PiDP-10 – a modern replica of the PDP-10(obsolescence.dev)

  Byte instructions

  In the PDP-10 a "byte" is some number of contiguous bits within one
  word.  A byte pointer is a quantity (which occupies a whole word)
  which describes the location of a byte.  There are three parts to the
  description of a byte: the word (i.e., address) in which the byte
  occurs, the position of the byte within the word, and the length of
  the byte.

  A byte pointer has the following format:

   000000 000011 1 1 1111 112222222222333333
   012345 678901 2 3 4567 890123456789012345
   _________________________________________
  |      |      | | |    |                  |
  | POS  | SIZE |U|I| X  |        Y         |
  |______|______|_|_|____|__________________|

  POS is the byte position: the number of bits from the right end of
  the byte to the right end of the word.

  SIZE is the byte size in bits.

  The U field is ignored by the byte instructions.

  The I, X and Y fields are used, just as in an instruction, to compute
  an effective address which specifies the location of the word
  containing the byte.


  Here are the byte instructions. 
  [...] LDB DPB IBP ILDB IDPB [...]

  Text strings are typically stored using seven-bit bytes, five per
  word.  ILDB and IDPB can then be used to step through a string.  The
  byte pointer should be initialized to 440700,,<address of string>.
  Then the first ILDB will increment it to point at the first character
  of the string.