Unit 4
Models of 8086 Assembly Language Program
The 8086 microprocessor supports different memory
models based on how much memory a program needs. These models determine how
code, data, and stack are organized in memory.
1. Types of Memory Models in 8086
The Intel 8086 processor operates in Real Mode,
where it can address up to 1MB of memory. Based on how memory is
divided, the following models are available:
|
Memory Model |
Code Size |
Data Size |
Stack Size |
Segments Used |
|
Tiny |
≤ 64 KB |
≤ 64 KB |
≤ 64 KB |
Single Segment (CS = DS = SS) |
|
Small |
≤ 64 KB |
≤ 64 KB |
≤ 64 KB |
Separate CS, DS, SS |
|
Medium |
Unlimited |
≤ 64 KB |
≤ 64 KB |
Separate CS, DS, SS |
|
Compact |
≤ 64 KB |
Unlimited |
≤ 64 KB |
Separate CS, DS, SS |
|
Large |
Unlimited |
Unlimited |
≤ 64 KB |
Separate CS, DS, SS |
|
Huge |
Unlimited |
Unlimited |
Unlimited |
Separate CS, DS, SS |
2. Explanation of Each Memory Model
a) Tiny Model
- Entire
program (code, data, stack) fits within a single 64 KB segment.
- CS
= DS = SS (single segment).
- Mostly
used for .COM files.
Example:
assembly
.MODEL TINY
.CODE
ORG 100H ; Required
for COM file execution
START:
MOV AH, 09H
MOV DX, OFFSET MSG
INT 21H ; Print message
MOV AX, 4C00H
INT 21H ; Exit program
MSG DB 'Hello, World!$'
END START
b) Small Model
- Code,
data, and stack have separate segments.
- Code
and data each fit in 64 KB.
- Used
for simple .EXE programs.
Example:
assembly
.MODEL SMALL
.STACK 100H
.DATA
MSG DB 'Hello, 8086!$'
.CODE
MAIN PROC
MOV AX, @DATA
MOV DS, AX ; Initialize data segment
MOV AH, 09H
MOV DX, OFFSET MSG
INT 21H ; Print message
MOV AX, 4C00H
INT 21H ; Exit program
MAIN ENDP
END MAIN
c) Medium Model
- Code
can exceed 64 KB (multiple code segments).
- Data
is limited to 64 KB.
- Used
for large programs with multiple functions.
Example:
assembly
.MODEL MEDIUM
.STACK 100H
.DATA
MSG DB 'Welcome to Medium Model$'
.CODE
MAIN PROC
MOV AX, @DATA
MOV DS, AX
MOV AH, 09H
MOV DX, OFFSET MSG
INT 21H
MOV AX, 4C00H
INT 21H
MAIN ENDP
END MAIN
d) Compact Model
- Multiple
data segments but single code segment.
- Used
when data is large but code remains within 64 KB.
Example:
assembly
.MODEL COMPACT
.STACK 100H
.DATA
BUFFER DB 256 DUP('$')
; Large data buffer
.CODE
MAIN PROC
MOV AX, @DATA
MOV DS, AX
; Your program
logic here
MOV AX, 4C00H
INT 21H
MAIN ENDP
END MAIN
e) Large Model
- Both
code and data can exceed 64 KB.
- Each
function and data block may be in different segments.
- Used
for very large programs.
Example:
assembly
.MODEL LARGE
.STACK 100H
.DATA
MSG DB 'Large Model Example$'
.CODE
MAIN PROC FAR
MOV AX, @DATA
MOV DS, AX
MOV AH, 09H
MOV DX, OFFSET MSG
INT 21H
MOV AX, 4C00H
INT 21H
MAIN ENDP
END MAIN
f) Huge Model
- Multiple
code and multiple data segments.
- Used
for handling large arrays beyond 64 KB.
- Similar
to Large Model, but pointers can access full segment
addressing.
assembly
.MODEL HUGE
.STACK 100H
.DATA
BIG_ARRAY DW 50000 DUP(0)
; Huge array
.CODE
MAIN PROC FAR
MOV AX, @DATA
MOV DS, AX
; Program logic
MOV AX, 4C00H
INT 21H
MAIN ENDP
END MAIN
3. Choosing the Right Model
- Tiny
→ Small programs, .COM files.
- Small
→ Standard programs, fits within 64 KB code & data.
- Medium
→ Large programs, multiple functions with a single data segment.
- Compact
→ Large data but small code.
- Large
→ Large programs requiring multiple code & data segments.
- Huge
→ Needed for very large arrays/structures (more than 64 KB).
4. Summary
- The 8086
memory model is selected based on code, data, and stack size.
- Tiny
model is for compact programs, small & medium for most
applications.
- Large
and huge models handle big programs and massive data structures.
- Use .MODEL directive in MASM (Microsoft Macro Assembler) to specify the model.
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