The following letters are used to identify stack items consumed or produced by xyw instructions or subroutines. Each letter can be followed by an integer counter to differentiate multiple values of the same type within the same signature. a -> 2-byte absolute address b -> 1-byte value d -> 1-byte direct page addresses f -> 1-byte boolean value (flag) n -> 1-byte or 2-byte value p -> 1-byte or 2-byte (direct page) value s -> 2-byte address to null-terminated string w -> 2-byte value === Example The signature for the equals subroutine is the following: s1 s2 -> f Where s1 and s2 are string addresses, and f is a boolean value.