Assembly Simulator

This project is a small simulator that combines BASIC like line numbers and assembly like instructions to help teach the basics of computing.

1. Installation
2. Usage
3. Architecture
4. Instruction Set
5. Documentation
6. Error Handling

Installation

npm install https://github.com/step/assembly_simulator.git

Usage

const Machine = require('assembly_simulator');
machine = new Machine();
machine.load('10 START\n20 PRN "HELLO"\n30 STOP');
machine.execute();
console.log(machine.getPrn().join("\n"));

For detailed instructions, see documentation

Architecture

The "Machine" has 4 registers A, B, C, D.

There are 4 flags that are set on the compare instruction.

1. EQ - this is set when two values are equal
2. NE - this is set when two values are not equal
3. GT - this is set when the value of the first argument is greater than the second one.
4. LT - this is set when the value of the first argument is lesser than the second one.

The "Machine" only recognises positive integers for now.

There is also a stack that you can PUSH and POP from.

Instruction Set

0. Comments

   Comments can be entered by starting a line with ;. The first non-space character has to be a ; for it to be considered a comment.

   Valid uses

   ; This is a comment
      ; this is also a comment

   Invalid uses

   10 START ; comments cannot be placed after an instruction
   this is also not a comment

1. START

   Starts execution and resets flags. Used at the start of a program, often as 10 START.

2. MOV

   Copies a value into a register. First argument has to be a register. Second argument can either be a register or a positive integer.

   Valid uses

   10 MOV A,2   ;; Copies 2 into A
   20 MOV B,A   ;; Copies A's value into B.

   Invalid uses

   10 MOV 2,2
   20 MOV 2,A

3. ADD

   Adds a value with a register and copies the new value into the register. First argument has to be a register. Second argument can either be a register or a positive integer.

   Valid uses

   10 ADD A,2   ;; Adds 2 to A and puts it back in A
   20 ADD B,A   ;; Adds A to B and puts it back in B

   Invalid uses

   10 ADD 2,2
   20 ADD 2,A

4. SUB

   Subtracts a value from a register and copies the new value into the register. First argument has to be a register. Second argument can either be a register or a positive integer.

   Valid uses

   10 SUB A,2   ;; Subtracts 2 from A and puts it back in A
   20 SUB B,A   ;; Subtracts A from B and puts it back in B

   Invalid uses

   10 SUB 2,2
   20 SUB 2,A

   Note that one can subtract a greater value from a lesser value and get a negative number even though one can't directly enter negative numbers.

5. CMP

   The compare instruction sets flags. Based on the arguments it sets one or more flags. The first argument has to be a register. The second argument can be either a positive integer or a register.

   CMP A,10 will set the EQ flag if A is 10.

   CMP A,10 will set the NE and GT flag if A is 12.

   CMP A,10 will set the NE and LT flag if A is 8.

   Valid uses

   10 CMP A, 2  ;; will set LT if A<2 or GT if A>2
   20 CMP A, B  ;; will set LT if A<B or GT if A>B

   Invalid uses

   10 CMP 2,2
   20 CMP 2,A

6. JMP

   The JMP instruction unconditionally continues execution from the line specified. JMP and all other jump instructions leave the registers and flags unmodified. JMP takes one argument and that argument has to be a line number.

   Valid uses

   70 JMP 80
   70 JMP 60

   Invalid uses

   10 JMP A
   20 JMP "10"

7. JE

   JE is a conditional jump instruction that only jumps to a specified line if the EQ flag is set. If EQ is not set, then the execution continues normally.

   Valid uses

   10 JE 50
   20 JE 10

   Invalid uses

   10 JE A
   20 JE "10"

8. JNE

   JNE is a conditional jump instruction that only jumps to a specified line if the NE flag is set. If NE is not set, then the execution continues normally.

   Valid uses

   10 JNE 50
   20 JNE 10

   Invalid uses

   10 JNE A
   20 JNE "10"

9. JGT

   JGT is a conditional jump instruction that only jumps to a specified line if the GT flag is set. If GT is not set, then the execution continues normally.

   Valid uses

   10 JGT 50
   20 JGT 10

   Invalid uses

   10 JGT A
   20 JGT "10"

10. JGE

JGE is a conditional jump instruction that only jumps to a specified line if the GT or EQ flags are set. If GT or EQ are not set, then the execution continues normally.

Valid uses

10 JGE 50
20 JGE 10

Invalid uses

10 JGE A
20 JGE "10"

11. JLT

JLT is a conditional jump instruction that only jumps to a specified line if the LT flag is set. If LT is not set, then the execution continues normally.

Valid uses

10 JLT 50
20 JLT 10

Invalid uses

10 JLT A
20 JLT "10"

12. JLE

JLE is a conditional jump instruction that only jumps to a specified line if the LT or EQ flags are set. If LT or EQ are not set, then the execution continues normally.

