Segmentation in Operating Systems - Step-by-Step Execution

Concept Flow - Segmentation

Program divided into segments

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Each segment has a base address and length

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Segment table stores base and length for each segment

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CPU uses segment number and offset to access memory

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Check if offset < segment length?

No→Error: Invalid memory access

Yes↓

Physical address = base + offset

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Access memory at physical address

Segmentation divides a program into logical parts called segments, each with its own base address and length, allowing memory access by segment number and offset.

Execution Sample

Operating Systems

Segment Table:
0: base=1000, length=500
1: base=2000, length=300
Access: segment=0, offset=450

This example shows accessing memory at segment 0 with offset 450 using the segment table.

Analysis Table

Step	Segment Number	Offset	Check Offset < Length	Physical Address Calculation	Result
1	0	450	450 < 500 = True	1000 + 450 = 1450	Access memory at 1450
2	1	350	350 < 300 = False	N/A	Error: Invalid memory access

💡 Step 2 stops because offset 350 is not less than segment length 300, causing an invalid access error.

State Tracker

Variable	Start	After Step 1	After Step 2
Segment Number	N/A	0	1
Offset	N/A	450	350
Segment Length	N/A	500	300
Base Address	N/A	1000	2000
Physical Address	N/A	1450	N/A

Key Insights - 2 Insights

Why do we check if the offset is less than the segment length?

Because accessing memory beyond the segment length causes an error. As shown in execution_table step 2, offset 350 is not less than length 300, so access is invalid.

How is the physical address calculated from segment and offset?

Physical address is the sum of the segment's base address and the offset. In step 1, base 1000 plus offset 450 equals physical address 1450.

Visual Quiz - 3 Questions

Look at the execution table, what is the physical address calculated at step 1?

A 1450

B 1000

C 450

D 500

💡 Hint

Check the 'Physical Address Calculation' column in execution_table row for step 1.

At which step does the offset exceed the segment length causing an error?

A Step 1

B Step 2

C Neither step

D Both steps

💡 Hint

Look at the 'Check Offset < Length' column in execution_table to find where the condition is false.

If the offset in step 2 was 250 instead of 350, what would happen?

A Access memory at physical address 350

B Error: Invalid memory access

C Access memory at physical address 2250

D No change, still error

💡 Hint

Refer to variable_tracker for base address 2000 and compare offset 250 with segment length 300.

Concept Snapshot

Segmentation divides a program into logical segments.
Each segment has a base address and length.
Memory access uses segment number + offset.
Check offset < segment length to avoid errors.
Physical address = base + offset.

Full Transcript

Segmentation is a memory management technique where a program is split into segments like code, data, and stack. Each segment has a base address and length stored in a segment table. When accessing memory, the CPU uses a segment number and an offset. It first checks if the offset is less than the segment length to ensure valid access. If valid, the physical address is calculated by adding the base address of the segment to the offset. If the offset is too large, an error occurs. This method helps organize memory logically and protects against invalid memory access.