For fun, run this at home

gistfile1.rb

["US-ASCII", "ASCII-8BIT", "UTF-8"].each do |encoding|
  Encoding.default_internal = encoding
  puts "With #{encoding} default internal:"
  [127, 255, 256].each do |number|
    no_arg = number.chr.encoding rescue "out of range"
    puts "#{number}.chr is: #{no_arg}"

    with_arg = number.chr(encoding).encoding rescue "out of range"
    puts "#{number}.chr('#{encoding}') is: #{with_arg}"
  end
  puts ""
end

Spoiler: here's the output:

With US-ASCII default internal:
127.chr is: US-ASCII
127.chr('US-ASCII') is: US-ASCII
255.chr is: ASCII-8BIT
255.chr('US-ASCII') is: US-ASCII
256.chr is: out of range
256.chr('US-ASCII') is: out of range

With ASCII-8BIT default internal:
127.chr is: US-ASCII
127.chr('ASCII-8BIT') is: ASCII-8BIT
255.chr is: ASCII-8BIT
255.chr('ASCII-8BIT') is: ASCII-8BIT
256.chr is: out of range
256.chr('ASCII-8BIT') is: out of range

With UTF-8 default internal:
127.chr is: US-ASCII
127.chr('UTF-8') is: UTF-8
255.chr is: ASCII-8BIT
255.chr('UTF-8') is: UTF-8
256.chr is: UTF-8
256.chr('UTF-8') is: UTF-8