Hack-Assembler

Part of nand2tetris project 6.

The Hack assembler is a software tool that comprises a series of five questions or steps. Each step is assigned a number between 1 and 5, and together they make up the entire assembler.

Questions

1. Write a program to read assembly program (.asm file) and print the same program to a new file by ignoring white space (_no_whitespace.asm).

2. Write a program to read the above generated no_whitespace.asm file and identify the label symbols and variables. Write those label symbols and variables to another file (symbolsandvariables.asm).

3. Write a program to construct a Symbol table for the given .asm file and print it to another file (symboltable.asm).

4. Write a program to convert a standalone A instruction into binary (generate .hack output file).

5. Write a program to convert a standalone C instruction into binary (generate .hack output file).

6. Combine all the above program and write an assembler program which convert a “.asm” file to binary hack file. [refer project 6 in the nand to tetris software)

Note : Question 6 is for Java; The project was intended to do in java or python. It was a two phase project and during the java part only I combined the all the programs and created a Hack Assembler.

Contents

• Description
• Intro to Hack Assembly
  • Predefined Symbols
  • Types of Instructions
    • A-INSTRUCTIONS
    • C-INSTRUCTIONS

Description

An assembler is a software program that converts simple computer instructions written in a human-readable programming language into a series of 1's and 0's, also known as binary code. This binary code is what a computer's processor can understand and use to execute the desired operations. The purpose of building an assembler for the Hack computer is to translate Hack Assembly code into its corresponding binary code, making it possible for the computer to execute the program.

Intro to Hack Assembly

The Hack Assembly Language is minimal, it mainly consists of 2 types of instructions. It ignores whitespace and allows programs to declare symbols with a single symbol declaration instruction. Symbols can either be labels or variables. It also allows the programmer to write comments in the source code, for example: // this is a single line comment.

Predefined Symbols

• A: Address Register.
• D: Data Register.
• M: Refers to the register in Main Memory whose address is currently stored in A.
• SP: RAM address 0.
• LCL: RAM address 1.
• ARG: RAM address 2.
• THIS: RAM address 3.
• THAT: RAM address 4.
• R0-R15: Addresses of 16 RAM Registers, mapped from 0 to 15.
• SCREEN: Base address of the Screen Map in Main Memory, which is equal to 16384.
• KBD: Keyboard Register address in Main Memory, which is equal to 24576.

Types of Instructions

1. A-Instruction: Addressing instructions.
2. C-Instruction: Computation instructions.

A-INSTRUCTIONS

Symbolic Syntax of an A-Instruction

@value, where value is either a decimal non-negative number or a Symbol.

Examples:

• @21
• @R0
• @SCREEN

Binary Syntax of an A-Instruction

0xxxxxxxxxxxxxxx, where x is a bit, either 0 or 1. A-Instructions always have their MSB set to 0.

Examples:

• 000000000001010
• 011111111111111

Effects of an A-Instruction

Sets the contents of the A register to the specified value. The value is either a non-negative number (i.e. 21) or a Symbol. If the value is a Symbol, then the contents of the A register is set to the value that the Symbol refers to but not the actual data in that Register or Memory Location.

Declaring Variables

Declaring variables is a straight forward A-Instruction, example:

@i
M=0

The instruction @i declares a variable "i", and the instruction M=0 sets the memory location of "i" in Main Memory to 0, the address "i" was automatically generated and stored in A Register by the instruction.

Declaring Labels

To declare a label we need to use the command (LABEL_NAME), where "LABEL_NAME" can be any name we desire to have for the label, as long as it's wraped between parentheses. For example:

(LOOP)
  // ...
  // instruction 1
  // instruction 2
  // instruction 3
  // ...
  @LOOP
  0;JMP

The instruction (LOOP) declares a new label called "LOOP", the assembler will resolve this label to the address of the next instruction (A or C instruction) on the following line.

The instruction @LOOP is a straight-forward A-Instruction that sets the contents of A Register to the instruction address the label refers to, whereas the 0;JMP instruction causes an unconditional jump to the address in A Register causing the program to execute the set of instructions between (LOOP) and 0;JMP infinitely.

C-INSTRUCTIONS

Symbolic Syntax of a C-Instruction

dest = comp ; jmp, where:

1. dest: Destination register in which the result of computation will be stored.
2. comp: Computation code.
3. jmp: The jump directive.

Examples:

• D=0
• M=1
• D=D+1;JMP
• M=M-D;JEQ

Binary Syntax of a C-Instruction

1 1 1 a c1 c2 c3 c4 c5 c6 d1 d2 d3 j1 j2 j3, where:

• 111 bits: C-Instructions always begin with bits 111.
• a bit: Chooses to load the contents of either A register or M (Main Memory register addressed by A) into the ALU for computation.
• Bits c1 through c6: Control bits expected by the ALU to perform arithmetic or bit-wise logic operations.
• Bits d1 through d3: Specify which memory location to store the result of ALU computation into: A, D or M.
• Bits j1 through j3: Specify which JUMP directive to execute (either conditional or uncoditional).

Effects of a C-Instruction

Performs a computation on the CPU (arithmetic or bit-wise logic) and stores it into a destination register or memory location, and then (optionally) JUMPS to an instruction memory location that is usually addressed by a value or a Symbol (label).