PHP - 1008

<?php
$_=trim(fread(STDIN,1e5));$z=$p=0;$t=$l=[];$q='';function s(){global$p,$t,$l,$_,$q,$z;switch($_[$z++]){case'>':$p++;break;case'<':$p--;break;case'+':@$t[$p]++;break;case'-':@$t[$p]--;break;case'.':$q.=chr($t[$p]);break;case',':echo"I>";$t[$p]=ord(trim(fread(STDIN,2)));break;case'[':$l[]=$z;if(!isset($t[$p])||$t[$p]==0){$c=1;while($c>0){$b=$_[$z++];if($b=='[')$c++;elseif($b==']')$c--;}array_pop($l);}break;case']':$z=array_pop($l)-1;break;}if($z==strlen($_)){if($q)echo'O>'.$q."\n";die("Done\n");}}for(;;){echo">";$c=trim(fread(STDIN,1e5));$g=strlen($c);switch($c[0]){case'L':echo str_repeat(' ',$z)."v\n".$_."\n";break;case'D':if($g)$x=$t;else{list($a,$b)=explode(' ',substr($c,1));$x=array_slice($t,$a-1,$b);}echo implode(', ',$x)."\n";break;case'S':if($g)s();for($i=0;$i<substr($c,1);$i++)s();echo $z."\n";break;case'G':if($c[1]=='-')$x=strlen($_)-substr($c,2);else$x=substr($c,1);while($z!=$x)s();break;case'P':echo$p."\n";default:if(is_numeric($c))echo$t[$c]."\n";}if($q)echo'O>'.$q."\n";$q='';}

The output for cells that were never written is slightly different:

[timwolla@/data/workspace/php/PCG]php pcg-22585.php
+
>D15 20

>

Large version:

<?php
$_=trim(fread(STDIN,1024));
$z=$p=0;
$t=$l=[];
$q='';

function step() {
    global $p, $t,$l,$_,$q,$z;
    switch ($_[$z++]){
        case '>': 
            $p++; 
        break;
        case '<': 
            $p--; 
        break;
        case '+': 
            @$t[$p]++; 
        break;
        case '-': 
            @$t[$p]--; 
        break;
        case '.': 
            $q.=chr($t[$p]); 
        break;
        case ',': 
            echo "I>";
            $t[$p] = ord(trim(fread(STDIN, 2)));
        break;
        case '[': 
            $l[] = $z;
            if (!isset($t[$p]) || $t[$p] == 0) { 
                $counter = 1; 
                while ($counter > 0) { 
                    $char = $_[$z++]; 
                    if ($char == '[') $counter++;
                    else if ($char == ']') $counter--;
                }
                array_pop($l);
            }
        break;
        case ']': 
            $z = array_pop($l)-1;
        break;
    }
    if($z==strlen($_)) {
        if ($q) echo 'O>'.$q."\n";
        die("Done\n");
    }
}

for(;;){
    echo ">";
    $c=trim(fread(STDIN,100));
    switch ($c[0]) {
        case 'L':
            echo str_repeat(' ', $z)."v\n".$_."\n";
        break;
        case 'D':
            if(strlen($c) == 1)$x=$t;
            else {
                list($a,$b)=explode(' ',substr($c,1));
                $x=array_slice($t,$a-1,$b);
            }
            echo implode(', ', $x)."\n";
        break;
        case 'S':
            if(strlen($c)==1)step();
            for($i=0;$i<substr($c,1);$i++)step();
            echo $z."\n";
        break;
        case 'G':
            if($c[1]=='-')$x=strlen($_)-substr($c,2);
            else $x=substr($c,1);
            while ($z!=$x)step();
        break;
        case 'P':
            echo $p."\n";
        default:
            if(is_numeric($c)) echo $t[$c]."\n";
    }
    if ($q) {
        echo 'O>'.$q."\n";
        $q='';
    }
}