Reference: "Assembly Language" by Wang Shuang, Chapter 10

Both call and ret are transfer instructions.

1. ret and retf

ret instruction: Use the data in the stack to modify the IP content, thereby achieving a near transfer

is equivalent to:

pop ip

retf instruction: Use the data in the stack to modify CS and IP to achieve far transfer

is equivalent to:

 

pop ip

 

 

pop cs

 

 

Example: ret

retf

2. Call instruction

Call instruction, execute operation:

    1. Push the current IP or CS and IP into the stack

    2. Jump

(1) Call instruction that transfers based on displacement

 

Format: call label

 

Push the next instruction's IP into the stack and go to the label

 

is equivalent to:

 

push ip

jmp near ptr label

(2) The transfer destination address is in the call instruction

Format:

call far ptr label

 

Push the CS and IP of the next instruction into the stack and jump to the label

 

is equivalent to:

push cs

push ip

jmp far ptr

(3) Call instruction with the transfer address in a register

Format: call 16-bit reg

 

is equivalent to:

push ip

jmp 16-bit reg

(4) Call instruction with transfer address in memory

   1. call word ptr memory unit

        is equivalent to:

             push ip

             jmp word ptr memory unit

   2. call dword ptr memory unit

        is equivalent to:

              push cs

              push ip

              jmp dword ptr memory location

3. mul instruction

mul is the multiplication instruction

 

Indicates the multiplication of two numbers, which must both be 8 bits or both 16 bits

 

The 8-bit multiplication result is stored in ax by default.

The high bit of the 16-bit multiplication result is stored in dx, and the low bit is stored in ax

See examples below.

3. Use call and ret together

Call combined with ret is equivalent to a function.

 

Example: Calculate the cube of the value in dw. Treat bx as the "function" parameter and ax as the return value of the "function".

The number of registers is limited. If there are too many parameters to be passed or returned, you can use memory or the stack.

Example: Convert lowercase to uppercase. (Use memory to store parameters)

Example: Calculate (a - b) ^3 Assume a=3, b=1 (Use stack to store parameters)

 

In the above code, ret 4 means:

pop ip

add sp,n

Example: Convert lowercase to uppercase, using 0 as the end. (Use stack to handle register conflicts)

Note: Use the stack to save cx

Example: Implement the show_str "function" to display a string on the screen. Use dh to specify the function, dl to specify the column number, and cl to specify the color.