Variables

This subtle and remarkable code was discovered by Peter. It provides for variables in RAM, retrieving them without involving an address register. Consider this self-modifying code

n! @p drop !p ;
n 0 ;

The words must be adjacent in this order. They occupy 3 words of RAM.

n!

Store value into variable n

Fetch the next word from RAM. That is the first instruction word of the variable's code. It must contain a @p to fetch the parameter word
Discard the instruction word
Since @p increments the p register, it now points to the parameter word
Store the new value
End

n

Calling n will return its value, initially 0.

n may execute additional code, so long as the parameter is fetched in the first instruction word. For example

n 0 . + ;

will add the variable instead of just fetching it.

n! may also execute additional code, so long as it preceeds the 4-word sequence @p drop !p ; aligned on a word boundary. I just used

pg! if drop 4000 then @p drop !p ;
pg 0 or ;