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https://github.com/catseye/zowie

An esoteric language where literally every operation is memory-mapped — even structured loops!
https://github.com/catseye/zowie

esolang esoteric-language esoteric-programming-language memory-mapped structured-programming

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An esoteric language where literally every operation is memory-mapped — even structured loops!

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README 

        
ZOWIE

=====



_Try it online_ [@ catseye.tc](https://catseye.tc/installation/ZOWIE)

| _Wiki entry_ [@ esolangs.org](https://esolangs.org/wiki/ZOWIE)

| _See also:_ [SMITH](https://codeberg.org/catseye/SMITH#smith)



- - - -



Introduction

------------



ZOWIE is a programming language where all control flow is both

*memory-mapped* and *structured*. It is memory-mapped in the sense that

changes in flow are triggered by changes made to memory locations, and

it is structured in the sense of structured programming – the programmer

never deals with `goto`s, offsets, or labels of any kind.



History

-------



The primary design goal of ZOWIE was to have memory-mapped structured

control flow. This goal was inspired by Jeffry Johnston's unsuccessful

attempt to reduce the number of instructions in

[BitChanger](http://esolangs.org/wiki/BitChanger) (while retaining

Turing-completeness) by memory-mapping the loop operation.



I initially thought that the difficulty lay in BitChanger's minimalism.

To do memory-mapped flow control in general sounded easy – just start a

loop when one memory location is written, and end it when some other

location is written, right? But no. It's not that simple, as I attempt

to explain below. ZOWIE is the last of several, sometimes painful,

attempts over the summer of 2009 to realize this design goal (and it is

not clear that the techniques used in ZOWIE could be usefully imported

back into BitChanger.) The final, workable idea crystallized at about

the time September turned into October.



The thing with loop structures is that there is usually some way to jump

to a known point in the loop (say, the start, or the end) – really,

that's what makes them structured. And to jump to a point in a loop, you

have to know where that point is. And if you can analyze the loop

structure statically, for example if your semantics are directed by

syntax as in almost all high-level languages, then it's not difficult to

know where that point is.



But if that point is defined by when some *memory* location is changed,

it is not in general possible to detect it statically (by Rice's

Theorem, which is just a generalization of the Halting Problem.) So it

is not, in general, possible to know ahead of time where the loop begins

or ends.



There are a few things to note about this.



One is that by "statically" I do not necessarily mean "at compile-time".

Many Brainfuck and Mouse interpreters seek out the end of the loop only

when they know they must exit it. However, because they are looking

through the program text, it is still a kind of static analysis.



Another thing is that it would of course be possible to detect some kind

of (fixed) command to change the memory location associated with ending

a loop – but that would be cheating! (Also, if memory locations can be

computed, it is still not fully general, because we cannot look for all

possible computations that would result in that memory location.)



Lastly, note that we really don't have a problem detecting the start of

a loop. As soon as we execute the start of a loop, we know it's a loop,

and we know where it is, so we can record that location. The problem is

any other point in the loop, like the end. A little reflection will

reveal that this means it will be more difficult to do a "WHILE" loop or

a structured conditional ("IF-THEN-ENDIF") than a "REPEAT" loop (where

the condition is at the end of the loop.) However, it is widely known

that "REPEAT" loops alone are not sufficient for a Turing-complete

language. We'll see below that ZOWIE manages to create generalized loops

through the use of transactions.



The secondary design goal of ZOWIE was to strike the perfect balance

between _It's a Mad Mad Mad Mad World_ and _The Party_. It is generally

considered a morbid failure in that regard, what with not being a madcap

60's movie and all.



Syntax and Semantics

--------------------



To mitigate retooling costs, ZOWIE borrows much of its archiecture and

instruction repertoire from [SMITH](http://catseye.tc/projects/smith/).

There are an unlimited number of registers, numbered from 0 upward; each

register contains a non-negative integer value of unbounded extent. The

contents of a register before it has ever been written is guaranteed to

be 0.



There are five instruction forms. The first register is the destination

register, and is written to; the second register (or immediate value) is

read from. As in SMITH, square brackets indicate indirect register

access. All instruction forms are case-sensitive; they must be given

using capital letters.



      MOV register, immediate       e.g.  MOV R8, 141

      MOV register, register              MOV R8, R9

      MOV [register], register            MOV R[R8], R9

      MOV register, [register]            MOV R8, R[R9]

      MOV [register], [register]          MOV R[R8], R[R9]



Not only flow control, but in fact all operations in ZOWIE are

memory-mapped. The lowest-numbered nine registers have special behaviour

when written to or read from:



`R0`



When a value is written into R0, the Unicode symbol represented by the

value is sent to the standard output channel.



Reading from R0 waits until a Unicode symbol is available on the

standard input, then offers its value as the value of this register.



This is similar to the `TTY` pseudo-register of SMITH.



