MiniMac is a virtual processor with a tiny but extendable memory and a tiny but extendable instruction set. The view panel on the right contains two lists $($javax$.$swing.JList$).$ The top list shows the contents of memory. The bottom list shows the program currently being executed. The control panel on the left contains three buttons $($javax$.$swing.JButton$).$ The first button prompts the user for a program to parse. The second button executes the program. The third button resets all memory cells to $0.$

These commands are duplicated under the Edit menu. The Help menu explains each command. The File menu contains the items: New, Save, Open, and Quit.

The MiniMax memory is an array of integers:

int size $=32$;

Integer$[]$ memory $=$ new Integer$[$size$]$;

A MiniMax program is a list of instructions. An instruction pointer $($ip$)$ indicates the position in the list of the next instruction to be executed.

Grammar

Here$$s the instruction set grammar $($note: $$~$$ means $$followed by$$ and all unquoted tokens are integers$)$:

load ::$=$load$$ ~ location ~ value $//$ memory$[$location$]=$ value

halt ::$=$halt$//$ terminates the program

add ::$=$add$$ ~ src$1$ ~ src$2$ ~ dest $//$ memory$[$dest$]=$ memory$[$src$1]+$ memory$[$src$2]$

mul ::$=$mul$$ ~ src$1$ ~ src$2$ ~ dest $//$ memory$[$dest$]=$ memory$[$src$1]*$ memory$[$src$2]$

bgt ::$=$bgt$$ ~ location ~ offset $//$ if $0<$ memory$[$location$]$ ip $+=$ offset

blt ::$=$blt$$ ~ location ~ offset $//$ if memory$[$location$]<0$ ip $+=$ offset

loop ::$=$loop ~ count ~ instruction $//$ executes instruction count times

A block is a list of one or more instructions separated by semicolons and bracketed by curly braces:

block ::$=\{$ ~ instruction ~ $($;$$ ~ instruction$)*$ ~ $\}$

Executing a block sequentially executes each instruction in the block$$s body.

For example, the program shown in the screenshot computes kn$,$ where n is stored in memory$[0].$ The base, k$,$ is stored in memory$[2].$ In this case k $=2,$ n was $7$ and the result, $128,$ is stored in memory$[1].$ Memory$[3]$ is the amount to decrement memory$[0]$ each time the result is multiplied by k$.$ Here$$s a partial implementation of MiniMacParser.java. Note that it is a utility class $($aka singleton$),$ all of its methods are static.

package minimac;

import java.util.$*$;

class ParserUtils $\{$

protected static Integer parseInteger$($Iterator it$)$ throws Exception $\{$

if $($it$.$hasNext$())\{$

return Integer.parseInt$($it$.$next$())$;

$\}$ else $\{$

throw new Exception$("$Parsing error, integer expected"$)$;

$\}$

$\}$

$\}$

public class MiniMacParser extends ParserUtils $\{$

static public List parse$($String program$)$ throws Exception $\{$

List result $=$ new ArrayList$()$;

$//$ split program into a list of tokens $($i$.$e$.$ numbers and operations$)$:

List tokens $=$ Arrays.asList$($program$.$split$("\backslash \backslash$s$*$;$\backslash \backslash$s$*|\backslash \backslash$s$+"))$;

Iterator it $=$ tokens.iterator$()$;

while$($it$.$hasNext$())\{$

result.add$($parseNext$($it$))$;

$\}$

return result;

$\}$

$//$ dispatcher method:

static public Instruction parseNext$($Iterator it$)$ throws Exception $\{$

String opcode $=$ it$.$next$()$;

if $($opcode$.$equalsIgnoreCase$("$halt$"))$ return new Halt$()$;

if $($opcode$.$equalsIgnoreCase$("$add$"))$ return parseAdd$($it$)$;

if $($opcode$.$equalsIgnoreCase$("$mul$"))$ return parseMul$($it$)$;

if $($opcode$.$equalsIgnoreCase$("$load$"))$ return parseLoad$($it$)$;

if $($opcode$.$equalsIgnoreCase$("$loop$"))$ return parseLoop$($it$)$;

if $($opcode$.$equalsIgnoreCase$("$Bgt$"))$ return parseBgt$($it$)$;

if $($opcode$.$equalsIgnoreCase$("$Blt$"))$ return parseBlt$($it$)$;

if $($opcode$.$equalsIgnoreCase$("\{"))$ return parseBlock$($it$)$;

if $($opcode$.$equalsIgnoreCase$("\}"))$ return null; $//$ used to indicate the end of a block

throw new Exception$("$Parsing error, unrecognized operator: $"+$ opcode$)$;

$\}$

static protected Add parseAdd$($Iterator it$)$ throws Exception $\{$

int src$1=$ parseInteger$($it$)$;

int src$2=$ parseInteger$($it$)$;

int dest $=$ parseInteger$($it$)$;

return new Add$($src$1,$ src$2,$ dest$)$;

$\}$

$//$ etc.

$\}$

Please finish the implementation code.