Monty Language: Implementing Stacks & Queues

Micah OndiwaMicah Ondiwa
8 min read

Table of contents

Introduction

The Monty Language

The Monty 0.98 is a scripting language that is first compiled into Monty byte codes (Just like Python). It relies on a unique stack, with specific instructions to manipulate it. The goal of this project is to create an interpreter for Monty ByteCodes files.

A. Monty byte code files

Files containing Monty byte codes usually have the .m extension. Most of the industry uses this standard but it is not required by the specification of the language. There is not more than one instruction per line. There can be any number of spaces before or after the opcode and its argument.

Monty byte code files can contain blank lines (empty or made of spaces only, and any additional text after the opcode or its required argument is not taken into account.

B. The Monty program

  • Usage: monty file

    • where file is the path to the file containing Monty byte code.

Compilation

$ gcc -Wall -Werror -Wextra -pedantic *.c -o monty
$

Run

$ ./monty monty_file.m
5
7
1
2

Interpreter Synopsis

$ ./monty [montyfilename]
$

  • If the user does not give any file or more than one argument to your program, print the error message USAGE: monty file, followed by a new line, and exit with the status EXIT_FAILURE

  • If, for any reason, it’s not possible to open the file, print the error message Error: Can't open file <file>, followed by a new line, and exit with the status EXIT_FAILURE.

    • where <file> is the name of the file.

  • If the file contains an invalid instruction, print the error message L<line_number>: unknown instruction <opcode>, followed by a new line, and exit with the status EXIT_FAILURE

    • where is the line number where the instruction appears.

    • Line numbers always start at 1

  • The monty program runs the bytecodes line by line and stop if either:

    • it executed properly every line of the file

    • it finds an error in the file

    • an error occured

  • If you can’t malloc anymore, the program prints the error message Error: malloc failed, followed by a new line, and exit with status EXIT_FAILURE.

C. Opcodes

  1. pint
  • The opcode pint prints the value at the top of the stack, followed by a new line.

Usage: pint

  • If the stack is empty, print the error message L<line_number>: can't pint, stack empty, followed by a new line, and exit with the status EXIT_FAILURE

  • Example:

    micahondiwa@ubuntu:~/monty$ cat bytecodes/06.m 
push 1
pint
push 2 
pint 
push 3 
pint 
micahondiwa@ubuntu:~/monty$ ./monty bytecodes/06.m 
1 
2 
3 
micahondiwa@ubuntu:~/monty$

    2. pop

  • The opcode pop removes the top element of the stack.

Usage: pop

  • If the stack is empty, print the error message L<line_number>: can't pop an empty stack, followed by a new line, and exit with the status EXIT_FAILURE

  • Example:

micahondiwa@ubuntu:~/monty$ cat bytecodes/07.m 
push 1
push 2
push 3
pall
pop
pall
pop
pall
pop
pall
micahondiwa@ubuntu:~/monty$ ./monty bytecodes/07.m 
3
2
1
2
1
1
micahondiwa@ubuntu:~/monty$
  1. swap
  • The opcode swap swaps the top two elements of the stack.

Usage: swap

  • If the stack contains less than two elements, print the error message L<line_number>: can't swap, stack too short, followed by a new line, and exit with the status EXIT_FAILURE -Example:
micahondiwa@ubuntu:~/monty$ cat bytecodes/09.m 
push 1
push 2
push 3
pall
swap
pall
micahondiwa@ubuntu:~/monty$ ./monty bytecodes/09.m 
3
2
1
2
3
1
micahondiwa@ubuntu:~/monty$
  1. add
  • The opcode add adds the top two elements of the stack.

Usage: add

  • If the stack contains less than two elements, print the error message L<line_number>: can't add, stack too short, followed by a new line, and exit with the status EXIT_FAILURE -The result is stored in the second top element of the stack, and the top element is removed, so that at the end:

    • The top element of the stack contains the result

    • The stack is one element shorter

  • Example:

    micahondiwa@ubuntu:~/monty$ cat bytecodes/12.m 
push 1 
push 2 
push 3 
pall 
add 
pall micahondiwa@ubuntu:~/monty$ ./monty bytecodes/12.m 
3 
2 
1 
5 
1 
micahondiwa@ubuntu:~/monty$
  1. nop
  • The opcode nop doesn’t do anything!

Usage: nop

  1. sub
  • The opcode sub subtracts the top element of the stack from the second top element of the stack.

Usage: sub

  • If the stack contains less than two elements, print the error message L<line_number>: can't sub, stack too short, followed by a new line, and exit with the status EXIT_FAILURE

  • The result is stored in the second top element of the stack, and the top element is removed, so that at the end:

    • The top element of the stack contains the result

    • The stack is one element shorter

  • Example:

micahondiwa@ubuntu:~/monty$ cat bytecodes/19.m 
push 1 
push 2 
push 10 
push 3 
sub 
pall 
micahondiwa@ubuntu:~/monty$ ./monty bytecodes/19.m 
7 
2 
1
  1. div
  • The opcode div divides the second top element of the stack by the top element of the stack.

