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|------|---|---|---|---|---|---|---|---|---|---|----|----|----|----|----|----|----|----|----|----|
| | * | * | * | * | * | * | * | * | * | * | - | - | - | - | - | - | - | - | - | - |
Return 7.
Allocate five blocks:
|block | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|------|---|---|---|---|---|---|---|---|---|---|----|----|----|----|----|----|----|----|----|----|
| | * | * | * | * | * | * | * | * | * | * | * | * | * | * | * | - | - | - | - | - |
Return 10.
Free the blocks starting from 0.
|block | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|------|---|---|---|---|---|---|---|---|---|---|----|----|----|----|----|----|----|----|----|----|
| | - | - | * | * | * | * | * | * | * | * | * | * | * | * | * | - | - | - | - | - |
Allocate two blocks:
|block | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|------|---|---|---|---|---|---|---|---|---|---|----|----|----|----|----|----|----|----|----|----|
| | * | * | * | * | * | * | * | * | * | * | * | * | * | * | * | - | - | - | - | - |
Return 0.
Free the blocks starting from 10.
|block | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|------|---|---|---|---|---|---|---|---|---|---|----|----|----|----|----|----|----|----|----|----|
| | * | * | * | * | * | * | * | * | * | * | - | - | - | - | - | - | - | - | - | - |
Allocate two blocks:
|block | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|------|---|---|---|---|---|---|---|---|---|---|----|----|----|----|----|----|----|----|----|----|
| | * | * | * | * | * | * | * | * | * | * | * | * | - | - | - | - | - | - | - | - |
Return 10.
Allocate three blocks:
Return 12.
|block | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|------|---|---|---|---|---|---|---|---|---|---|----|----|----|----|----|----|----|----|----|----|
| | * | * | * | * | * | * | * | * | * | * | * | * | * | * | * | - | - | - | - | - |
Free the blocks starting from 2.
|block | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|------|---|---|---|---|---|---|---|---|---|---|----|----|----|----|----|----|----|----|----|----|
| | * | * | - | - | - | - | - | * | * | * | * | * | * | * | * | - | - | - | - | - |
Allocate four blocks:
|block | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|------|---|---|---|---|---|---|---|---|---|---|----|----|----|----|----|----|----|----|----|----|
| | * | * | * | * | * | * | - | * | * | * | * | * | * | * | * | - | - | - | - | - |
Return 2.
Required Functions
==================
Please read `memory.c` carefully and implement the needed functions.
For this assignment, you need to design and implement five
functions. Among them, `allocateMemory`, `saveOccupancy`, and
`freeMemory` are particularly important.
- `allocateMemory` takes two arguments: a memory object and the number
of blocks to allocate. It returns the index of the starting block of
one (or several) block large enough. If it is not possible to find
consecutive blocks for the size, this function returns -1.
- `freeMemory` takes two arguments: a memory object and the starting
index of one (or several) consecutive blocks. This starting address
should be returned by `allocateMemory` as a previously allocated
blocks of memory. If the starting address is invalid, this function
does nothing.
Consider the following memory:
|block | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|------|---|---|---|---|---|---|---|---|---|---|----|----|----|----|----|----|----|----|----|----|
| | - | - | - | - | - | - | * | * | * | * | - | - | - | - | - | - | - | - | - | - |
Allocate two blocks:
|block | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|------|---|---|---|---|---|---|---|---|---|---|----|----|----|----|----|----|----|----|----|----|
| | - | - | - | - | - | - | * | * | * | * | - | - | - | - | - | - | - | - | - | - |
`allocateMemory` returns 0.
|block | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|------|---|---|---|---|---|---|---|---|---|---|----|----|----|----|----|----|----|----|----|----|
| | * | * | - | - | - | - | * | * | * | * | - | - | - | - | - | - | - | - | - | - |
If `freeMemory`'s second argument is 1, this argument is invalid because `allocateMemory` did not return 1.
If `freeMemory`'s second argument is 12, this argument is invalid because block 13 is unoccupied.
- `saveOccupancy`: saves the occupancies of each block to a *binary*
file. **Each block is represented by one bit**. The memory shown
above will be saved as
(binary) 1100 0011 1100 0000 0000
Since this is not a multiple of 8, 0's are added to the end:
(binary) 1100 0011 1100 0000 0000 0000
These bytes are saved to the file (3 bytes).
Useful Tool
===========
You will likely find `xxd -b` very helpful. It shows the bits stored in a binary file.
Encapsulation
=============
This assignment gives you a lot of flexibility. You design how to keep
track of the occupied blocks. You design how to manage these blocks, as long
the functions behave correctly from *external* viewpoints (to the callers and
the output file).
This concept is called **encapsulation** in object-oriented programming:
You have the freedom to change the internal representations of data
structures, as long as externally the program behaves correctly.
It is strongly suggested that you think carefully about how to design
this program. Efficiency (using memory or time) is not particularly
important here. Thus, you may use more memory or slow methods, as long
your program behaves correctly. In fact, it is advised that you start
with really simple solutions without worrying about efficiency too
much. After you understand how to design the program, you may choose
to improve efficiency (in memory, or time, or both). Obviously, your
program must be fast enough so it can be graded. Please make sure your
program does not enter an infinite loop in any case.
Create Your Own Test Cases
==========================
You need to know how to create test cases now.
DO NOT ASK FOR TEST CASES.
Submission
==========
For this assignment, you need to provide all necessary files **and**
`Makefile`. Your program will be tested using the following commands
`make`: generate an executable called `hw20`. Please remember that the
executable must be called `hw20`. Any other name will be rejected.
If `make` fails, you will receive no score in this assignment. Each
warning message has 10% penalty of the score.
```
zip hw20.zip Makefile + all necessary files
```
Upload hw20.zip.
