Assembly language Note call and RET instructions

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? Modular programming

1. Modular programming

   Assembly language implements modular programming through call and RET instructions. It is possible to implement multiple interconnected, functionally independent subroutines to solve a complex problem.

2. Sub-Program Framework

   1 Assume cs:code 2 code segment 3     main:     : 4               : 5               call Sub1          ; Calling subroutine Sub1 6               : 7               : 8               mov ax, 4c 00h 9               int 21h10     sub1:     :                  ; subroutine sub1 start               :          call sub2; Calling subroutine sub214               : +               ret                ; sub-program returned to     sub2:     :                  ; subroutine Sub2 start               : £               º               RET The                subroutine returns the code ENDS23 end main

? Call and RET directives

Instructions	Function	Compilation grammar explanation	Note
Ret	Use the data in the stack to modify the contents of the IP, achieve near-transfer.	Pop IP
Retf	Use the data in the stack to modify CS and IP content, Achieve far transfer.	Pop IP Pop CS
Retn		Pop IP Add SP, n	See the back.
Pager	The call instruction is divided into two steps: (1) Press the current IP or CS and IP into the stack. (2) Transfer.	Format Compilation grammar explanation Call label Push IP JMP near PTR label Call Far label Push CS Push IP JMP far PTR designator Call 16-bit Reg Push IP JMP 16-bit Reg Call word PTR memory cell address Push IP JMP Word PTR memory cell address Call DWORD PTR memory Unit address Push CS Push IP JMP DWORD PTR memory Unit address	Pager Means Make No Yes Real Is Short Turn Move

? mul and DIV commands

Instruction format	Explain	Example
MUL Reg Mul Memory Unit	(1) 8-bit multiplication: The default multiplier one is placed in Al and the other is placed in a 8-bit reg or memory byte cell. Default results are placed in Microsoft Dynamics AX (2) 16-bit multiplication: The default multiplier is placed in Microsoft Dynamics AX and the other is placed in a 16-bit reg or memory word cell. Default results are placed in dx| In Ax.	Mul byte ptr ds:[0] ; (ax) = (AL) * ((DS) *16+0) Mul word ptr [bx+si+8] ; (dx|ax) = (AX) * ((DS) *16+ (BX) + (SI) +8)
DIV Reg div Memory Unit	(1) 16-bit/8-bit: Dividend By default in AX, the divisor is placed in a 8-bit reg or memory byte cell. Results Al (quotient), AH (remainder). (2) 32-bit/16-bit: dividend default dx| In Ax, the divisor is placed in a 16-bit reg or memory word cell. Results ax (quotient), DX (remainder). (3) Cross-border issues (how to design a security div see back)	div byte ptr ds:[0] ; (AL) = (AX)/((DS) *16+0) ; (AH) = remainder of (AX)/((DS) *16+0) div word ptr [bx+si+8] ; (AX) = (AX)/((DS) *16+ (BX) + (SI) +8) ; (dx) = (AX)/((DS) *16+ (BX) + (SI) +8) remainder

? parameters and result passing

Position	Advantages	Disadvantages	Example
Put it in the register.	The most common method Fast	Limited number of registers, Register may be conflicting	; Description: Calculates the 3-th square of n ; Parameter: (BX) =n ; Results: (DX:AX) =n^3 Cube:mov AX, BX Mul BX Mul BX Ret
Put it in the data segment	Delivery of bulk data	Slow speed	; Ds:si points to the first address of the space where the string (bulk data) resides ; The length of the CX store string Capital:and byte ptr [si], 11011111b Inc si Loop capital Ret
Put it on the stack.	The way of C language Simple and convenient		Explain in detail

• attached 1: Use the stack pass parameter

  Combined with the C language function to learn the idea of stack transfer parameters.

   

   1; Description: Calculates (A-B) ^3,a, 2 for font data, and parameter: When entering subroutines, the top of the stack holds the IP, followed by a, B 3;              Result: (Dx:ax) = (A-B) ^3 4 difcube:push BP 5 mov bp, SP 6 mov ax, [bp+4]; the value of a in the stack is fed into AX 7 Sub Ax, [bp+6];              A-B 8 mov bp, ax 9 mul bp10 mul bp11 pop BP12 ret 4 ;    Popup parameter (modify the top pointer to the value before the call) 13 14; The meaning of the directive ret N: 15;    Pop ip16; The Add sp,n 

  uses the stack for parameter passing, so the caller presses the arguments into the stack when assembling the program, and the subroutine can use RET when returning The n instruction modifies the stack-top pointer to the value before the call.

• Attached 2: C language Processing method (local variable, also use stack)

  1 void Add (int, int, int); 2      3 void Main () 4 {5     int a = 1; 6     int b = 2; 7     int c = 0; 8     Add (A, B, c); 9     C + +;}11     ID Add (int a, int b, int c), {      + c = a + b;15}

Assembly language Note call and RET instructions