TrapScreen

TRAPSCR is an Assembler program, which I wrote many years ago.

It demonstrates the two undocumented methods for calling DOS functions from TSR (Terminate and Stay Resident) programs.

The needed files are contained in TRAPSCR.ZIP.

The program intercepts interrupts 5, 28h and 2Fh.
Interrupt 5 (PrintScreen key) triggers the resident part.
The Busy flag is checked and if it is turned on, that means the program is currently saving a file, and the keypress is ignored.
If the program is not busy, then the DOS reentrancy flag is checked.
If it is off (meaning DOS function can be safely used), then the procedure for saving the screen to file is called.
If it is on, then the Request flag is turned on, and the program waits for interrupt 28h.
When the program receives interrupt 28h and the Request flag is turned on, then it calls the procedure for saving the screen to file.
More information about DOS interrupts can be found at Ralf Brown's Interrupt List, or try to find a copy of the Technical Reference Manual by Dan Rollins.



; file JASM.MAC - useful macros

; get the interrupt vector
@get_vec MACRO	vec
	MOV	AH,35h
	MOV	AL,vec
	INT	21h
	ENDM

; set the interrupt vector
@set_vec MACRO	vec
	MOV	AH,25h
	MOV	AL,vec
	INT	21h
	ENDM

; print string to screen
@print	MACRO	str
	LOCAL	around,msg
	PUSH	AX
	PUSH	DX
	MOV	AH,9
	MOV	DX,OFFSET msg
	INT	21h
	POP	DX
	POP	AX
	JMP	SHORT around

msg	DB	str,0Dh,0Ah,'$'
around:
	ENDM

; print string to screen	
@print_r MACRO	addr
	PUSH	DX
	MOV	DX,addr
	MOV	AH,9
	INT	21h
	POP	DX
	ENDM

; quit the program with a given exit code
@exit	MACRO	ret_code
	MOV	AH,4Ch
	MOV	AL,ret_code
	INT	21h
	ENDM
	
; end of file JASM.MAC

--------------------------

; file TRAPSCR.ASM

prog_id EQU	0
prog_ver_hi EQU 2
prog_ver_lo EQU 6
prog_name EQU	"TRAPSCR v2.63"

	INCLUDE jasm.mac

attr	EQU	0Fh

len	EQU	40

escape	EQU	1
bkspace EQU	14
retrn	EQU	28

video	SEGMENT AT 0B800h
video	ENDS

code	SEGMENT
	ASSUME	CS:code,DS:code

	ORG	2Ch
env	LABEL	WORD

	ORG	5Ch
data	LABEL	BYTE
psp	DW	?
old_5	DD	?		
old_28	DD	?
old_2f	DD	?
reentr	DD	?
request DB	0
busy	DB	0
s_curs	DB	0
string	DB	'File name: '
fname	DB	len DUP(0)
enddata LABEL	BYTE
buf	DB	len*2 DUP(?)

	ORG	80h
argc	DB	?
argv	LABEL	BYTE

	ORG	100h
start:
	JMP	main
; data
id	DB	0C0h
p_name	DB	prog_name,0

;***************************************************************
; Interrupt communication
;***************************************************************

int_5:
	CLI
	PUSH	DS
	PUSH	ES
	PUSH	AX
	PUSH	BX
	PUSH	CX
	PUSH	DX
	TEST	BYTE PTR CS:[busy],1
	JNZ	int5_ret
	MOV	BYTE PTR CS:[request],0
	MOV	BX,WORD PTR CS:[reentr+2]
	MOV	ES,BX
	MOV	BX,WORD PTR CS:[reentr]
	MOV	AL,BYTE PTR ES:[BX]
	OR	AL,AL
	JNZ	set_request
	CALL	save
	JMP	SHORT int5_ret
set_request:
	MOV	BYTE PTR CS:[request],1
int5_ret:
	POP	DX
	POP	CX
	POP	BX
	POP	AX
	POP	ES
	POP	DS
	STI
	IRET

