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The Reustmann machine is a tiny programming language and abstract processing engine designed for educational use for experimentation with automatic program generation. It has the following properties:

  • Any bit pattern in Reustmann memory is a legal Reustmann program – there are no instruction faults
  • All operations address existing memory – there are no memory faults
  • Reustmann source code consists of a string of characters, one character per instruction

A corollary is that any string of random characters is the source code for a legal, executable Reustmann program. This allows, for example, a genetic algorithm to use an Reustmann program in source form as its genome, evolving the source code string until the Reustmann program produces the desired output.


The Reustmann abstract machine is a Von Neumann architecture where all memory locations are accessed through a program counter (PC) or stack pointer (SP). The machine architecture can be characterized as a processor with three special-purpose registers (PC, SP, and NZ flag), L memory locations ("words") W bits wide, and an instruction set of about 40 opcodes. All possible states of memory and registers comprise legal Reustmann programs – there is no possibility of an illegal instruction or a memory fault.

Allowing all possible bit patterns to result in legal (but not necessarily interesting) programs simplifies experimentation with automatic program generation. AI programs, such as neural nets and genetic algorithms, can regard Reustmann programs as arbitrary bit patterns, evaluate them with an Reustmann simulator, and compare the results against some fitness metric, without concern for syntactic program correctness.

The Reustmann language is similar to a rudimentary assembly language. Reustmann source code can be expressed using either long or short mnemonics. Here is a simple Reustmann program that copies its input to its output as long as the data is nonzero, shown with long mnemonics with added comments:

LOOP    // loop until the ENDL opcode
IN      // read one char from stdin and push it on the stack
BNZ     // skip the next instruction if nonzero
OUT     // pop a word from the stack and write it to stdout
ENDL    // loop forever

All memory addressing is through the stack pointer (SP) and is implied in each instruction, so there are no operands specified in the source code. Using the short mnemonics, each Reustmann instruction can be represented by a single character, and is the preferred format for entering and saving programs. The sample program above is represented in source code form using short mnemonics as the string:


An Reustmann abstract machine contains L memory locations, W bits wide, where

1 ≤ L ≤ 2^32 and 6 ≤ W ≤ 32.

This Reustmann architecture specifies a set of possible abstract machines, where the rank of a machine is determined by unique combinations of L and W. An Reustmann program runs differently on machines of different ranks. For example, an Reustmann program that runs correctly on an abstract machine of rank L = 16, W = 16 will not necessarily produce the same results on a machine with rank L = 20, W = 8.

Memory and addressing model

The L words of memory are addressed as locations 0 through L – 1. The Stack Pointer (SP) seamlessly wraps around, treating the entire address space as a circular buffer. Address calculations relating to the Program Counter (PC) (e.g., branch targets) are performed using unsigned 32-bit arithmetic and then adjusted if necessary to fit in the range [0 .. L). When the PC would have been advanced to address A where A > L, the address is immediately readjusted to the address modulo L. This implies that it is impossible to address a non-existent memory location.

Each memory location can store unsigned values in the range [0 .. 2 W). The memory locations are all writable. Instruction opcodes are stored in memory by their numeric values as defined below.

Execution model

After machine reset, the three Reustmann registers have these initial values, and are described in more detail in a later section.

PC = 0, SP = 0 and NZ = false

For each simulation cycle, if the value at the memory location indexed by PC is a defined opcode number, the opcode is executed in simulation as described in the tables below. If the memory location indexed by PC is not a defined opcode number, it is executed as if it were a NOP opcode. This implies there are no instruction faults.

Input and Output

The Reustmann instruction set has one IN and one OUT instruction. These are character-oriented, regardless of the value of W. Each time an IN instruction is executed, one character is consumed from a user-specified input string or stream and pushed onto the Reustmann stack, narrowing or widening the value to W bits. Each OUT instruction pops the top of the Reustmann stack, casts the value to a (char) type, and sends it to the abstract output stream.

Source Code Representation

For Reustmann machines of rank W ≤ 8, the preferred source code format is a string of characters of length ≤ L. When loaded into Reustmann memory, the source string S is interpreted as follows:

for each character C in S:
    if C is a defined short mnemonic:
        store the opcode by opcode number
        store the character value narrowed to W bits

When a short-mnemonic source code disassembly is generated by an Reustmann abstract machine of any rank, the listing will render each memory value N as a single character as follows:

if N is an assigned opcode number:
    show the short opcode mnemonic
else if N is representable as a single character:
    show the literal character
    show a ';' (NOP) opcode

PC – Program Counter

The program counter PC contains a value in the range [0 .. L). Values wrap around the address space, such that when the instruction execution reaches the end of memory, execution by default continues at location zero. After machine reset, the PC starts at zero, and by default, is incremented by one after each instruction is executed. If any branching or looping opcode results in a calculated address outside the range [0 .. L), the address is changed to the calculated address modulo L so that it always refers to a valid memory location.

SP – Stack Pointer

The stack pointer SP contains a value in the range [0 .. L). Values wrap around, so that the entire memory becomes a circular buffer for the SP. After machine reset, the SP starts at zero, and always points at the last item stacked (the “top” of the stack). The SP grows toward lower addresses. If the SP changes to an address outside the range [0 .. L), it is changed to the value modulo L so that it always refers to a valid memory location. This implies that the first item pushed onto the stack after machine reset will be written to the top of memory at location L-1, and the new SP will contain the value L-1 and will grow downward from there.

NZ – Non-Zero Flag

The non-zero flag NZ reflects the non-zeroness of the most recent instruction that read or wrote memory, as defined in the details below. NZ = true means that the result was nonzero. The NZ flag can be tested with the BZ and BNZ instructions. On machine reset, NZ is initialized to false.


Values in memory are regarded as unsigned integers in the range [0 .. 2 W). The arithmetic opcodes evaluate their operands as if using 32-bit unsigned integers with the results truncated to the least significant W bits. See individual opcodes below for more details.

Copyright 2012 David R. Miller. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts and no Back-Cover Texts. A copy of the license is included in the accompanying file named COPYING and online at http://www.gnu.org/licenses/fdl.txt.



Instruction Set Summary




A Debug structure to help debugging :)


The main interpreter, execute instructions, read from input, write to output


A set of instructions that can be given to an interpreter.


Type used to return the opcode executed with its execution status