1. Presented By Dr. Shazzad Hosain Asst. Prof. EECS, NSU
Lecture 7
Presented By
Dr. Shazzad Hosain
Asst. Prof. EECS, NSU

2. Real Mode Memory Addressing
The first 1MB memory is Real memory or the Conventional memory *
Segment 1
Segment 2
Segment n
0000h CS
8000h DS
A000h SS
1MB
offset
16 bit Segment registers
1 MB requires 20 bit address
Each segment is 64 KB
Offset address is 16 bit or 2 byte
Actual address = segment address + offset address

3. Real Mode Memory Addressing
Real mode operation allows to address 1MB of memory space – even for the Pentium microprocessor
This first 1MB memory is called the real memory or the conventional memory
A combination of segment and offset address access the real memory
Segment registers contains the beginning address of any 64KB memory segment
The offset address selects the any location within the 64KB memory space

4. Segment Plus Offset Determines Address
To get the real address
Pad 0H at the end of segment register
Add the offset value
10000H
F000H
1F000H
CS = 1000H
Offset = F000H
12340H
245FH
1479FH
DS = 1234H
Offset = 245FH
Since each segment is 64 K, the offset address can take maximum of FFFFH
Once, the beginning address is found in segment registers, ending address is calculated by adding FFFFH with the value of segment register after padding 0H after it.
From Intel Microprocessor

5. Default Segment and Offset Registers
If CS = 1400H and IP/EIP = 1200 H
The microprocessor access instruction from H+ 1200H = 15200H.

6. Allows relocation Suppose 1000H bytes of code 190H bytes of data
Figure 2-4: A memory system showing the placement of four memory segments
Suppose
1000H bytes of code
190H bytes of data
200H bytes of stack
Allows
relocation
Figure 2-5

7. Protected Mode The segment register now contains a selector
Selector selects a descriptor from a descriptor table
The descriptor describes the memory segment’s location
Two descriptor table
Global Descriptor Table (GDT)
Local Descriptor Table (LDT)

8. Selectors and Descriptors*
GDT/LDT
8191 * 2 1 16 8 31 15 7
Offset
Segment Registers
As Selector
8192 number of descriptors in each table
Each descriptor 8 bytes long, thus table size is 64 KB
Selector selects one descriptor
Descriptor describes the segment

9. Selectors and Descriptors
CS/DS/ES
Offset 16 8 31 15 7
GDT/LDT
8191 * 2 1
Segment Register
0012H
0044H
FFF0H

10. Selectors and Descriptors
CS/DS/ES
Offset 16 8 31 15 7
GDT/LDT
8191 * 2 1

11. Access Right Bits

12. Selectors and Descriptors
CS/DS/ES
Offset 16 8 31 15 7
GDT/LDT
8191 * 2 1 7 5 3 1 6 4 2
00H
A0H C2H
10H H

13. Selectors and Descriptors
CS/DS/ES
Offset 16 8 31 15 7
GDT/LDT
8191 * 2 1
Code a descriptor for that starts at H and ends at 21001FH. This memory segment is a code segment that can be read 7 5 3 1 6 4 2
21H
00H H
00H FH
Limit = 21001FH – H = 001FH

14. Selectors and Descriptors
Code a descriptor for that starts at H and ends at 21001FH. This memory segment is a code segment that can be read 7 5 3 1 6 4 2
1xx x11
21H
00H H
00H FH
Limit = 21001FH – H = 001FH

15. Selectors and Descriptors
32 bit / 4 byte base, memory size is 4 GB = 22x210x210x210
AV = 1, means available, 0 means not available
D = 1, means 32 bit instructions, 0 means 16 bit instructions
G bit or the Granularity bit
G = 0, the limit is from 1 to 1MB in length
G = 1, the limit is multiplied by 4K bytes (appended with 000H). So segment length is 4K to 4G bytes in steps of 4K bytes

16. Selectors and Descriptors7 5 3 1 6 4 2
01H AV D O 0H
00H
00H H
FFH FFH
Start = H
Limit = FFFFH
End = FFFFH
G bit or the Granularity bit
G = 0, the limit is from 1 to 1MB in length
G = 1, the limit is multiplied by 4K bytes (appended with 000H). So segment length is 4K to 4G bytes in steps of 4K bytes

17. Selectors and Descriptors7 5 3 1 6 4 2
00H AV 1 D O 0H
28H
00H H
00H H
Start = H
Limit =
End =
00010
000 H
G bit or the Granularity bit
G = 0, the limit is from 1 to 1MB in length
G = 1, the limit is multiplied by 4K bytes (appended with 000H). So segment length is 4K to 4G bytes in steps of 4K bytes H

18. G bit or the Granularity bit7 5 3 1 6 4 2
03H AV G 1 D O 0H
00H
00H H
2FH FFH
End = 05FFFFFFH
Start = H
G bit or the Granularity bit
G = 0, the limit is from 1 to 1MB in length
G = 1, the limit is multiplied by 4K bytes (appended with 000H). So segment length is 4K to 4G bytes in steps of 4K bytes
Size = 02FFFFFFH
Limit = 02FFFH

19. 03H 1 0H 1xx1 001x 00H 00H 00H 2FH FFH End = 05FFFFFFH7 5 3 1 6 4 2
03H AV G 1 D O 0H
1xx x
00H
00H H
2FH FFH
End = 05FFFFFFH
Start = H
Size = 02FFFFFFH
Limit = 02FFFH

20. References
Chapter 2, Intel Microprocessors – by Brey