int_28:
	STI
	TEST	BYTE PTR CS:[request],1
	JZ	old_int28
	TEST	BYTE PTR CS:[busy],1
	JNZ	old_int28
	CALL	save
old_int28:
	JMP	CS:[old_28]

int_2f:
	STI
	CMP	AH,CS:[id]
	JNZ	old_int2f
	CMP	DL,'J'
	JNZ	old_int2f
	CMP	DH,prog_id
	JNZ	old_int2f
	CMP	AL,1
	JZ	uninstall
	CMP	AL,2
	JE	curs
	CMP	AL,3
	JE	ret_curs
	MOV	AH,1
	MOV	DH,DL
	MOV	DL,'j'
	PUSH	CS
	POP	ES
	ASSUME	ES:code
	LEA	BX,p_name
	MOV	CH,prog_ver_hi
	MOV	CL,prog_ver_lo
	IRET
old_int2f:
	JMP	CS:[old_2f]

curs:
	MOV	AH,CS:[s_curs]
	XOR	AH,1
	MOV	CS:[s_curs],AH
	IRET
ret_curs:
	MOV	AH,CS:[s_curs]
	IRET

uninstall:
	PUSH	ES
	@get_vec 2Fh
	MOV	AX,CS
	MOV	BX,ES
	CMP	AX,BX
	JNE	cannot_unload
	@get_vec 28h
	MOV	AX,CS
	MOV	BX,ES
	CMP	AX,BX
	JNE	cannot_unload
	@get_vec 05h
	MOV	AX,CS
	MOV	BX,ES
	CMP	AX,BX
	JNE	cannot_unload
	POP	ES
	PUSH	DS
	MOV	DS,WORD PTR CS:[old_2f+2]
	MOV	DX,WORD PTR CS:[old_2f]
	CLI
	@set_vec 2Fh
	MOV	DS,WORD PTR CS:[old_28+2]
	MOV	DX,WORD PTR CS:[old_28]
	@set_vec 28h
	MOV	DS,WORD PTR CS:[old_5+2]
	MOV	DX,WORD PTR CS:[old_5]
	@set_vec 05h
	MOV	AX,WORD PTR CS:[psp]
	MOV	ES,AX
	MOV	AH,49h
	INT	21h
	POP	DS
	MOV	AH,1
	STI
	IRET
cannot_unload:
	POP	ES
	XOR	AH,AH
	IRET


;***************************************************************
; Screen saving
;***************************************************************

save	PROC	NEAR
	PUSH	AX
	PUSH	BX
	PUSH	CX
	PUSH	DX
	PUSH	DS
	PUSH	ES
	MOV	BYTE PTR CS:[request],0
	MOV	BYTE PTR CS:[busy],1
	MOV	AX,CS
	MOV	DS,AX
	MOV	ES,AX
	ASSUME	DS:code,ES:code
	LEA	DX,fname
	CALL	getname
	MOV	AH,3Ch
	LEA	DX,fname
	XOR	CX,CX
	INT	21h		; open file
	JC	error
	MOV	BX,AX
	MOV	AX,video
	MOV	DS,AX
	ASSUME	DS:video
	XOR	DX,DX
	MOV	AH,40h
	MOV	CX,4000
	INT	21h		; write screen
	JC	error
	MOV	AH,CS:[s_curs]
	CMP	AH,1
	JNE	no_save_cursor
	PUSH	BX
	MOV	AH,03h
	MOV	BH,0
	INT	10h		; get cursor position and size
	PUSH	CS
	POP	DS
	ASSUME	DS:code
	MOV	DS:[buf],DH
	MOV	DS:[buf+1],DL
	MOV	DS:[buf+2],CH
	MOV	DS:[buf+3],CL
	MOV	DX,OFFSET buf
	POP	BX
	MOV	AH,40h
	MOV	CX,4
	INT	21h		; save cursor position and size
	JC	error
no_save_cursor:
	MOV	AH,3Eh
	INT	21h		; close file
error:
	MOV	BYTE PTR CS:[busy],0
	POP	ES
	POP	DS
	POP	DX
	POP	CX
	POP	BX
	POP	AX
	RET
save	ENDP

