Overview of Simplified Segment Definition

In assembly language programming, the new version of the assembler provides a method for simplified segment definition, which is more convenient compared to complete segment definitions. The simplified segment definition automatically handles segment attributes through predefined storage models, significantly reducing the burden on programmers.

Core Advantages of Simplified Segment Definition:

1. Simplified Syntax: Reduces redundant segment attribute definitions
2. Automatic Management: The assembler automatically handles segment combinations based on the storage model
3. Improved Readability: Code structure is clearer
4. Efficient Development: Quickly build program frameworks

Storage Model Specification Pseudoinstruction

Before using the simplified segment definition, you must first specify the storage model using the <span>.MODEL</span> pseudoinstruction:

.MODEL storage_model [, language_type] [, os_type] [, stack_options]

Comparison of Common Storage Models

Model	Data Segment Limit	Code Segment Limit	Applicable Scenarios
SMALL	≤64KB (single segment)	≤64KB (single segment)	Small standalone assembly programs
MEDIUM	≤64KB (single segment)	>64KB (multiple segments)	Medium-sized code programs
COMPACT	>64KB (multiple segments)	≤64KB (single segment)	Programs with large data processing
LARGE	>64KB (multiple segments)	>64KB (multiple segments)	Large programs (arrays ≤64KB)
HUGE	>64KB (multiple segments)	>64KB (multiple segments)	Very large programs (arrays can >64KB)

Simplified Segment Definition Pseudoinstructions

1. Code Segment Definition

.CODE [segment_name]

• If no segment name is specified, the default name for the small model is <span>_TEXT</span>

2. Data Segment Definition

.DATA

• Initializes the data segment, the default name for the small model is <span>_DATA</span>

.DATA?

• Uninitialized data segment (BSS segment)

.CONST

• Constant data segment

3. Stack Segment Definition

.STACK [size]

• If no size is specified, the default is 1KB

Complete Example Program

Example of SMALL Model

; Program Name: SIMPLE.ASM
; Function: Demonstrate simplified segment definition of SMALL model

.MODEL SMALL    ; Small storage model

.STACK 100H     ; Define 256-byte stack

.DATA           ; Data segment
    MSG DB 'Hello, Assembly!', '$'
    COUNT DW ?

.CODE           ; Code segment
MAIN PROC
    MOV AX, @DATA   ; Initialize DS
    MOV DS, AX

    ; Display string
    MOV AH, 09H
    LEA DX, MSG
    INT 21H

    ; Set counter
    MOV COUNT, 100

    ; Program exit
    MOV AH, 4CH
    INT 21H
MAIN ENDP

END MAIN

Example of MEDIUM Model

; Example of large code program
.MODEL MEDIUM

.STACK 200H     ; 512-byte stack

.DATA
    BUFFER DB 1024 DUP(?)

.CODE
MAIN PROC FAR   ; Requires FAR call
    MOV AX, @DATA
    MOV DS, AX

    ; Content of large code program...

    RET
MAIN ENDP

SUB_MODULE PROC FAR
    ; Submodule code...
    RET
SUB_MODULE ENDP

END MAIN

Example of COMPACT Model

; Example of large data program
.MODEL COMPACT

.STACK 300H     ; 768-byte stack

.DATA
    ARRAY1 DB 32768 DUP(?)  ; 32KB array
    ARRAY2 DB 32768 DUP(?)  ; Another 32KB array

.CODE
MAIN PROC
    MOV AX, @DATA
    MOV DS, AX

    ; Process large data...

    MOV AH, 4CH
    INT 21H
MAIN ENDP

END MAIN

Advanced Features of Simplified Segment Definition

1. Handling Multiple Code Segments

.MODEL MEDIUM

.CODE
MAIN PROC FAR
    ; Main module code
    CALL SUB1
    RET
MAIN ENDP

.CODE SUB_PROCS  ; Additional code segment
SUB1 PROC FAR
    ; Subroutine code
    RET
SUB1 ENDP

END MAIN

2. Mixed Data Segment Usage

.MODEL SMALL

.DATA
    INIT_VAR DB 10H

.DATA?
    UNINIT_VAR DW ?

.CONST
    PI REAL4 3.1415926

.CODE
; Code...

3. Mixed Use with Complete Segment Definitions

.MODEL SMALL

EXTRN _printf:NEAR  ; Reference C function

_DATA SEGMENT PUBLIC 'DATA'  ; Complete definition
    FORMAT DB '%d', 0AH, 0
_DATA ENDS

.CODE
MAIN PROC
    MOV AX, @DATA
    MOV DS, AX

    PUSH 1234
    PUSH OFFSET FORMAT
    CALL _printf
    ADD SP, 4

    MOV AH, 4CH
    INT 21H
MAIN ENDP

END MAIN

Practical Application Recommendations

1. Model Selection:

• Prefer the SMALL model unless there are special requirements
• For large projects, consider MEDIUM or LARGE models

Segment Usage Specifications:

• Initialize data in <span>.DATA</span>
• Uninitialized data in <span>.DATA?</span>
• Constant data in <span>.CONST</span>

Mixed Programming:

• Pay attention to naming conventions when interacting with C language
• Use <span>EXTRN</span> and <span>PUBLIC</span> to manage external symbols

Debugging Tips:

• Use <span>/Fl</span> option to generate a listing file to check segment layout
• Check the MAP file to confirm memory allocation

Performance Optimization Considerations

1. Advantages of SMALL Model:

• All transfers are of NEAR type
• Data access does not require segment crossing
• High code density

Large Data Processing:

• Large data in COMPACT model needs to be processed in segments
• Consider using pointers and segment registers for optimization

Code Organization:

• Hot path code should be placed in the main code segment
• Less frequently used code can be placed in other segments

Simplified segment definitions greatly enhance the efficiency of assembly language program development, especially in modern development environments. By reasonably selecting storage models and utilizing simplified pseudoinstructions, programmers can focus on algorithm implementation rather than low-level detail management.