1. Presentation on Real Mode Memory Addressing
Presented By

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. Road Map Real Mode Memory Addressing Array, Loop and Accessing Memory

8. Array, Loop and Accessing Memory
DATA DW 50 ; DATA is a word with value 50
DATA1 DW 50 DUP (?) ; array of 50 uninitialized words
DATA2 DW 50 DUP (0) ; array of 50 words initialized with 0 50
DATA
DATA1
DATA2
Accessing memory
MOV AX, 1020h
MOV AX, [1020h]
MOV [AX], 20 AX
1020h
1021h
0000h
0001h
MOV CX, 10 ; loop count
XYZ:
; statements
LOOP XYZ

9. Example Program 1
Write a program that will initialize 50 bytes array with values 1 to 50
.MODEL SMALL
.DATA
DATAS DB 50 DUP (?) ; setup array of 50 bytes
.CODE ; start of code segment
.STARTUP ; start of program
MOV AX, 1
MOV CX, 50 ; load counter with 50
MOV BX, OFFSET DATAS ; address of DATAS array
AGAIN:
MOV [BX], AX ; save values to array positions
INC AX ; increment AX to next values
INC BX ; increment BX to next elements
LOOP AGAIN ; repeat 50 times
.EXIT ; exit to DOS
END ; end program

10. Example Program 2
Write a program that will initialize 50 bytes array in the following form
.MODEL SMALL
.DATA
DATAS DB 50 DUP (?) ; setup array of 50 bytes
.CODE ; start of code segment
.STARTUP ; start of program
MOV AX, 1
MOV CX, 50 ; load counter with 50
MOV BX, OFFSET DATAS ; address of DATAS array
AGAIN:
MOV [BX], AX ; save values to array positions
INC AX ; increment AX to next values
INC BX ; increment BX to next elements
LOOP AGAIN ; repeat 50 times
.EXIT ; exit to DOS
END ; end program 50
DEC
decrement

11. Example Program 2, Alternate Way
Write a program that will initialize 50 bytes array in the following form
.MODEL SMALL
.DATA
DATAS DB 50 DUP (?) ; setup array of 50 bytes
.CODE ; start of code segment
.STARTUP ; start of program
MOV AX, 1
MOV DI, 49
MOV CX, 50 ; load counter with 50
MOV BX, OFFSET DATAS ; address of DATAS array
AGAIN:
MOV [BX+DI], AX ; save values to array positions
INC AX ; increment AX to next values
DEC DI ; decrement DI
LOOP AGAIN ; repeat 50 times
.EXIT ; exit to DOS
END ; end program

12. Example 3 Move array element 10H into array element 20H **** ***** ***
**** ***** ***
ARRAY
****
29H 16
****
.MODEL SMALL
.DATA
Array DB 16 DUP (?) ; setup array
DB 29H
DB 30 DUP (?)
.CODE
.STARTUP ; start of program
MOV BX, OFFSET ARRAY ; address of ARRAY
MOV DI, 10H ; address element 10H
MOV AL, [BX + DI] ; get element 10H
MOV DI, 20H ; address element 20H
MOV [BX+DI], AL ; save in element 20H
.EXIT ; exit to DOS
END ; end program
Example 3-7: Intel Microprocessors - by Brey

13. Example 3, Alternate Way Move array element 10H into array element 20H
****
29H 16
****
Example: 3-8, Brey

14. DATA Addressing Modes: Figure 3-2: by Brey