a20.asm

是关于保护模式与实模式之间的相互切换,是用汇编写的源程序

OPEN_A20    PROC
            PUSH  AX
            IN    AL,92H
            OR    AL,2
            OUT   92H,AL
            POP   AX
            RET
OPEN_A20    ENDP
CLOSE_A20   PROC
            PUSH AX
            IN  AL,92H
            AND AL,0FDH
            OUT 92H,AL
            POP AX
            RET
CLOSE_A20   ENDP