Befunge-93 Documentation

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 forward thereafter, 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 that can be specified in the program source; Befunge-93 programs have a maximum size of 80x25 total commands and data bytes. There are no run-time variables, only a single run-time stack. Befunge-93 programs allow for self-modification. Due to the 2-dimensional nature of the PC, they also 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 (that is, each cell of the stack can hold as much as a C language signed long int on the same platform.) 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:

• + addition
• - subtraction
• / integer division
• * multiplication
• % modulo
• ! logical negation

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 :

99*76*+

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

The Program Counter in Detail

There are 5 commands which unconditionally 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:

><

>v
^<

>v>v
 >^
^  <

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

<

Decision Making

The standard 'if' statement in Befunge-93 is either _ or |, depending on how you want to branch. Both of these instructions 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 the value is true or > if it is false; and
• | acts like ^ if the value is true or v if it is false.

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

>_@

Input

The & (ampersand) command will get a numeric value (in decimal) from the standard input and push it on the stack. ~ (tilde) will get the next ASCII character from standard input and push it on the stack.

For example,

&,

~.

Output

The . command will pop a value off the stack and output it as a decimal integer, followed by a space, somewhat like Forth. , will pop a value, interpret it as the ASCII value of a character, and output that character (not followed by a space.)

For example,

665+*1-,

665+*1-.

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,

>123...@

>123#...@

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

v.<>:| @

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

123.$.@

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

123\...@

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

65`.

25`.

Self-Modification

The last two commands that need to be explained are the ones that allow you to examine and change the contents of the torus where the program is stored. This 'playfield' can be used for auxilliary storage when the stack alone will not suffice, but keep in mind that it also contains the running program.

The g command examines the contents of the playfield. It pops a y coordinate off the stack, then an x coordinate. It pushes the value found at (x, y) onto the stack. If the thing at (x, y) is a Befunge-93 instruction, the value pushed will be the ASCII value of that character. From the point of view of the program text, x determines the column and y determines the row; (0, 0) refers to the first (leftmost) column and the first (topmost) row of the program source.

The p command alters the contents of the playfield. It pops a y coordinate off the stack, then an x coordinate, and then a value. It places the value into the torus at (x, y). If the program, at some later point, executes the instruction at (x, y), it will be the interpreted as the Befunge instruction in the ASCII character set with the same value as was put there with the p instruction.

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.