MODULE ; CATCH.ACT

; copyright (c) 1984
; by Action Computer Services
; All Rights Reserved

; This module provides two PROCs
; (Catch and Throw) which can be used
; for error trapping (and flow
; control, yeck!) in ACTION!.  To
; use them, you must call the Catch
; PROC to indicate where you want
; the program to continue when you
; call Throw.  When throw is called,
; execution will continue following
; the last call to Catch with the
; same index as the call to Throw.
; Calling Catch is similar (but not
; identical) to TRAP in BASIC.  It
; differs in that the actual trapping
; is generated by the user (by
; calling Throw) and that you can
; have multiple Catch'ers active at
; one time.  Also, you cannot Throw
; to a Catcher that is no longer 
; active (the PROC/FUNC containing
; it has RETURN to it's caller).  The
; Throw procedure tries to check for
; this error, but it is possible to
; fool it into thinking it's OK.  If
; you want to solve this problem, you
; can set 'c_t_sp(index)' to zero
; before you return from the PROC
; that contained the Catch(index).
; If index is greater than 24 or
; if there is no matching Catch index
; for the Throw, then Error will be
; called with a value of CTERR
; (defined below to be 71).  If you
; setup your own Error procedure and
; use Catch and Throw, your error
; procedure should handle this error
; as well or your program will most
; likely "go off the deep end".


DEFINE CTERR = "71" 

BYTE ARRAY c_t_sp(25)=[0 0 0 0 0 0 0
 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
BYTE ARRAY c_t_hi(25), c_t_lo(25)


PROC Catch(BYTE index)
  DEFINE TSX="$BA", TXA="$8A",
         LDYA="$AC", STAY="$99",
         PLA="$68", LDAY="$B9",
         PHA="$48"

  IF index>=25 THEN
    Error(CTERR,0,CTERR) FI 

  [
    LDYA index  
    PLA
    STAY c_t_hi
    PLA
    STAY c_t_lo
    TSX
    TXA
    STAY c_t_sp
    LDAY c_t_lo
    PHA
    LDAY c_t_hi
    PHA
  ]
RETURN


PROC Throw(BYTE index)
  DEFINE TXS="$9A", PHA="$48",
         LDYA="$AC", STX="$86",
         TSX="$BA", TAX="$AA",
         LDAY="$B9"

  BYTE sp=$A2

; get current stack pointer
  [ TSX : STX sp ]

  IF index>=25 OR sp+2>c_t_sp(index)
    THEN Error(CTERR,0,CTERR) FI 

  [
    LDYA index  
    LDAY c_t_sp
    TAX
    TXS
    LDAY c_t_lo
    PHA
    LDAY c_t_hi
    PHA
  ]
RETURN

MODULE ; just in case