Parsing binary data in PowerShell

Question

I have read the contents of a binary file as an array of unsigned bytes but I want to interpret some of them as signed bytes. How do I do that?

#An array of unsigned bytes
[byte[]]$b = @(255)
#This doesn't work
[sbyte]$s = $b[0]
#Neither does this
[sbyte]$s = [convert]::ToSByte($b[0])

What I wanted was the value -1.

function AsSignedByte { param([Byte]$Byte) [sbyte]$(if($Byte -ge 128) { $Byte - 256 } else { $Byte }) } — user4003407
– user4003407, Commented Jan 3, 2018 at 23:01
Casting to an invalid value is, well, not valid (hence the error). The value is not in range. If you really want to do it, you can work around it as @PetSerAl notes. — Bill_Stewart
– Bill_Stewart, Commented Jan 3, 2018 at 23:03

JosefZ · Accepted Answer · 2018-01-04 02:24:34Z

2

The \[System.Convert\]::ToSByte(value) method raises an OverflowException if value is greater than [sbyte]::MaxValue.

Return to theory (Converting from two's complement representation):

$mask = [byte]  0x80    # [math]::pow(2,7)
$b    = [byte]  255
$s    = [sbyte] ( -($b -band $mask) + ($b -band ( -bnot $mask )) )

'result: {0,4}, {1,4} ( = 0x{0:x2}, 0x{1:x2} )' -f $b, $s

Output:

result:  255,   -1 (= 0xff, 0xff )

Of course, you could simplify the calculation as

$s    = [sbyte] ( -($b -band 0x80) + ($b -band 0x7F) )

Edit ~~to refute PetSerAl's doubts about [sbyte] -($b -band $mask) + ($b -band ( -bnot $mask )) formula correctness~~. PetSerAl's doubts are right: results in Powershell-ISE differ from those in Powershell! The formula should be (see labelled parentheses)

#       ↓                                                ↓
[sbyte] ( -($b -band $mask) + ($b -band ( -bnot $mask )) )
#       ↑                                                ↑

Proof:

function AsSignedByte {     ### taken from PetSerAl's comment to the OQ
    param([Byte]$Byte) 
    [sbyte]$( if($Byte -ge 128) { $Byte - 256 } else { $Byte }) 
}

$mask = [byte]  0x80    # [math]::pow(2,7)
$good = 0
For ($i  = 0 ; $i -lt 256; $i++) {
    $b    = [byte]  $i
    $s    = [sbyte] ( -($b -band $mask) + ($b -band ( -bnot $mask )) )
    $ss   = AsSignedByte $b 
    if ( ($s -ne $ss) -or 
         ($s.GetTypeCode() -ne $ss.GetTypeCode()) -or 
         ($s -isnot [sbyte])) {
        'result: {0,4}, {1,4} ( = 0x{0:x2}, 0x{1:x2} )' -f $s, $ss
    } else { 
        $good +=1
    }
}
$good

Output:

PS D:\PShell> D:\PShell\SO\48085510.ps1
256

PS D:\PShell>

edited Jan 4, 2018 at 2:24

answered Jan 3, 2018 at 23:44

JosefZ

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8 Comments

user4003407 Over a year ago

I think ([sbyte] -($b -band $mask)) + ($b -band ( -bnot $mask )) is not right.

JosefZ Over a year ago

@PetSerAl yes, your formula with added parentheses is not right :)

user4003407 Over a year ago

But them added according to PowerShell precedence rules to match your formula.

JosefZ Over a year ago

And what about my post edit (formula unchanged)? Maybe you think [sbyte] (-($b -band $mask) + ($b -band ( -bnot $mask )))?

user4003407 Over a year ago

Change if ( $s -ne $ss ) { to if ( ($s -isnot [sbyte]) -or ($s -ne $ss) ) {

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