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VB程序中實現(xiàn)字節(jié)移位操作 VB程序中實現(xiàn)字節(jié)移位操作 按位左移 右移

在利用VB系統(tǒng)開發(fā)數(shù)據(jù)采集或工業(yè)控制軟件，或?qū)ξ募M行低級操作時，常需要對字節(jié)進行移位操作，但VB系統(tǒng)中沒有提供字節(jié)移位操作的指令和函數(shù)，只提供了And（與）、Or（或）、Xor（異或）、Equ（同或）、Not（非）等幾個邏輯運算指令。筆者在用VB系統(tǒng)開發(fā)工業(yè)控制軟件的過程中，就遇到了這個問題，于是利用VB中已有的邏輯運算指令，模擬匯編語言的字節(jié)移位指令，編制了七個字節(jié)移位函數(shù)：邏輯左移、邏輯右移、算術(shù)右移、循環(huán)左移、循環(huán)右移、進位循環(huán)左移和進位循環(huán)右移。


在匯編語言指令中，邏輯左移的功能相當于乘2, 邏輯右移的功能相當于除2，利用這個特性，在VB程序中用乘2和除2方法實現(xiàn)位的左移和右移，然后再用And（與）和Or（或）邏輯運算指令，判斷移位過程中是否有進位發(fā)生，將進行標志置位。 


程序清單

CF是進位標志，采用Boolean類型邏輯變量，如果CF為True表示有進位，為False則表示無進位。


[code]
Public CF As Boolean   '進位標志



'1.邏輯左移



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 '將D7位屏蔽左移，防止字節(jié)溢出

Next i

CF = BD And &H80 '判斷D7位是否進位

SHL = (BD And &H7F) * 2

End Function



'2.邏輯右移



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 '判斷D0位是否進位

SHR = BD \ 2

End Function



'3.算術(shù)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 '判斷D0位是否進位

BD = BD \ 2 '右移

SAR = BD Or Fg1

End Function



'4.循環(huán)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 '判斷D7位是否進位

BD = ((BD And &H7F) * 2) Or Fg1 '帶進位左移

Next i

CF = Fg1

ROL = BD

End Function



'5.循環(huán)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 '判斷D0位是否進位

BD = (BD \ 2) Or Fg1 '帶進位右移

Next i

CF = Fg1

ROR = BD

End Function



'6.進位循環(huán)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 '判斷D7位是否進位

BD = ((BD And &H7F) * 2) Or Fg2 '帶進位左移

Fg2 = Fg1

Next i

CF = Fg1

RCL = BD

End Function



'7.進位循環(huán)右移



Public Function RCR(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 128

For i = 1 To n

Fg1 = (BD And 1) * 128 '判斷D0位是否進位

BD = (BD \ 2) Or Fg2 '帶進位右移

Fg2 = Fg1

Next i

CF = Fg1

RCR = BD

End Function
[/code]


結(jié)束語

以上七個字節(jié)移位操作函數(shù)的功能和用法同宏匯編語言的移位操作指令基本相同，不過只對單字節(jié)進行操作，但上述程序稍加改動后，就可對雙字節(jié)Integer類型和四字節(jié)Long類型進行移位操作。