Article Directory

1. Variables and labels
2. Constant
3. Type indication
4. Segment definition
5. Process definition
6. Structure type
7. Record Type
8. Macro
9. Conditional Assembly
10. Simplify segment definition
11.Reference

1. Variables and labels

1.1 identifier

Alphanumeric and special symbol strings starting with letters and special characters (_, $, ?, @)

1.2 Reserved words

Identifier with a specific purpose, the user cannot change its meaning

1.3 Variables

1.3.1 Four attributes

The offset is the byte distance relative to the segment start address (0)
Types include basic types such as BYTE, WORD and DWORD, as well as composite types such as struct
The segment address is the base address given when the program is loaded
The value is the actual
content and can be changed

1.3.2 Definition

[Variable name] DB/DW/DD expression

  例：
  msg DB "How are you?",0 ;//共13个字节，DB表示以byte类型（1个字节）定义变量
  
  magic DB 50 dup('1','2','3')   
  		DB '1','2','3'; 共占53字节，并初始化为字符串"123123…"；50表示容量；dup重复数据  
  
  DATA7  DB ?  ; 占1字节；‘?’，表示该变量的值不确定,即:该变量所表示的内存单元中的内容是不确定的,或者说是,当表达式为问号时,变量所对应的内存区中并没有存入新的值,而只是预留出了相应的存储空间;
  
  num dw 1,2,3,4,5,6,7,8,9,10 ;共20字节
                              ;相当于C风格的 WORD num[10]={1,2,3,4,5,6,7,8,9,10};

1.4 Label

1.4.1 Properties

Segment address refers to the segment address of the segment where the instruction corresponding to the label is located
Offset address within segment offset address
There are two types: NEAR and FAR
- NEAR type, which means that the label is used in the segment
- FAR type means that the label can be used between segments

1.4.2 Definition: Add an identifier and a colon ":" in front of the instruction code;

For example: START: PUSH DS

2. Constant

2.1 pseudo-instruction EQU

PI EQU 31415926; similar to C language #define PI 31415926

2.2 pseudo-instruction name = expression

PIX = PI+2 //The compiler replaces name with expression in the preprocessing stage

2.3 Pseudo-instruction ORG

The starting address of the specified program starts
from 0000H by default
ORG 2000H; the specified program instructions are stored from address 2000H

2.4 Number System

Binary: 11101011B
Octal: 345Q
Decimal: 8097D, 8097
Hexadecimal: 1234H, 0EFDAH

2.5 Constant operations

+ - * \ 
AND OR XOR NOT 
SHL SHR //shift left  \  shift right
EQ NE   //equal  \ not equal
LT LE   //lower than  \lower equal小于等于
GT GE   //greater than  \greater equal大于等于

Example: MOV AL, ((11110000B AND 64) +5) GT 32 //Transfer the value of the comparison result to AL

3. Type indication

3.1 Operator PTR (forced type conversion)

MOV BYTE PTR AGE, 20 ;//Indicates the type of AGE

mov ax,bx; is to assign the value in the BX register to AX, since both are word types, there is no need to add "WORD"
mov ax,word ptr [bx]; is to set the memory address to "the value of the BX register" The data stored in the place is assigned ax. Since only a memory address is given, I don't know whether we want to assign ax to byte or word, so we need to specify it with word!

PTR的作用域只在当前语句中

3.2 Operator THIS (current address)

COUNTW EQU THIS WORD
COUNTB DB 51,96,78,90
AGAINF EQU THIS FAR  
AGAINN: MOV AL, 90

3.3操作符LENGTH,TYPE,SIZE(SIZE=LENGTH*TYPE)，WIDTH

LENGTH
- LENGTH variable name
- Take out the length of the variable
TYPE
- TYPE variable name or label
- Get the type of variable name or label (the number of bytes occupied by the variable)
- TYPE的值：BYTE:1, WORD:2, DWORD:4，NEAR:-1, FAR:-2
SIZE
- SIZE variable name
- Get the size of the variable
WIDTH
- WIDTH variable name
- Get the width of the record field

3.4HIGH,LOW

MOV AL, HIGH 56761; take the high byte of the constant
MOV AL, LOW 56761; take the low byte of the constant

3.5SEG,OFFSET

SEG
- SEG variable name or label
- Get the segment address of the segment where the variable name or label is located
OFFSET
- OFFSET variable name or label
- Take out the offset address in the segment where the variable name or label is located
For example:
MOV AX, SEG COUNT //Remove the segment address of the COUNT segment and put it in the AX register
MOV AX, OFFSET COUNT /Remove the offset address of the COUNT segment and put it in the AX register

4. Segment definition

END indicates the end of the entire assembly source program and gives the execution position of the first instruction ()

END　START

DATA 1 SEGMENTS ;一段的开始  
  name1 DB 50DUP(?)  
        DB 2,3,4,5  
DATA 1 ENDS ;一段的结束

DATA 2 SEGMENTS ;一段的开始  
   name2 DB 50DUP(?)  
DATA 2 ENDS ;一段的结束  

STACK SEGMENTS ;一段的开始  
      DB 5000 DUP(?)
STACK ENDS ;一段的结束  

CODE SEGMENT  
    ;把段与段寄存器相关连  
    ASSUME CS:CODE, SS: STACK; DS:DATA1, ES:DATA2  
START: MOV AX,DATA1  
       MOV DS,AX  
       MOV AX,DATA2  
       MOV ES,AX  
       MOV AL,name1 ;name1隐含使用DS  
       MOV BL,name2 ;name2隐含使用ES。这里需要段超越。  
         ……
CODE ENDS
    END START

5. Process definition

definition

Process name PROC [FAR or NEAR]; The default is NEAR
……
RET
process name ENDP
transfer
- CALL procedure name
Example:
call delay; call delay subroutine
...
delay proc near; delay subroutine definition
mov bx,200
lll: mov cx,0
ll: loop ll
dec bx
jne lll
ret
delay endp

6. Structure type

definition

结构类型名 STRUC [对齐类型Alignment][,NONUNIQUE]
    Field1 Type1 Exp1
    Field2 Type2 Exp2
    ......
    FieldN TypeN ExpN
 结构类型名 ENDS

FieldN: Variable name, it can be omitted, no variable name is accessed through the offset in the structure

use
- [Variable name] Structure type name <[Field value list]>
- The field value list is not re-initialized to use the default value, separated by ","

7. Record Type

Record name RECORD field[,field,…]
Field field name: width[=initial value expression] //width: binary digits, default to 0 without initialization

use

[Variable name] Structure type name <[Field value list]>

Variable name. Field name

WEATHER RECORD A:5=10, B:2=3, C:3 ;按位，不能超过16位。
TOR WEATHER <1, ,3>
MOV AL, TOR.B

8. Macro