The Programmer's Guide to TRSDOS Version 6 - Tim Mann's Home ...

8.14 TRAP Filter Illustrated
;TRAP/ASM - Filter to trap a single character - 07/31/83
;
COM ''
;*=*=*
; This FILTER will trap a single character
; as specified by the command line entry.
;
; A single byte to trap can be passed in the
; command line as a parameter. If not entered,
; it will default to X'0E', the infamous cursor
; on character which if sent to a printer, will
; cause expanded character mode on a lot of dot
; matrix printers if CURSOR ON is sent to *PR.
;
; To filter the printer output, issue:
; SET *TP to TRAP (CHAR=dd)
; FILTER *PR using *TP
;
;*=*=*
LF EQU 10 ;Line feed
CR EQU 13 ;Carriage return
@CHNIO EQU 20
@HIGH$ EQU 100
@DSPLY EQU 10
@FLAGS$ EQU 101
@PARAM EQU 17
@LOGOT EQU 12
;
ORG 3000H
BEGIN PUSH DE
POP IX ;Get DCB pointer into IX
LD (MODDCB),DE ;Stuff DCB pointer
PUSH HL ;Save command line ptr
LD HL,HELLO$
LD A,@DSPLY ;Display hello
RST 40
POP HL ;Rcvr command line ptr
;*=*=*
; Check if entry from SET command
;*=*=*
LD A,@FLAGS$ ;Get flag pointer
RST 40
BIT 3,(IY+'C'-'A') ;System request?
JP Z,VIASET
LD DE,PRMTBL$ ;Point to parameter table
LD A,@PARAM ;Get parms if any
RST 40
JR NZ,PRMERR
CPARM LD BC,14 ;Init to X'0E'
LD A,(CRESP) ;P/u the response
OR A ; & see if any entry
JR Z,CDEFLT ;Default if none
BIT 7,A ;Value entry?
JR NZ,CDEFLT ;Value is in reg C
BIT 5,A ;String value?
JP NZ,PRMERR ;Error if anything else
LD A,(BC) ;BC contains a pointer
LD C,A ;Shorter than a jump
CDEFLT LD A,C ;Xfer the value to reg A
LD (TRAPBYT+1),A ; & stuff in filter
;*=*=*
; install new HIGH$ and move filter code
;*=*=*
LD HL,0 ;Get current HIGH$
LD B,L
LD A,@HIGH$
RST 40
JR NZ,NOMEM
LD (OLDHI),HL ;Put in filter header
;*=*=*
; Move module into memory
;*=*=*
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