title: 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.