import java.io$.*$;

import java.util.$*$;

public class Assemble

$\{$

public static String pad$($int n$,$ int w$)$

$\{$

String zeros $=""$;

$//$ determine length of $\text{'}$n$\text{'}$

len $=$ String.valueof$($n$).$length$()$

use len to calculate #zeros

$//$ calculate number of leading $0\text{'}$s to pad with

for$($numzeros$,$ i$++)$

$\{$

zero$=$zero$+"0"$;

$\}$

return $($zero$+$n$)$

$//$ create and return string of leading $0\text{'}$s$,$ and $\text{'}$n$\text{'}$

$\}$

static void readSrc$($String fname$)$ throws IOException

$\{$

$//$ open BufferedReader FileIO.java

BufferedReader br $=$ new BufferedReader$($new FileReader$($fname$))$;

String buffer;

$//$for each line of assembly code:

$\{$

$//$ skip any lines of length zero or starting with $\text{'}$#$\text{'}$

while $(($buffer $=$ br$.$readLine$())!=$ null$)\{$

System.out.println$("["+$ buffer $+"]")$;

$//$ Call ST tokenize string class. $1$st token is operator, $2$nd is operand

$???=$ new ST$($buffer$)$

$//$ If operand is invalid, display error and abort.

$//$ If operator is invalid, display error and abort.

VAR$1=$ stok.next$()$;

VAR$2=$ stok.next$()$;

$!$VAR$2=$ parseInt...

print VAR$1$ and VAR$2$

if$($operator $==$ "HALT"$)$opcode $=0$

else if$($operator $==$ "ADD"$)$opcode$=1$

else if$($operator $==$ "SUB"$)$opcode$=3$

$...$

else print an error message and break$/$return$/$exit$...$

operator $=$ "MULT";

String operator$[]=\{"$HALT$",$ "ADD", "SUB", $"$MLT$",$ "DIV","ILOAD", "LOAD", "STOR", "READ"...

$//$Indexs: $012345678$

looping variable: i

to conver operator to numeric opcode:

opcode $=-1$

loop though all of the operator

$\{$

if we find it$,$ we assign opcode variable

$\}$

$//....$println$($pad$($opcode$,2)+$ pad$($ioperand$,2))$

pad opcode and operand

$\}$

$\}$

$//$ convert string operator to numeric form.

$//$ output opcode and operand, each padded to two characters.

$\}$

$\}$

public static void main$($String argv$[])$ throws IOException

$\{$

if $($argv$.$length $!=1)$

$\{$

System.out.println$("$usage: java Assemble INPUTFILE"$)$;

System.exit$(0)$;

$\}$

readSrc$($argv$[0])$;

$\}$

$\}$