Assembler

Table of Contents

Hack language specification

A-instruction

Symbolic syntax

@value

Where value is either

  • a non-negative decimal constant
  • a symbol referring to such a constant

Example

@21, @foo

Binary syntax

0valueInBinary
Example:
0000000000010101

C-instruction

Symbolic syntax

dest = comp ; jump

Binary syntax

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

Symbols(Pre-defined symbols)

Label declaration: (label)
Variable declaration: @variableName

The Hack language specification describes 23 pre-defined symbols

Symbol Value Symbol Value
R0 0 SP 0
R1 1 LCL 1
R2 2 ARG 2
THIS 3
R15 15 THAT 4
SCREEN 16384
KBD 24576

A translator’s perspective

Assembly program elements:

White space

  • q Empty lines / indentation
  • Linecomments
  • In-line comments

Instructions

  • A-instructions
  • C-instructions

Symbols

  • References
  • Label declarations

The assembly process: instructions

For each instruction

  1. Parse the instruction: break it into its underlying fields
  2. A-instruction: translate the decimal value into a binary value
  3. C-instruction: for each field in the instruction, generate the corresponding binary code;
  4. Assemble the translated binary codes into a complete 16-bit machine instruction
  5. Write the 16-bit instruction to the output file.

The assembly process: symbols

  1. Translating @preDefinedSymbol: Replace preDefinedSymbol with its value.
  2. Label symbols
    • Used to label destinations of goto commands
    • Declared by the pseudo-command (XXX)
    • This directive defines the symbol XXX to refer to the memory location holding the next instruction in the program
    • Translating @labelSymbol : Replace _labelSymbol_ with its value
LOOP 4
STOP 18
END 22
  1. Variable symbols
    • Any symbol XXX appearing in an assembly program which is not predefined and is not defined elsewhere using the (XXX) directive is treated as a variable
    • Each variable is assigned a unique memory address, starting at 16
    • Translating @variableSymbol :
      • If seen for the first time, assign a unique memory address
      • Replace _variableSymbol_ with this address
i 16
sum 17

Symbol table

To resolve a symbol, look up its value in the symbol table

The assembly process

  1. Initialization
  • Construct an empty symbol table
  • Add the pre-defined symbols to the symbol table
  1. First pass
  • Scan the entire program;
  • For each instruction of the form (xxx):
    • Add the pair (xxx, address) to the symbol table, where address is the number of the instruction following (xxx)
  1. Second pass
  • Set n to 16
  • Scan the entire program again, for each instruction:
    • If the instruction is @symbol, look up symbol in the symbol table;
      • If (symbol, value) is found, use value to complete the instruction’s translation;
      • If not found:
        • Add (symbol, n) to the symbol table,
        • Use n to complete the instruction’s translation,
        • n++
    • If the instruction is a C-instruction, complete the instruction’s translation
    • Write the translated instruction to the output file.
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