Valid uses

10 JLE 50
20 JLE 10

Invalid uses

10 JLE A
20 JLE "10"

13. STOP

The STOP instruction halts execution entirely. It does not take an argument.

Valid uses

100 STOP

Invalid uses

100 STOP 10
100 STOP A

14. PUSH

The PUSH instruction pushes the value of a register onto the stack. It takes exactly one argument that has to be a register.

Valid uses

10 PUSH A
20 PUSH B

Invalid uses

10 PUSH
20 PUSH 20
30 PUSH A,B

15. POP

The POP instruction pops the stack into the register specified. It takes exactly one argument that has to be a register.

Valid uses

10 POP A
20 POP B

Invalid uses

10 POP
20 POP 20
30 POP A,B

16. FUNC

The FUNC instruction is a special instruction. It has no effect on execution and leaves flags and registers unchanged. It is used to indicate the beginning of a function. It takes one argument that has to be a valid function name.

A valid function name begins with an alphabet but can consequently contain alphabets or numbers.

Function names are case insensitive. So, mul is the same as MUL.

The RET instruction is used to return to the point of execution from where the previous call was invoked.

Valid uses

10 FUNC MUL
20 ADD A,A
30 RET

Invalid uses

10 FUNC 1MUL
20 FUNC "ABCD"
30 FUNC A,B

17. CALL

The CALL instruction calls the given function. It takes exactly one argument which has to be a valid function name.

A valid function name begins with an alphabet but can consequently contain alphabets or numbers.

Valid uses

70 CALL MUL
80 CALL GREATEST

Invalid uses

70 CALL 
80 CALL 1MUL

18. RET

The RET instruction is used to return to the point of execution from where CALL was last called from. RET is usually the last instruction executed in a function. The instruction accepts no arguments.

Valid uses

30 RET

Invalid uses

30 RET A
40 RET 20
50 RET "ABC"

Documentation

The simulator can be accessed and controlled using the Machine object. The machine object is instantiated as

const machine = new Machine();

To load a program into the machine, use the load method.

let program = fs.readFileSync("add.asm","utf8");
machine.load(program);

To execute a program in the machine, use the execute method.

let program = fs.readFileSync("add.asm","utf8");
machine.load(program);
machine.execute();

Once the machine has finished execution, you can get all the output that has been generated using the PRN instruction by using the method getPrn.

// code to load and execute here
let lines = machine.getPrn();
console.log(lines.join("\n"));

getPrn returns an array of lines. This is preferred instead of giving a string as one may wish to use this library either on the command line or on the frontend where having separate lines available makes it easy to format/render

You can also get the trace table of the execution by using the getTable method

// code to load and execute here
let table = machine.getTable();
table.forEach(({CL,NL,A,B,C,D,EQ,NE,GT,LT,SL,INST,PRN})=>{
  console.log([CL,NL,SL,INST,A,B,C,D,EQ,NE,GT,LT,PRN],join(", "));
})

This table is very useful to help debug and trace execution. Each row of the table consists of:

1. The registers A, B, C and D

2. The flags EQ,NE,GT and LT

3. Print output from the PRN instruction.

4. The line number from the source file or program that was executed in SL

5. The actual instruction being executed in INST

6. The stack as an array contained in STK

Stepwise Execution

The Machine has the capability of executing stepwise. This feature allows one to 'step through' the program.

const machine = new Machine();
let program = fs.readFileSync("add.asm","utf8");
let callBack = (state) => {
  let { A, B, C, D } = state;
  let { EQ, NE, GT, LT } = state;
  let { PRN } = state;
  let { CL, NL } = state;
  let { SL, INST } = state;
  let { STK } = state;
  console.log(`A : ${A}, B : ${B}, C : ${C}, D : ${D}`);
  console.log(`EQ : ${EQ}, NE : ${NE}, GT : ${GT}, LT : ${LT}`);
  console.log(`CL : ${CL}, NL : ${NL}`);
  console.log(`SL : ${SL}, INST : ${INST}`);
  console.log(`INST : ${INST}`);
  console.log(`STK : ${STK.join(" ")}`);
} 
let executor;
try {
  machine.load(program);
  executor = machine.executeStepWise();
} catch(e) {
  console.log("Error on ", e.lineNumber, e.instruction);
}

try {
  machine.nextStep();
  machine.nextStep();
} catch(e) {
  // do whatever with the error
}

As shown above, in order to use it, you are required to pass a callback. The callback will be called with the same fields that getTable fetches. The important thing to note here is that once the machine executes the program completely, the callback will no longer be called no matter how many times you call nextStep.

This manner of execution is very useful to debug things like infinite loops.

Error Handling

All errors currently are encapsulated by the InvalidInstructionException class. All exceptions have two pieces of information currently.

1. Source line number where the exception occurred in lineNumber
2. The offending instruction in instruction

Always surround your machine's load and execute in try catch blocks.

try {
  machine.load(program);
  machine.execute();
} catch(e) {
  console.log("Error on ", e.lineNumber, e.instruction);
}