spartacus5329 / hack-assembler Goto Github PK

1. Instruction
   1. A instruction
   2. C instruction
2. White space -> ignore
3. Symbols
   1. A instructions
   2. Labels
   3. Predefined symbols

// example 1
@21

// example 2
@x

1. value is decimal constant
   1. Convert to 15-bit binary equivalent and leftPad a 0 to denote an A instruction
2. value is a symbol
   1. Discussed in symbol handling

// example
MD = D + 1

1. All C instructions start with 111
2. Next 7 bits correspond to comp
   1. Refer table for exact code
3. Next 3 bits correspond to dest
   1. Refer table for exact code
4. Next 3 bits correspond to dest
   1. Refer table for exact code

1. Add all the label symbols to the symbol table
2. Add all the variable symbols to the symbol table if they haven't been encountered before
3. To resolve a symbol, look it up from the symbol table

// example 1
@R0

// example 2
@KBD

Convert to corresponding value

// example 1
(LOOP)

// example 2
@STOP

1. (XXX) will refer to the next instruction in the program
2. @labelSymbol can just be replaced with its corresponding value

// example
@i

1. The variable gets mapped to an address in the RAM starting from 16 at first occurrence
2. If the variable has already been seen, it gets replaced by the corresponding value

1. Initialisation
   1. Construct an empty symbol table
   2. Add the pre-defined symbols to the symbol table

   3. 

2. First pass
   1. Scan the entire program
   2. For instruction of the form (XXX)
      1. Add the pair (XXX, address) to the symbol table, where address is the number of the instruction following (XXX)
3. Second pass
   1. Set n = 16
   2. Scan the entire program again
   3. For each instruction
      1. If the instruction is @symbol, look up the symbol in the symbol table
         1. If (symbol, value) is found, use value to complete the instruction's translation
         2. If not found
            1. Add (symbol, n) to the symbol table
            2. Use n to complete the instruction translation
            3. n++
      2. If the instruction is a C-instruction, complete the translation
      3. Write the instruction in the output file