colon-sys

description:: According to the ANS Forth standard, : (colon) leaves colon-sys on the control-flow stack and ; (semi-colon) consumes it. One might think that the !CSP ... ?CSP approach is not ANS-compliant. The mentioned approach is based on checking the stack depth by comparing the values returned by the word SP@ (read the data stack pointer) in the beginning and the end of colon definition. \ the following code is from Win32Forth source (public domain) VARIABLE CSP \ Current Stack Pointer variable : !CSP ( -- ) \ save current stack pointer for later stack depth check SP@ CSP ! ; : ?CSP ( -- ) \ check current stack pointer against saved stack pointer SP@ CSP @ XOR ABORT" stack changed" ; The word : (colon) calls !CSP and the word ; (semi-colon) calls ?CSP to verify that no control structures have been left unfinished. This implementation is ANS-compliant with the following definition of the control-flow stack (c.f. stack): the c.f. stack consists of the value kept in the variable CSP (the bottom element) and the values placed on the data stack above the value that was on the stack top when the word !CSP was performed; outside of a colon definition, the c.f. stack does not exist.