class N65::Instruction

Represents a single 6502 Instruction

Constants

AddressingModes

Attributes

address[R]
arg[R]
boundry_add[R]
cycle[R]
description[R]
flags[R]
hex[R]
length[R]
mode[R]
op[R]

Public Class Methods

new(op, arg, mode, byte_selector = nil) click to toggle source

Create an instruction. Having the instruction op a downcased symbol is nice because that can later be used to index into our opcodes hash in OpCodes OpCodes contains the definitions of each OpCode

# File lib/n65/instruction.rb, line 172
def initialize(op, arg, mode, byte_selector = nil)

  ##  Lookup the definition of this opcode, otherwise it is an invalid instruction
  @byte_selector = byte_selector.nil? ? nil : byte_selector.to_sym
  fail(InvalidInstruction, "Bad Byte selector: #{byte_selector}") unless [:>, :<, nil].include?(@byte_selector)

  @op = op.downcase.to_sym
  definition = OpCodes[@op]
  fail(InvalidInstruction, op) if definition.nil?

  @arg = arg

  ##  Be sure the mode is an actually supported mode.
  @mode = mode.to_sym
  fail(InvalidAddressingMode, mode) unless AddressingModes.has_key?(@mode)

  if definition[@mode].nil?
    fail(InvalidInstruction, "#{op} cannot be used in #{mode} mode")
  end

  @description, @flags = definition.values_at(:description, :flags)
  @hex, @length, @cycles, @boundry_add = definition[@mode].values_at(:hex, :len, :cycles, :boundry_add)
end
parse(line) click to toggle source

Parse one line of assembly, returns nil if the line is ultimately empty of asm instructions Raises SyntaxError if the line is malformed in some way

# File lib/n65/instruction.rb, line 110
def self.parse(line)

  ##  Try to parse this line in each addressing mode
  AddressingModes.each do |mode, parse_info|

    ##  We have regexes that match each addressing mode
    match_data = parse_info[:regex].match(line)

    unless match_data.nil?
      ##  We must have a straight instruction without symbols, construct
      ##  an Instruction from the match_data, and return it
      _, op, arg_hex, arg_bin = match_data.to_a

      ##  Until I think of something better, it seems that the union regex
      ##  puts a hexidecimal argument in one capture, and a binary in the next
      ##  This is annoying, but still not as annoying as using Treetop to parse
      if arg_hex != nil
        return Instruction.new(op, arg_hex.to_i(16), mode)
      elsif arg_bin != nil
        return Instruction.new(op, arg_bin.to_i(2), mode)
      else
        return Instruction.new(op, nil, mode)
      end

    else
      ##  Can this addressing mode even use labels?
      unless parse_info[:regex_label].nil?

        ##  See if it does in fact have a symbolic argument
        match_data = parse_info[:regex_label].match(line)

        unless match_data.nil?
          ##  We have found an assembly instruction containing a symbolic
          ##  argument.  We can resolve this symbol later by looking at the
          ##  symbol table in the #exec method
          match_array = match_data.to_a

          ##  If we have a 4 element array, this means we matched something
          ##  like LDA #<label, which is a legal immediate one byte value
          ##  by taking the msb.  We need to make that distinction in the
          ##  Instruction, by passing an extra argument
          if match_array.size == 4
            _, op, byte_selector, arg = match_array
            return Instruction.new(op, arg, mode, byte_selector.to_sym)
          else
            _, op, arg = match_array
            return Instruction.new(op, arg, mode)
          end
        end
      end
    end
  end

  ##  We just don't recognize this line of asm, it must be a Syntax Error
  fail(SyntaxError, line)
end

Public Instance Methods

apply_byte_selector(byte_selector, value) click to toggle source

Apply a byte selector to an argument

# File lib/n65/instruction.rb, line 243
def apply_byte_selector(byte_selector, value)
  return value if byte_selector.nil?
  case byte_selector
  when :>
    high_byte(value)
  when :<
    low_byte(value)
  end
end
emit_bytes() click to toggle source

Emit bytes from asm structure

# File lib/n65/instruction.rb, line 256
def emit_bytes
  case @length
  when 1
    [@hex]
  when 2
    if zero_page_instruction? && @arg < 0 || @arg > 0xff
      fail(ArgumentTooLarge, "For #{@op} in #{@mode} mode, only 8-bit values are allowed")
    end
    [@hex, @arg]
  when 3
    [@hex] + break_16(@arg)
  else
    fail("Can't handle instructions > 3 bytes")
  end
end
exec(assembler) click to toggle source

Execute writes the emitted bytes to virtual memory, and updates PC If there is a symbolic argument, we can try to resolve it now, or promise to resolve it later.

# File lib/n65/instruction.rb, line 208
def exec(assembler)

  promise = assembler.with_saved_state do |saved_assembler|
    @arg = saved_assembler.symbol_table.resolve_symbol(@arg)

    ##  If the instruction uses a byte selector, we need to apply that.
    @arg = apply_byte_selector(@byte_selector, @arg)

    ##  If the instruction is relative we need to work out how far away it is
    @arg = @arg - saved_assembler.program_counter - 2 if @mode == :relative

    saved_assembler.write_memory(emit_bytes)
  end

  case @arg
  when Fixnum, NilClass
    assembler.write_memory(emit_bytes)
  when String
    begin
      ##  This is a bug, I don't believe it will ever get here.
      ##  I think it always resolves every symbol later.
      promise.call
    rescue SymbolTable::UndefinedSymbol
      placeholder = [@hex, 0xDE, 0xAD][0...@length]
      ##  I still have to write a placeholder instruction of the right
      ##  length.  The promise will come back and resolve the address.
      assembler.write_memory(placeholder)
      return promise
    end
  end
end
to_s() click to toggle source

Pretty Print

# File lib/n65/instruction.rb, line 275
def to_s
  #display = AddressingModes[@mode][:display]
  #if @arg.kind_of?(String)
    #sprintf("#{display} (#{@mode}, #{@arg})", @op, 0x0)
  #else
    #sprintf("#{display} (#{@mode})", @op, @arg)
  #end
end
zero_page_instruction?() click to toggle source

Return if this instruction is a zero page instruction

# File lib/n65/instruction.rb, line 199
def zero_page_instruction?
  [:zero_page, :zero_page_x, :zero_page_y].include?(@mode)
end

Private Instance Methods

break_16(integer) click to toggle source

Break an integer into two 8-bit parts

# File lib/n65/instruction.rb, line 288
def break_16(integer)
  [integer & 0x00FF, (integer & 0xFF00) >> 8]
end
high_byte(word) click to toggle source

Take the high byte of a 16-bit integer

# File lib/n65/instruction.rb, line 295
def high_byte(word)
  (word & 0xFF00) >> 8
end
low_byte(word) click to toggle source

Take the low byte of a 16-bit integer

# File lib/n65/instruction.rb, line 302
def low_byte(word)
  word & 0xFF
end