第6章 程序控制指令－上课.ppt

CHAPTER 6 Program Control Instructions(程序控制指令)(P. 183) 6-1 THE JUMP GROUP Unconditional Jump Conditional Jumps LOOP 6-1-1 Unconditional Jump(无条件转移指令)(P. 184) The unconditional jump instruction, JMP, allows the programmer to skip sections of a program and branch to any part of the memory for the next instruction. Three types of unconditional jump instructions are available to the microprocessor: Short jump Near jump Far jump Jumps with Register Operands Indirect jumps Using an Index Short jump (短转移)(P. 184) Format JMP SHORT LABEL Machine language Note: The short jump displacement is a distance represented by a one-byte signed number whose value ranges between +127 and –128. The short jump is an intrasegment jump that allows branches to memory locations within +127 and –128 bytes from the address following the jump. Short jump is relative program memory addressing. 0000 33 DB XOR BX,BX 0002 B8 0001 START: MOV AX,1 0005 03 C3 ADD AX,BX 0007 EB 17 JMP SHORT NEXT 0020 8B D8 NEXT: MOV BX,AX 0022 EB DE JMP SHORT START Near jump (近转移)(P. 185) Format JMP (NEAR PTR) LABEL Machine language Note: The near jump displacement is a distance represented by a signed 16-bit number whose value ranges between +32767 and –32768. A near jump is an intrasegment jump that can jump to any memory location within the current real mode code segment. Short jump is relative program memory addressing. 0000 33 DB XOR BX,BX 0002 B8 0001 START: MOV AX,1 0005 03 C3 ADD AX,BX 0007 E9 F601 JMP NEXT 0200 8B D8 NEXT: MOV BX,AX 0202 E9 FDFD JMP START Far jump (远转移)(P. 186) Format JMP FAR PTR LABEL Machine language A far jump instruction obtains a new segment and offset address to accomplish the jump. A far jump is an intersegment jump that accesses any location within the first 1M byte of memory in the real mode. A far jump is direct program memory addressing. EXTRN UP:FAR 0000 33 DB XOR BX,BX 0002