Uxntal Stacks
All programming in Uxntal is done by manipulating the working stack,
and return stack, each stack contains 256 bytes. Here are some stack
primitives assuming the initial state of the stack is a b c where
c is the top of the stack:
| POP | a b | Discard top item. |
|---|---|---|
| NIP | a c | Discard second item. |
| SWP | a c b | Move second item to top. |
| ROT | b c a | Move third item to top. |
| DUP | a b c c | Copy top item. |
| OVR | a b c b | Copy second item to top. |
A byte is a number between 0-255(256 values), a short is a number between 0-65535(65536 values) made of two bytes, each byte in a short can be manipulated individually:
#0a #0b POP 0a #12 #3456 NIP 12 56 #1234 DUP 12 34 34
The two stacks are circular, to pop an empty stack does not trigger an error, but merely means to set the stack pointer to 255. There are no invalid programs, any sequence of bytes is a potential Uxn program. Values are moved between stacks with the STH opcode.
WST 00 00 00 00 00 00|12 34 <02 RST 00 00 00 00 00 00 00|56 <01
The program above contains 12 and 34 on the working stack, and 56 on the return stack. The stack content can always be printed by sending a non-null byte to the System/debug port.