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

EX DE,HL ;Destination address to DE
LD HL,MODDCB-MODEND
ADD HL,DE ;Relocate one address
LD (RX01),HL
LD HL,MODEND ;Last byte of module
LD BC,LENGTH ;Length of filter
LDDR
EX DE,HL ;Move new HIGH$ to HL
LD A,@HIGH$ ;Set new HIGH$ into the system
RST 40
INC HL ;Bump to filter entry
LD (IX+0),40H.OR.7 ;Stuff TYPE byte
LD (IX+1),L
LD (IX+2),H ;Install addr into DCB
LD HL,0 ;Successful...
RET
;
PRMERR LD HL,PRMERR$
DB 0DDH
VIASET LD HL,VIASET$
DB 0DDH
NOMEM LD HL,NOMEM$
LD A,@LOGOT
RST 40
LD HL,-1 ;Indicate extended error
RET
;
HELLO$ DB LF,'TRAP filter to trap a character code',CR
PRMERR$ DB 'Bad parameters!',CR
NOMEM$ DB 'High memory is not available!',CR
VIASET$ DB 'Must install via SET!',CR
;
PRMTBL$ DB 80H
DB 80H.OR.20H.OR.10H.OR.4
DB 'CHAR' ;Parameter word
CRESP DB 0 ;Response byte
DW CPARM+1 ;Storage address
NOP ;Table end indicator
;
;*****
; Actual FILTER routine to shift up to HIGH$
;*****
TRAP JR START
OLDHI DW $-$ ;HIGH$ before filtering
DB MODDCB-TRAP-5
DB 'TRAP'
MODDCB DW $-$ ;Loaded with DCB pointer
DW 0
;
START JR NZ,OUTP1 ;Go if not PUT
LD A,C
TRAPBYT SUB 0 ;Space for trap char
RET Z ;Back with Z & A=0 if trapped
OUTP1 PUSH IX ;Save current pointer
LD IX,(MODDCB) ;P/u this module's DCB
RX01 EQU $-2
LD A,@CHNIO ;Chain to the next
RST 40
POP IX
MODEND RET
LENGTH EQU $-TRAP
END BEGIN
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