Befunge-93 Documentation

Twisted, Deranged Programming Language in the Tradition of BrainF*** and False

Chris Pressey
Original document September, 1993
Updated December, 1996

The Basics of Befunge-93

Most likely the most unique element of Befunge-93 programming is the Program Counter (PC.) In almost all computer programming languages, the program counter is continually moving forward through the program, occassionally jumping to another spot in the code (but continuing forwardthereafter nonetheless.)

The PC in Befunge-93, however, is subject to different rules. It may go forward, backward, left or right. A Befunge-93 program is treated as an 80x25 torus (a page which wraps around on the edges) of ASCII text. Certain commands change the direction of the progress of the PC. By default, the PC points to the upper-left corner of the program, and is oriented to travel left-to-right.

Each command in Befunge-93 is a single character, as is the largest data unit; Befunge-93 programs have a maximum size of 80x25 total commands and data bytes. There are no run-time variables and a single run-timestack. Befunge-93 programs allow for self-modification. Due to the 2-dimensional nature of the PC, they allow for some extremely quirky code.

The Stack

Something like Forth and PostScript, Befunge-93 supports a LIFO, Reverse Polish Notation (RPN or postfix) stack of signed long integers. The act of placing a value on the stack is called pushing, and the act of taking a value off the stack is called popping. The digits from '0' to '9' are valid Befunge-93 commands which push their respective values onto the stack. A double quote '"', when encountered, toggles stringmode, and while stringmode is active, all character cells will have their ASCII value pushed onto the stack until another '"' is located.

There are a few basic calculation commands:

These are explained in greater detail in the Commands section.

In order to push a number greater than 9 on the stack, calculations must be done with numbers less than or equal to 9. In any other language this would be a pain. In Befunge-93 it is a joy. For example, to push '123' onto the stack, one might push 9, then 9, then multiply (leaving 81), then push 7, then 6, then multiply (leaving 81 and 42,) then add (leaving 123.) In Befunge, this would look something like :

This is, of course, assuming (correctly) that the PC starts at or before the first '9' and is working towards the right.

NB. If the stack is be empty when you pop something off, be warned that this will not generate an underflow! It will simply return '0' to you. Hope you can live with it!

The Program Counter in Detail

There are 5 commands which directly control the PC direction: '>', '<', 'v', '^', and '?'. '>' makes the PC travel to the right; '<' to the left;'v' down; '^' up; and '?' in a random direction. So, the following example is an infinite loop :

As is :

As is :

Note that ' ' (space) is a null command which does nothing.

Should the PC encounter the 'edge' of the program, such as if you were to try to execute:

The PC will 'wrap around' to the other 'edge' of the program. This, too, is an infinite loop.

Decision Making

The standard 'if' statement in Befunge is either '_' or '|', depending on how you want to branch. Both pop a value off the stack and check to see if it is true (non-zero,) and change the direction of the PC accordingly.

'_' acts like '<' if it is true, and '>' if it is false.
'|' acts like '^' if it is true, and 'v' if it is false.

'While' loops can be made by sticking an 'if' in an infinite loop. For example,

(This program fragment pops all of the non-zero values off the stack, and the first zero value, then exits ['@' is the exit command.])

Input

The '&' command will get a numeric value from the standard input and push it on the stack. '~' will get the next ASCII character from standard input and push it on the stack.

eg.

...prints out "A" if the user types "65 ", and...

...prints out "65 " if the user types "A".

Output

The '.' command will pop a value off the stack and output it as an integer, followed by a space. (somewhat like Forth.) ',' will pop avalue and output as ASCII with no space.

eg.

...prints out ASCII 65 ("A".), and...

...prints out "65 ".

Special Commands

'#' is the 'bridge' command... it causes the next command which would normally be executed to be skipped over, and not executed. For example,

would output "3 2 1 " but

would output "3 2 " with one of the '.''s being skipped. Judicious use of '#' can make for very interesting code!

':' is the duplicating command. It makes a copy of the top element of the stack. This is useful, as demonstrated in the program below.

This makes duplicates of each value on the stacked, which is checked, and if non-zero, printed.

'$' pops a value off the stack, but does nothing with it. So,

results in "3 1 ".

'\' swaps the top two elements of the stack. So,

results in "2 3 1 ".

'`' (back-quote) is the 'greater' command. It compares the top two values on the stack, and returns '1' if the first is greater than the second.

eg.

...outputs "1 " and... ...outputs "0 ".

Appendix A. Command Summary

COMMAND         INITIAL STACK (bot->top)RESULT (STACK)
-------         -------------           -----------------
+ (add)         <value1> <value2>       <value1 + value2>
- (subtract)    <value1> <value2>       <value1 - value2>
* (multiply)    <value1> <value2>       <value1 * value2>
/ (divide)      <value1> <value2>       <value1 / value2> (nb. integer)
% (modulo)      <value1> <value2>       <value1 mod value2>
! (not)         <value>                 <0 if value non-zero, 1 otherwise>
` (greater)     <value1> <value2>       <1 if value1 > value2, 0 otherwise>
> (right)                               PC -> right
< (left)                                PC -> left
^ (up)                                  PC -> up
v (down)                                PC -> down
? (random)                              PC -> right? left? up? down? ???
_ (horizontal if) <boolean value>       PC->left if <value>, else PC->right
| (vertical if)   <boolean value>       PC->up if <value>, else PC->down
" (stringmode)                          Toggles 'stringmode'
: (dup)         <value>                 <value> <value>
\ (swap)        <value1> <value2>       <value2> <value1>
$ (pop)         <value>                 pops <value> but does nothing
. (pop)         <value>                 outputs <value> as integer
, (pop)         <value>                 outputs <value> as ASCII
# (bridge)                              'jumps' PC one farther; skips
                                        over next command
g (get)         <x> <y>                 <value at (x,y)>
p (put)         <value> <x> <y>         puts <value> at (x,y)
& (input value)                         <value user entered>
~ (input character)                     <character user entered>
@ (end)                                 ends program

The People Who Helped Make the Dream Reality

Special thanks to Curtis Coleman, Jason Goga, Kalyna Zazelenchuk,Shawn Vincent, Mike Veroukis, Urban Mueller, and Wouter van Oortmerssen.