VB程序中实现字节移位操作




 

 

在利用VB系统开发数据采集或工业控制软件，或对文件进行低级操作时，常需要对字节进行移位操作，但VB系统中没有提供字节移位操作的指令和函数，只提供了And（与）、Or（或）、Xor（异或）、Equ（同或）、Not（非）等几个逻辑运算指令。笔者在用VB系统开发工业控制软件的过程中，就遇到了这个问题，于是利用VB中已有的逻辑运算指令，模拟汇编语言的字节移位指令，编制了七个字节移位函数：逻辑左移、逻辑右移、算术右移、循环左移、循环右移、进位循环左移和进位循环右移。 

 

在汇编语言指令中，逻辑左移的功能相当于乘2, 逻辑右移的功能相当于除2，利用这个特性，在VB程序中用乘2和除2方法实现位的左移和右移，然后再用And（与）和Or（或）逻辑运算指令，判断移位过程中是否有进位发生，将进行标志置位。 

 

程序清单 

CF是进位标志，采用Boolean类型逻辑变量，如果CF为True表示有进位，为False则表示无进位。 

 

Public CF As Boolean　　 进位标志 

 

1.逻辑左移 

 

Public Function SHL(OPR As Byte, n As Integer) As Byte

Dim BD As Byte

Dim I As Integer

BD = OPR

For i = 1 To n - 1

BD = (BD And &H7F) * 2 将D7位屏蔽左移，防止字节溢出

Next i

CF = BD And &H80 判断D7位是否进位

SHL = (BD And &H7F) * 2

End Function

 

2.逻辑右移 

 

Public Function SHR(OPR As Byte, n As Integer) As Byte

Dim BD As Byte

Dim I As Integer

BD = OPR

For i = 1 To n - 1

BD = BD  2 右移

Next i

CF = BD And 1 判断D0位是否进位

SHR = BD  2

End Function

 

3.算术右移 

 

Public Function SAR(OPR As Byte, n As Integer) As Byte

Dim BD As Byte

Dim I As Integer

Dim Fg1 As Byte

BD = OPR

Fg1 = BD And &H80

For i = 1 To n - 1

BD = BD  2 右移

Next i

CF = BD And 1 判断D0位是否进位

BD = BD  2 右移

SAR = BD Or Fg1

End Function

 

4.循环左移 

 

Public Function ROL(OPR As Byte, n As Integer) As Byte

Dim BD As Byte

Dim I As Integer

Dim Fg1 As Byte

BD = OPR

For i = 1 To n

Fg1 = (BD And &H80)  128 判断D7位是否进位

BD = ((BD And &H7F) * 2) Or Fg1 带进位左移

Next i

CF = Fg1

ROL = BD

End Function

 

5.循环右移 

 

Public Function ROR(OPR As Byte, n As Integer) As Byte

Dim BD As Byte

Dim I As Integer

Dim Fg1 As Byte

Dim Fg2 As Byte

BD = OPR

For i = 1 To n

Fg1 = (BD And 1) * 128 判断D0位是否进位

BD = (BD  2) Or Fg1 带进位右移

Next i

CF = Fg1

ROR = BD

End Function

 

6.进位循环左移 

 

Public Function RCL(OPR As Byte, n As Integer) As Byte

Dim BD As Byte

Dim I As Integer

Dim Fg1 As Byte

Dim Fg2 As Byte

BD = OPR

Fg2 = CF And 1

For i = 1 To n

Fg1 = (BD And &H80)  128 判断D7位是否进位

BD = ((BD And &H7F) * 2) Or Fg2 带进位左移

补充：软件开发 , Vb ,