bound_variable/data/um-spec.txt

  Order for Construction          Standard Sand of Pennsylvania Co.

  Client: Cult of the Bound Variable
  Object: UM-32 "Universal Machine"
  -----------------------------------------------------------------
                                                      21 July 19106

  Physical Specifications.
  ------------------------

  The machine shall consist of the following components:

    * An infinite supply of sandstone platters, with room on each
      for thirty-two small marks, which we call "bits."

                           least meaningful bit
                                              |
                                              v
              .--------------------------------.
              |VUTSRQPONMLKJIHGFEDCBA9876543210|
              `--------------------------------'
               ^
               |
               most meaningful bit

              Figure 0. Platters
      
      Each bit may be the 0 bit or the 1 bit. Using the system of
      "unsigned 32-bit numbers" (see patent #4,294,967,295) the
      markings on these platters may also denote numbers.

    * Eight distinct general-purpose registers, capable of holding one
      platter each.

    * A collection of arrays of platters, each referenced by a distinct
      32-bit identifier. One distinguished array is referenced by 0
      and stores the "program." This array will be referred to as the
      '0' array.

    * A 1x1 character resolution console capable of displaying glyphs
      from the "ASCII character set" (see patent #127) and performing
      input and output of "unsigned 8-bit characters" (see patent
      #255).
  

  Behavior.
  ---------

  The machine shall be initialized with a '0' array whose contents
  shall be read from a "program" scroll. All registers shall be
  initialized with platters of value '0'. The execution finger shall
  point to the first platter of the '0' array, which has offset zero.

  When reading programs from legacy "unsigned 8-bit character"
  scrolls, a series of four bytes A,B,C,D should be interpreted with
  'A' as the most magnificent byte, and 'D' as the most shoddy, with
  'B' and 'C' considered lovely and mediocre respectively.

  Once initialized, the machine begins its Spin Cycle. In each cycle
  of the Universal Machine, an Operator shall be retrieved from the
  platter that is indicated by the execution finger. The sections
  below describe the operators that may obtain. Before this operator
  is discharged, the execution finger shall be advanced to the next
  platter, if any.

  Operators.
  ----------

  The Universal Machine may produce 14 Operators. The number of the
  operator is described by the most meaningful four bits of the
  instruction platter.

              .--------------------------------.
              |VUTSRQPONMLKJIHGFEDCBA9876543210|
              `--------------------------------'
               ^^^^
               |
               operator number

              Figure 1. Operator Description


  Standard Operators.
  -------------------

  Each Standard Operator performs an errand using three registers,
  called A, B, and C. Each register is described by a three bit
  segment of the instruction platter. The register C is described by
  the three least meaningful bits, the register B by the three next
  more meaningful than those, and the register A by the three next
  more meaningful than those.

                                      A     C
                                      |     |
                                      vvv   vvv                    
              .--------------------------------.
              |VUTSRQPONMLKJIHGFEDCBA9876543210|
              `--------------------------------'
               ^^^^                      ^^^
               |                         |
               operator number           B

              Figure 2. Standard Operators


  A description of each basic Operator follows.

  Operator #0. Conditional Move.

                  The register A receives the value in register B,
                  unless the register C contains 0.

           #1. Array Index.

                  The register A receives the value stored at offset
                  in register C in the array identified by B.

           #2. Array Amendment.

                  The array identified by A is amended at the offset
                  in register B to store the value in register C.

           #3. Addition.

                  The register A receives the value in register B plus 
                  the value in register C, modulo 2^32.

           #4. Multiplication.

                  The register A receives the value in register B times
                  the value in register C, modulo 2^32.

           #5. Division.

                  The register A receives the value in register B
                  divided by the value in register C, if any, where
                  each quantity is treated treated as an unsigned 32
                  bit number.

           #6. Not-And.

                  Each bit in the register A receives the 1 bit if
                  either register B or register C has a 0 bit in that
                  position.  Otherwise the bit in register A receives
                  the 0 bit.

  Other Operators.
  ----------------

  The following instructions ignore some or all of the A, B and C
  registers.

           #7. Halt.

                  The universal machine stops computation.

           #8. Allocation.

                  A new array is created with a capacity of platters
                  commensurate to the value in the register C. This
                  new array is initialized entirely with platters
                  holding the value 0. A bit pattern not consisting of
                  exclusively the 0 bit, and that identifies no other
                  active allocated array, is placed in the B register.

           #9. Abandonment.

                  The array identified by the register C is abandoned.
                  Future allocations may then reuse that identifier.

          #10. Output.

                  The value in the register C is displayed on the console
                  immediately. Only values between and including 0 and 255
                  are allowed.

          #11. Input.

                  The universal machine waits for input on the console.
                  When input arrives, the register C is loaded with the
                  input, which must be between and including 0 and 255.
                  If the end of input has been signaled, then the 
                  register C is endowed with a uniform value pattern
                  where every place is pregnant with the 1 bit.

          #12. Load Program.

                  The array identified by the B register is duplicated
                  and the duplicate shall replace the '0' array,
                  regardless of size. The execution finger is placed
                  to indicate the platter of this array that is
                  described by the offset given in C, where the value
                  0 denotes the first platter, 1 the second, et
                  cetera.

                  The '0' array shall be the most sublime choice for
                  loading, and shall be handled with the utmost
                  velocity.

  Special Operators.
  ------------------

  One special operator does not describe registers in the same way.
  Instead the three bits immediately less significant than the four
  instruction indicator bits describe a single register A. The
  remainder twenty five bits indicate a value, which is loaded
  forthwith into the register A.

                   A  
                   |  
                   vvv
              .--------------------------------.
              |VUTSRQPONMLKJIHGFEDCBA9876543210|
              `--------------------------------'
               ^^^^   ^^^^^^^^^^^^^^^^^^^^^^^^^
               |      |
               |      value
               |
               operator number

               Figure 3. Special Operators

          #13. Orthography.

                  The value indicated is loaded into the register A
                  forthwith.

  Cost-Cutting Measures.
  ----------------------

  As per our meeting on 13 Febtober 19106, certain "impossible
  behaviors" may be unimplemented in the furnished device. An
  exhaustive list of these Exceptions is given below. Our contractual
  agreement dictates that the machine may Fail under no other
  circumstances.


  If at the beginning of a cycle, the execution finger does not indicate
  a platter that describes a valid instruction, then the machine may Fail.

  If the program decides to index or amend an array that is not
  active, because it has not been allocated or it has been abandoned,
  or if the offset supplied for the access lies outside the array's
  capacity, then the machine may Fail.

  If the program decides to abandon the '0' array, or to abandon an array
  that is not active, then the machine may Fail.
  
  If the program sets out to divide by a value of 0, then the machine
  may Fail.

  If the program decides to load a program from an array that is not
  active, then the machine may Fail.

  If the program decides to Output a value that is larger than 255, the
  machine may Fail.

  If at the beginning of a machine cycle the execution finger aims
  outside the capacity of the 0 array, the machine may Fail.