use strict;
use warnings;

while (<DATA>) {
    my ($hb, $lb) = split /\s+/;
    print "$hb $lb -> " . decode_d2b($hb, $lb) . "\n";
}

sub decode_d2b {
    my $hb = hex($_[0]);
    my $lb = hex($_[1]);

    if ($hb & 0x80) {
        return -( (~$hb & 255) + ((~$lb & 255) + 1)/256 );
    }
    else {
        return $hb + $lb/256;
    }
}

__DATA__
00 00 -> 0
00 01 -> 1/256
FF FF -> -1/256
FF F0 -> -1
80 00 -> -128
80 01 -> -127,996
7F FF -> 127,996