pgm6_7.asm

汇编编程艺术

; Unconditional Jumps

		.386
		option	segment:use16

dseg		segment	para public 'data'


; Pointers to statements in the code segment

IndPtr1		word	IndTarget2
IndPtr2		dword	IndTarget3



dseg		ends


cseg		segment	para public 'code'
		assume	cs:cseg, ds:dseg

Main		proc
		mov	ax, dseg
		mov	ds, ax
		mov	es, ax

; JMP instructions transfer control to the
; location specified in the operand field.
; This is typically a label that appears
; in the program.
;
; There are many variants of the JMP
; instruction.  The first is a two-byte
; opcode that transfers control to +/-128
; bytes around the current instruction:

		jmp	CloseLoc
		nop
CloseLoc:


; The next form is a three-byte instruction
; that allows you to jump anywhere within
; the current code segment.  Normally, the
; assembler would pick the shortest version
; of a given JMP instruction, the "near ptr"
; operand on the following instruction
; forces a near (three byte) JMP:


		jmp	near ptr NearLoc
		nop
NearLoc:


; The third form to consider is a five-byte
; instruction that provides a full segmented
; address operand.  This form of the JMP
; instruction lets you transfer control any-
; where in the program, even to another
; segment.  The "far ptr" operand forces
; this form of the JMP instruction:

		jmp	far ptr FarLoc
		nop
FarLoc:


; You can also load the target address of a
; near JMP into a register and jump indirectly
; to the target location.  Note that you can
; use any 80x86 general purpose register to
; hold this address; you are not limited to
; the BX, SI, DI, or BP registers.

		lea	dx, IndTarget
		jmp	dx
		nop
IndTarget:


; You can even jump indirect through a memory
; variable.  That is, you can jump though a
; pointer variable directly without having to
; first load the pointer variable into a reg-
; ister (Chapter Six describes why the following
; labels need two colons).

		jmp	IndPtr1
		nop
IndTarget2::


; You can even execute a far jump indirect
; through memory.  Just specify a dword
; variable in the operand field of a JMP
; instruction:

		jmp	IndPtr2
		nop
IndTarget3::



Quit:		mov	ah, 4ch
		int	21h
Main		endp

cseg		ends

sseg		segment	para stack 'stack'
stk		byte	1024 dup ("stack   ")
sseg		ends

zzzzzzseg	segment	para public 'zzzzzz'
LastBytes	byte	16 dup (?)
zzzzzzseg	ends
		end	Main