Hack Assembler

About Assembler

Assembler is a program that acts as a simple language translator (convert assembly language to machine language)

assembler

Hack Assembler

program that translates programs written in the symbolic Hack assembly language into binary code.
input file: xyz.asm
output file: xyz.hack

Hack assembly language consist of two types of instruction

A-instruction: @value
A-instruction (addressing instruction) assigns value of const to register A i.e A <- const
ROM[A] register is selected (ROM : Read only memory act as instruction memory)
C-instruction: dest=comp;jump
C-instruction (compute instruction) does basic computation assign valuse to destination.
```
D = M+1 // where D and M are register
```
We can also perform branching based on value comp and opcode jump
```
D;JGT // if D > 0 jump to ROM[A]
```

How to build Hack Assembler

Hack assembler can be build using the idea of 2 pass assembler.
pre-processing :- handle all white space and ignore comments
1-pass :- iterate over all instructions and build up symbol table
2-pass :- using symbol table and instruction translate into binary code.

General approach

Preprocess by parse each line of .hack code ignoring white space (i.e. empty line, comments, identation)
can be done using file handling and processing in Python.
.hack code (input)
```
// Computes R1=1 + ... + R0
// i = 1
     @i
     M=1
...
```
symbolic code (output)
```
@i
M=1
...
```
Create a symbol table and add all 23 pre-defined hack symbols.

First pass
If the instruction contain a symbol (i.e could be pre-defined, label or variable) add it to symbol table if only variable . eg)
```
| symbol | value |
| -------| ------|
 ....
| LOOP   |   4   |
| STOP   |   18  |
|   i    |   16  |
```
Second pass

start from begining
Translate each instruction (if w/o any symbol) directly . Need to figure out what type of instruction it is A-type or C-type can be done by just checking whether inst. start with @ or not.

Then if A-TYPE:

 @16            =>  0000000000010000

and if C-type (find what is destn,comp and jump of inst. and decode acc.)

M=1             =>  1110111111001000

If the instruction contains symbols use the symbol table to parse it.
Keep writing output to the new file as we go through each instruction. #####

Resources

Nand2Tetris
Hack assembler guide
Wiki