2.7 Memory Management

Posted Oct 29, 2025 Updated Nov 24, 2025

By So Hee Park 11 min read

⭐️ Practical exercise of segmentation, paging
practice in aula virtual

✅ Memory Management

Memory Management is for managing the RAM
does not apply for SM, HD

how to save processes in the RAM
when we open the files

RAM and Secondary Memory are very far away
To communicate between RAM and Secondary memory, we need an intermediate block to help

✔️ Intermediate blocks to help RAM and SM communicate

1️⃣ segmentation unit
2️⃣ paging unit
❓ Where is the segmentation unit and paging unit located?
these units are inside the Microprocessor

✔️ Two units, two techniques to manage the RAM

As there are two units, there are two techniques to maange the RAM
1️⃣ segmentation
2️⃣ paging
each of the units proposes a way of placing processes in the RAM
from the two techniques that places more processes⬆️ in the RAM using small amount⬇️ of RAM

❓ Tick the elements involved in placing processes inside the RAM?

👉🏻
Hard Disk (Has the OS so it’s involved)
RAM is also involved,
CPU is also involved (More specifically, segmentation unit and paging unit).

0️⃣ Swapping

🚌 Processes travel through the front side bus

1️⃣ Segmentation

CANNOT break memory
ALL or NONE
IN or OUT of the memory
Order: big to small recommended
👎🏻 external fragmentation

No possibility of breaking the memory
One process goes completely goes inside the RAM, or cannot go on the RAM at all
If a process cannot go on a RAM, and it is left outside the RAM, you will not be able to run the process
last space left in the RAM that is free is called FREE SPACE

❓ Why does my program not running?
- No space on the RAM, cannot go on the RAM

❓ Swap out
My RAM looks like this
P1 300MB
P2 200MB
P3 400MB
P4 500MB

If I want to run P5, which is 300MB, which process should I swap out?
- P1 or P3 or P4

☑️ Free Space

The free empty portions in the middle of the RAM are known as FREE SPACE
However, when free spaces are seperate they remain seperate
you cannot join them into one big free space

❓ Imagine future processes P6, 7, 8,,,,,are all greater than 200MB, and you cannot swap out.
Can they swap in?

- NO

☑️ External Fragmentation

So the wasted areas that are not occupied anymore is wasted forever
they are called External fragmentation
empty portions in the middle of the RAM
⚠️ (only the middle ones, the empty portion in the end is still called Free space)
they are normally colored in 🔴 RED, to show that they are wasted
External fragmentation = 100MB

☑️ Order of putting program on the RAM

If you use segemtation,
it is mandatoey to open the big programs first,
then the small programs
If you open the small programs first, later with external fragmentation, you will not be able to open in big problems.

❓  Why in some companies, big programs like database, HR applications and the invoicing applications
are opened automatically in the begining of the session?

- Bc that system use segmentation

2️⃣ Paging

RAM fragmented into frames, process into pages
ALL or NONE
Order of the processes going on the RAM does not matter
👎🏻 internal fragmentation
page table

RAM is divided into portions, which is called FRAMES(Marcos)
your processes will also be divided into portions called Pages
the size of the frames need to be greater than the size of pages size of the frames >= pages
One page per frame
The process SWAPS IN if ALL its pages can go in. ALL or NONE
Pages do not need to be in order when it goes into the frames

Process1 = 300MB: Divide Process1 into 3 pages of 100MB each
Process2 = 200MB: Divide Process2 into 2 pages of 100MB each
Process3 = 400MB: Divide Process3 into 4 pages of 100MB each
Process4 = 500MB: Divide Process4 into 5 pages of 100MB each

If the whole process cannot go ALL on the RAM, then it cannot go on the RAM at all
Process4 of 5 pages(500MB) cannot go on the RAM at all, even if there is one frame frame 10 left
The frames that are not used at the end are called FREE SPACE

☑️ Internal Fragmentation

when the frame is bigger than the page

Problem of Paging
If frames are bigger than pages
the frames are fragmented to 100MB and 50MB
we are wasting space in each of the frames
they are called Internal Fragmentation
they are called internal as frames inside are fragmented, they have wasted portions

⭐️⭐️⭐️ EXAM ⭐️⭐️⭐️
❓ How much data is internal fragmentation in P1?
👉🏻 50MB

❓ How much total internal fragmentation of the RAM?
👉🏻 450MB
- Internal fragmentation does not take free space at the end into account
- Free space at the end is not Internal Fragmentation

❓ Can you get rid of internal fragmentation?
Yes, make the size of the page to the size of the frame
size of the page = size of the frame
However, the size of the frame is a factory setting that comes with a RAM
so when you buy a RAM, pay attention to size of the frame
and adopt/modify OS size of the page to the size of the frame
you cannot change the size of the frame ❌
but you can adopt the size of the page ⭕️

⭐️⭐️⭐️ EXAM ⭐️⭐️⭐️
❓ How can you minimize  external fragmentation?
👉🏻 Open processes in a good order,
first big then small processes

In general, external fragmentation and internal fragmentation is difficult to get rid of it
Thats why sometimes RAM seems to be smaller, less capacity then I think

☑️ Swap out, swap in

⭐️⭐️⭐️ EXAM ⭐️⭐️⭐️
❓ Swap P1 out, and decide if P4 can fit in.
P4: Process 4 fragmented into 5 pages of 100MB each
P4: Process4 into 5 pages of 100MB each

