Base64 lang is a simple esolang made by User:1048576.
We have 1 register val with default 0, and a sequence of length 512(start is a_0).
A command is two characters(12 bits when decoding in Base64 encoding):
(A is short for a_(addr in binary))
|000||val <- A|
|001||A <- val|
|010||A <- !(A and val)|
|011||if val, then jump to before A-th command running|
|101||input a bit from stdin, then save it in A|
Explanation: when it writes in binary, it looks at follows:
000 000000001 # Set val to 0 001 000000000 # Set a_0 to val(0) 010 000000000 # Set a_0 to (a_0 and val), which is 1 000 000000000 # set val to a_0(1) 011 000000000 # jump to before the first command running, so it is a cat program.