Assembly language - Wang Shuang Chapter 9 Principles of transfer instructions - Notes

assume cs:codesg
codesg segment

	start:	mov ax, 2000H
			mov ds, ax
			mov bx, 0

	s:		mov cl, [bx]
			mov ch, 0
			inc cx
			inc bx
			loop s
	ok:		dec bx
			mov dx, bx
			mov ax, 4c00h
			int 21h

codesg ends

From the code point of view , It should be analyzed in this way , First of all, the entry of the program is start Where to point , That is to say mov ax,0 here , Instead of the first two statements to exit the program .

Then two nop, Each account 1 Bytes .

then mov di,offset s This sentence is to di Register transfer s The offset address of ;

then mov si,offset s2, This is the Wang register si Pass on s2 The offset address of

then mov ax,cs：[si], therefore ax Save the execution s2 Pointer address of .

mov cs：[di] ,ax And then put s2 The execution address of is given to cs：[di] This memory location .

Here's the main point s2：jmp short s1, From the perspective of effect , The next step is to execute s1 The code for segment . And machine instructions EBF6, Will be copied to cs：[di] Memory here . and EBF6 The effect is actually to jump back 10 Bytes （ s1 here mov ax,0 Occupy 3 Bytes ,int 21h Occupy 2 Bytes , mov ax,0 Occupy 3 Bytes , This is the 8 Bytes . and s2 Of jmp short s1 It's up to you 2 Bytes . This sentence is finished , Go back 10 Bytes , just s1 The location of ）

So when EBF6 Copy to cs：[di] after , amount to cs：[di] Actually saved s The execution address of , So it's to put EBF6 This machine instruction is copied to s Of nop nop here . and EBF6 The actual meaning of "jump back" is to jump back 10 Bytes , Not literally jmp short s1 It means , That is, it will not jump down to execute s1, But jump back 10 Bytes

Calculate it like this EBF6 In itself 2 Bytes ,mov ax, 4c00h 3 Bytes , int 21h 2 Bytes , mov ax, 0 3 Bytes , All together 10 Bytes . So the execution is over s Section of the code , The pointer is there mov ax,4c00h here . So the program exits normally .
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assume cs:codesg ds:datasg

datasg segment
	
	db 'Welcome to masm!'
	db	2H, 24H, 71H						; String attribute value 

datasg ends

stack segment

	dw 8 dup (0)

stack ends


codesg segment

	start:	mov ax, stack
			mov ss, ax
			mov sp, 10h

			mov ax, datasg
			mov ds, ax
			mov bx, 0

			mov ax, 0B800H
			mov es, ax						;80×25 The color character mode shows the buffer section address input es
			
			mov di, 10H						; This offset is used for both string and color   
			mov cx 3

	s0:		push cx
			mov ah, ds:[di]
			push di

			mov di, 0
			mov si, 0
			mov cx, 16

	s:		mov al, ds:[di]
			mov es:[bx+si], al
			mov es:[bx+si+1], ah
			add si, 2
			inc di
			loop s

			pop di
			inc di
			pop cx
			add bxm 0a0h					; Line break , Each row 80 character ,160 Bytes 
			loop s0


	all:	jmp short all					; Cycle all the time , In order to see the effect 

			mov ax, 4c00h
			int 21h

codesg ends

end start