👉🏻 No, still only 4 frames
so P4 cannot go in
P4 stays out of the RAM

⭐️⭐️⭐️ EXAM ⭐️⭐️⭐️
❓ Then what happens to P5,
P5: Process5 fragmented into 4 pages of 100MB each
👉🏻 YES, as there are 4 frames
and in Paging, order does not matter
but you can go on the RAM if there is sapce

When process go in the RAM
ALL or NONE
but ORDER does not matter

☑️ Program Counter in Paging

In paging, the processes are not in order ❌
so the computer should somehow know from P5 and jump over P2 and jump to P5
so you need to know the order of the processes
So paging needs Program Counter of the Neuman Machine
so PC tells you which instruction comes next
Goal: give user a smooth experience
I want my video to run smoothly, not suddenly pop up word file

☑️ Pages Table

inside the OS in RAM

I need a schema that has the order of the processes
This Pages Table is located in the forbidden area of the RAM, the OS in RAM
Pages Table is a file with .csv file
.csv file: Comma Seperated Values
everything in this file is seperated with commas ,
Pages Table contains the following
PID, Frames of the process, Base Register, Limit Register, ...rest of the processes..., Base Register of the free areas, Limit Register of the free areas

✔️ Contents of the Pages table

PID:
Frames of the process
Base Register: beginning
Limit Register: size of the process, how many frames does this process have

Base Register and Limit Register is redundant(repeated)
as you can know start and size with Frames of the process
👍🏻 But we repeat for security!

📄 Design a Pages Table

✔️ Design a pages table for this RAM

📄 Pages Table 📄
process name,
frames where is process has portions,
base register,
limit register


P5 , 1, 2, 3, 10 , BRP5 , 1 , LRP5, 4,
P2, 4, 5, BRP2, 4, LRP2, 2,
P3, 6, 7, 8, 9, BRP6, LRP3, 4,
FBR(Free Base Register, there is none free),
FLR(Free limit Register, there is no free limit register)

✔️ Design a pages table for this RAM

📄 Pages Table 📄
P1, 1, 2, 3, BRP1, 1, LRP1, 3,
P11, 6, 7, 8, BRP11, 6, LRP11, 3,
P3, 11, 12, 13, 14, BRP3, 11, LRP3, 4,
P4, 15, BRP4, 15, LRP4, 1,
P12, 17, 18, BRP12, 17, LRP12, 2,
P7, 23, BRP7, 23, LRP7, 1,
P8, 24, 25, BRP8, 24, LRP8, 2,
P13, 26, BRP13, 26, LRP13, 1,
P10, 27, 28, BRP10, 27, LRP10, 2,
FBR, 4, 9, 16, 19,
FLR, 2, 2, 1, 4

All the free areas of the RAM Base Registers go together
- FBR, 4, 9, 16, 19
and all the free areas of Limit Registers also go together
- FLR, 2, 2, 1, 4
❓ Who uses the Pages Table?
1️⃣ Program Counter will use the Pages Table to jump around the RAM
to find the next instructions
Program COunter is in the CPU
2️⃣ The Paging Unit of the CPU will use the Pages Table for memory management
to check where to put the next process on the RAM

⭐️⭐️⭐️ EXAM ⭐️⭐️⭐️
❓ Which block of the computer uses the `Pages Table`?
- Read by the Program Counter in CPU to jump in the RAM
- The Paging Unit in CPU to know if the process fits
- so the CPU

⭐️⭐️⭐️ EXAM ⭐️⭐️⭐️
❓ Should we protect the paging table?
👉🏻 YES

❓ OSRAM and macro virus

What happens if the macro virus is trying to enter the OS in RAM?

Paging Unit never sends a process to a OS is RAM
the macro virus will look like a normal process P20
Paging Unit will send the process to a normal address in RAM, not OS in RAM
the virus will try to move internally in the RAM
However, in RAM, onced placed, you cannot move up and down internally
❓ So what does the virus do?
If the virus tries to move,
it creates corruption in the RAM
it makes MS corrupt
and the virus convinces the MS to go up and down the RAM
and use MS to go up to the forbiden area, the OS in RAM
and corrupts the Pages Table by changing the commas, and changing the numbers
so I make a huge mess of the Pages Table
and we cannot run processes in order
❓ How do I convince the MS to be corrupt?
If the destination address for MS was 1111,
the macro virus changes the number, to 0000
so that MS goes to the OS in RAM
❓ What should we do if the process has been intercepted by a virus?
So if process is intercepted by the macro virus
the macro virus should be swapped out
However, the virus process could have created a child process
the virus process could also have had a parent process
👉🏻 Thus, you need to swap out 3 generations(the process, parent and children process)
swap out ALL the related processes
👉🏻 Maybe swapping out 3 generations is too tough, but its for more security
as parent/child process of a virus process was not corrupted.
👉🏻 But we will still swap out three generations, related processes for security
This is called 3 Level Security

📌 Segmented Paging

Paging is more complicated for the CPU
If things work with segmentation, they use segmentation
Computer first tries segmentation
However, if the process cannot SWAP IN
if things do not work with segmentation,
then the computer uses paging
you page the free space of the RAM
then use paging

✅

1️⃣

2️⃣

3️⃣

4️⃣

1️⃣
2️⃣
3️⃣
4️⃣ 👍🏻 👎🏻

⭐️⭐️⭐️ EXAM ⭐️⭐️⭐️
❓
👉🏻

DAW bilingual, Computer System

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