Assembly language basics 4-Basic commands

Logic Operation commands

The following binary operations can be used:

Operator	Meaning	OperationAlgorithmThen
And	And	0 & 0 = 0; 0 & 1 = 0; 1 & 0 = 0; 1 & 1 = 1
Or	Or	0 | 0 = 0; 0 | 1 = 1; 1 | 0 = 1; 1 | 1 = 1
XOR	Exclusive or	0 then 0 = 0; 0 then 1 = 1; 1 then 0 = 1; 1 then 1 = 0
Not	Non	~ 0 = 1 ;~ 1 = 0

These are bitwise operations.

Note:

1 DWORD = 2 word = 4 byte = 32 bit

1 word = 2 byte = 16 Bit

1 byte = 8 bit

 

Arithmetic Operation commands

Add	Add
Sub	Subtraction
Mul	Multiplication
Div	Division
SHL	Shift
INC	Add 1
Dec	Decrease 1
Neg	Complement	Reverse lookup and add one more

 

Note:

The compiler generally combines these operations in a strange way, because it can achieve faster execution speed with the same results. Generally, the compiler compares complex commands to achieve faster execution speed. And the execution speed of all commands on different CPUs is different. Some of them can be done in one clock cycle, and some may be much better.

String command

Command	Full name	Command mnemonic	Meaning
Rep	Repeat	Repeated	It allows repeated execution of string commands (ECx is executed when it is not equal to 0). Each execution time, the content of ECx is reduced by 1.
Repe	Repeat equal	Equal and repeated	When the two strings are equal, that is, ZF = 1, it can execute the string commands repeatedly (if ECx is not equal to 0). Each execution time, the content of ECx is reduced by 1. (Exactly the same as repz)
Repne	Repeat not equal	Not repeated	When the two strings are not equal, that is, ZF = 0, it can execute the string commands repeatedly (if ECx is not equal to 0). Each execution time, the content of ECx is reduced by 1. (Exactly the same as repnz)
Movs	Move string	String Transmission	Similar to movsb and movsw, but the data string type (word or byte) must be specified ).
Movsb	Move string by byte	Transmit strings by byte	Transmit one byte of data in the data segment directed by ESI to a byte storage unit in the additional segment directed by EDI, and modify the addresses in ESI and EDI according to the direction signs. When df = 0, the address is added with 1; When df = 1, the address is reduced by 1.
Movsw	Move string by word	Send string by word	Transmit a word term in the data segment directed by Si to a word storage unit in the additional segment directed by DI, and modify the address in Si and di according to the direction indicator. When df = 0, the address is added with 2; When df = 1, the address is reduced by 2.
CMPS	Compare string	String comparison	Cmpsb or cmpsw is usually used.
Cmpsb	Compare string by byte	Byte comparison string	Subtract the byte data specified by the DS segment Si from the byte data specified by the es segment di (the result is not saved), and then set the flag Based on the subtraction result (if the two data are equal, ZF = 1 ). And the direction signs DF to modify the address in Si and Di, that is, when df = 0, the address is added with 1; When df = 1, the address is reduced by 1.
Cmpsw	Compare string by word	Comparing strings by words	Subtract the word data specified by Si in the DS segment from the word data specified by DI in the es segment (the result is not saved), and then set the flag Based on the subtraction result (if the two data are equal, ZF = 1 ). And the direction signs DF to modify the address in Si and Di, that is, when df = 0, the address is added with 2; When df = 1, the address is reduced by 2.
STOs	Store Accumulator	Storage Accumulators	This command transfers the value in the accumulators to the memory specified by DS: Di. After the storage ends, the CPU either increases or reduces EDI, depending on the DF flag.
Lods	Load Accumulator	Load Accumulators	The lods command is unique in string commands. You never need to use a duplicate prefix to modify the command. The lods Command copies the byte or word directed by DS: Si to Al, ax, or eax. After the copy is completed, the SI value is increased or decreased Based on the DF flag. Repeat this command does not make any sense. The value in eax will be continuously washed away, leaving only the last loaded value.
SCAs	Scan string	Scan string	This command can be used to find a specific element in a string. The SCAs command only needs the address es: Di of the target string, instead of the Source and Destination strings. The value of the source string is in Al, ax, or eax. This command compares the value pointed to by ES: di with the value (Al, ax, eax) in the accumulators, and then increases or decreases di 1, 2, or 4 According to DF.

Note:

The table above involves some of the Flag spaces in the sign register. For convenience, they are listed below:

ZF = zero flag, zero flag, and DF = Direction Flag;