Usage: div

  • If the stack contains less than two elements, print the error message L<line_number>: can't div, stack too short, followed by a new line, and exit with the status EXIT_FAILURE

  • The result is stored in the second top element of the stack, and the top element is removed, so that at the end:

    • The top element of the stack contains the result

    • The stack is one element shorter

  • If the top element of the stack is 0, print the error message L<line_number>: division by zero, followed by a new line, and exit with the status EXIT_FAILURE

  1. mul
  • The opcode ```mul`` multiplies the second top element of the stack with the top element of the stack.

Usage: mul

  • If the stack contains less than two elements, print the error message L<line_number>: can't mul, stack too short, followed by a new line, and exit with the status EXIT_FAILURE.

  • The result is stored in the second top element of the stack, and the top element is removed, so that at the end:

    • The top element of the stack contains the result

    • The stack is one element shorter

  1. mod
  • The opcode mod computes the rest of the division of the second top element of the stack by the top element of the stack.

Usage: mod

  • If the stack contains less than two elements, print the error message L<line_number>: can't mod, stack too short, followed by a new line, and exit with the status EXIT_FAILURE.

  • The result is stored in the second top element of the stack, and the top element is removed, so that at the end:

    • The top element of the stack contains the result

    • The stack is one element shorter

  • If the top element of the stack is 0, print the error message L<line_number>: division by zero, followed by a new line, and exit with the status EXIT_FAILURE

  1. pchar

  • The opcode pchar prints the char at the top of the stack, followed by a new line.

  • Usage: pchar

  • The integer stored at the top of the stack is treated as the ascii value of the character to be printed

  • If the value is not in the ascii table (man ascii) print the error message L<line_number>: can't pchar, value out of range, followed by a new line, and exit with the status EXIT_FAILURE

  • If the stack is empty, print the error message L<line_number>: can't pchar, stack empty, followed by a new line, and exit with the status EXIT_FAILURE

  • Example:

    micahondiwa@ubuntu:~/monty$ cat bytecodes/28.m 
push 72 
pchar 
micahondiwa@ubuntu:~/monty$ ./monty bytecodes/28.m 
H 
micahondiwa@ubuntu:~/monty$
  1. pstr
  • The opcode pstr prints the string starting at the top of the stack, followed by a new line.

Usage: pstr

  • The integer stored in each element of the stack is treated as the ascii value of the character to be printed

  • The string stops when either:

    • the stack is over

    • the value of the element is 0

    • the value of the element is not in the ascii table

  • If the stack is empty, print only a new line

  • Example:

micahondiwa@ubuntu:~/monty$ cat bytecodes/31.m 
push 1 
push 2 
push 3 
push 4 
push 0 
push 110 
push 0 
push 108 
push 111 
push 111 
push 104 
push 99 
push 83 
pstr 
micahondiwa@ubuntu:~/monty$ ./monty bytecodes/31.m 
School 
micahondiwa@ubuntu:~/monty$
  1. rotl
  • The opcode rotl rotates the stack to the top.

Usage: rotl

  • The top element of the stack becomes the last one, and the second top element of the stack becomes the first one rotl never fails

  • Example:

micahondiwa@ubuntu:~/monty$ cat bytecodes/35.m 
push 1 
push 2 
push 3 
push 4 
push 5 
push 6 
push 7 
push 8 
push 9 
push 0 
pall 
rotl 
pall 
micahondiwa@ubuntu:~/monty$ ./monty bytecodes/35.m 
0 
9 
8 
7 
6 
5 
4 
3 
2 
1 
9 
8 
7 
6 
5 
4 
3 
2 
1 
0 
micahondiwa@ubuntu:~/monty$
  1. rotr
  • The opcode rotr rotates the stack to the bottom.

Usage: rotr

  • The last element of the stack becomes the top element of the stack rotr never fails
  1. stack, queue

The stack opcode

  • The opcode stack sets the format of the data to a stack (LIFO). This is the default behavior of the program.

  • Usage: ```stack``

The queue opcode

  • The opcode queue sets the format of the data to a queue (FIFO).

  • Usage: queue

When switching mode:

  • The top of the stack becomes the front of the queue

  • The front of the queue becomes the top of the stack

-Example:

micahondiwa@ubuntu:~/monty$ cat bytecodes/47.m 
queue 
push 1 
push 2 
push 3 
pall 
stack 
push 4 
push 5 
push 6 
pall 
add 
pall 
queue 
push 11111 
add 
pall 
micahondiwa@ubuntu:~/monty$ ./monty bytecodes/47.m 
1 
2 
3 
6 
5 
4 
1 
2 
3 
11 
4 
1 
2 
3 
15 
1 
2 
3 
11111 
micahondiwa@ubuntu:~/monty$

Conclusion

The

Source Code: https://github.com/micahondiwa/monty

3
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monty

Written by

Micah Ondiwa
Micah Ondiwa

Software Engineer at IBM Research | Africa.