*Note: although implementations should make a best effort, the external

encoding and representation of Unicode characters is ultimately

implementation-defined, especially on systems which are only capable of

accurately displaying a subset of the Unicode character set. For

example, on a strict ASCII teletype or other device incapable of

displaying the DOWNWARDS ARROW (↓) symbol, it would be reasonable to

output `↓` or some similar "escape sequence" when executing

`MOV R0, 8595`.*



`R1`



When a value is written into R1, a **BEGIN TRANSACTION** occurs;

conceptually, a copy of the program state, including all registers and

the location of the currently executing instruction, is made, and pushed

onto a stack.



Reading from R1 always offers the value 1.



`R2`



When a value is written into R2, what happens depends on the value.



If the value is greater than zero, the current transaction is

**COMMIT**ted; conceptually, the topmost program state is popped from

the stack and discarded.



If the value is equal to zero, the current transaction is

**ROLLBACK**ed. Conceptually, the topmost program state is popped; the

contents of all registers are reset to what they were in the popped

program state; but the location of the currently executing instruction

is unchanged.



Reading from R2 always offers the value 2.



`R3`



When a value is written into R3, what happens depends on the value.



If the value is greater than zero, the current transaction is **COMMIT

AND REPEAT**ed; conceptually, the topmost program state is popped from

the stack; the location of the currently executing instruction is reset

to what it was in the program state; and a copy of this new program

state is pushed once more onto the stack.



If the value is equal to zero, the current transaction is **COMMIT**ed

(described previously in R2).



Reading from R3 always offers the value 3.



`R4`



When a value is written into R4, that value is added to the value in R8,

and the result is written into R8. Reading from R4 always offers the

value 4.



`R5`



When a value is written into R5, that value is subtracted from the value

in R8, and the result is written into R8. If the result would be

negative, the result will be zero. Reading from R5 always offers the

value 5.



`R6`



When a value is written into R6, the product of that value and the value

in R8 is written into R8. Reading from R6 always offers the value 6.



`R7`



When a value is written into R7, the boolean negation of that value is

written into R8: 1 if the value was 0, and 0 otherwise. Reading from R7

always offers the value 7.



`R8`



Not really memory-mapped, but used as an "accumulator" by the registers

R4 through R7.



Because the reading and writing of registers can have side-effects, the

order of reads and writes during the execution of a single instruction

is strictly defined as follows:



-   The indirect source register, if any, is read (to discover the

    direct source register.)

-   The direct source register is read.

-   The indirect destination register, if any, is read (to discover the

    direct destination register.)

-   The direct destination register is written.



Computational Class

-------------------



I believe ZOWIE is Turing-complete because the transactions can simulate

both "IF" and "REPEAT" control structures, which, taken together, can

simulate a "WHILE", which is widely known to be sufficient, along with

the usual arithmetical operations on an unbounded number of unbounded

integer registers, to have a system that is Turing-complete.



For example, a crude translation of Brainfuck into ZOWIE might go like:



    preamble MOV R10, 100        ; the Brainfuck tape index

             MOV R11, 101        ; the saved-test-value stack pointer



    >        MOV R8, R10         ; inc tape index by two

             MOV R4, R2

             MOV R10, R8



    <        MOV R8, R10         ; dec tape index by two

             MOV R5, R2

             MOV R10, R8



    +        MOV R8, R[R10]      ; inc value on tape

             MOV R4, R1

             MOV R[R10], R8

             

    -        MOV R8, R[R10]      ; dec value on tape

             MOV R5, R1

             MOV R[R10], R8



    .        MOV R0, R[R10]      ; output



    ,        MOV R[R10], R0      ; input



    [        MOV R1, R1          ; BEGIN TRANSACTION for "REPEAT"

             MOV R8, R11         ; bump up the saved-value stack pointer

             MOV R4, R2

             MOV R11, R8

             MOV R[R11], R[R10]  ; save the value we are testing

             MOV R1, R1          ; BEGIN TRANSACTION for "IF"



    ]        MOV R2, R[R11]      ; COMMIT if non-zero or ROLLBACK otherwise

             MOV R12, R11        ; retain a copy of the saved-stack pointer

             MOV R8, R11         ; bump down the saved-stack pointer

             MOV R5, R2

             MOV R11, R8

             MOV R3, R[R12]      ; COMMIT AND REPEAT if non-zero



Three things to note:



-   In this translation, the simulated Brainfuck tape and the

    saved-value stack are interleaved.

-   It is important to save the value being tested *before* the "IF"

    transaction is begun – otherwise, the value will be rolled back

    before it can be tested for the **COMMIT AND REPEAT**.

-   The input-output behaviour of ZOWIE programs produced by this

    translation does differ from Brainfuck. If the value on the tape is

    initially zero, a Brainfuck "while" loop will never be executed at

    all, whereas a ZOWIE transaction *will* be executed, but afterwards

    undone – everything, that is, except input and output, because being

    interactions with the outside world, those can't be undone. This

    limitation does not affect whether ZOWIE is Turing-complete or not

    (you could just refrain from outputting anything until the very end

    of the computation), but it does imply that ZOWIE has limitations on

    how it can communicate.



That's all.



Happy *«deleted by black helicopters»*!  

Chris Pressey  

December 29th, 2009 CE  

Evanston, IL