getname PROC	NEAR
	PUSH	SI
	PUSH	DI
	PUSH	DX
	LEA	DI,buf
	MOV	CX,len
	CALL	getline
	MOV	CX,len
	CALL	clrline
	POP	DI
	MOV	CX,len
	CALL	getstr
	MOV	CX,len
	LEA	SI,buf
	CALL	putline
	POP	DI
	POP	SI
	RET
getname ENDP

clrline PROC  NEAR
	PUSH	DI
	PUSH	ES
	MOV	AX,video
	MOV	ES,AX
	ASSUME	ES:video
	XOR	AX,AX
	XOR	DI,DI
	REP	STOSW
	POP	ES
	POP	DI
	RET
clrline ENDP

getline PROC	NEAR
	PUSH	SI
	PUSH	DI
	PUSH	DS
	MOV	AX,video
	MOV	DS,AX
	XOR	SI,SI
	ASSUME	DS:video
	REP	MOVSW
	POP	DS
	POP	DI
	POP	SI
	RET
getline ENDP

putline PROC	NEAR
	PUSH	SI
	PUSH	DI
	PUSH	ES
	MOV	AX,video
	MOV	ES,AX
	XOR	DI,DI
	ASSUME	ES:video
	REP	MOVSW
	POP	DS
	POP	DI
	POP	SI
	RET
putline ENDP

getstr	PROC	NEAR
	PUSH	SI
	PUSH	DI
	PUSH	DS
	PUSH	ES
	POP	DS
	MOV	SI,DI
	MOV	BH,attr
	XOR	AL,AL
	PUSH	CX
	REPNE	SCASB
	DEC	DI
	POP	CX
next_key:
	CALL	printstr
	STI
	XOR	AX,AX
	INT	16h
	CLI
	CMP	AH,escape
	JE	esc_key
	CMP	AH,bkspace
	JE	bk_char
	CMP	AH,retrn
	JE	return
	OR	CX,CX
	JZ	next_key
	STOSB
	DEC	CX
	JMP	next_key
esc_key:
	MOV	BYTE PTR ES:[SI],0
	JMP	SHORT return
bk_char:
	CMP	DI,SI
	JE	next_key
	DEC	DI
	MOV	BYTE PTR ES:[DI],0
	INC	CX
	JMP	next_key
return:
	MOV	BYTE PTR ES:[DI],0
	POP	DS
	POP	DI
	POP	SI
	RET
getstr	ENDP

printstr PROC	NEAR
	PUSH	SI
	PUSH	DI
	PUSH	ES
	MOV	AX,video
	MOV	ES,AX
	ASSUME	ES:video
	XOR	DI,DI
	LEA	SI,string
	MOV	AH,BH
next_char:
	LODSB
	STOSW
	OR	AL,AL
	JNZ	next_char
	DEC	DI
	DEC	DI
	MOV	AL,'_'
	MOV	AH,10001111b
	STOSW
	XOR	AX,AX
	STOSW
	POP	ES
	POP	DI
	POP	SI
	RET
printstr ENDP

;***************************************************************
; Instalation
;***************************************************************

main:
	@print_r <OFFSET jstr>

; Check parameters
	MOV	CL,CS:[argc]
	OR	CL,CL
	JZ	nopar
	XOR	CH,CH
	LEA	SI,argv
next:
	LODSB
	CMP	AL,'?'
	JE	help
	CMP	AL,'h'
	JE	help
	CMP	AL,'t'
	JE	inst_test
	CMP	AL,'u'
	JE	jmp_unload
	CMP	AL,'c'
	JE	s_cursor
	CMP	AL,0Dh
	JE	nopar
	CMP	AL,0Ah
	JE	nopar
	OR	CX,CX
	JZ	nopar
	JMP	next
nopar:
	JMP	begin
jmp_unload:
	JMP	unload
help:
	@print_r <OFFSET helpmsg>
	@exit	0

inst_test:
	CALL	checkinst
	MOV	AL,AH
	MOV	AH,4Ch
	INT	21h

s_cursor:
	CALL	checkinst
	CMP	AH,1
	JE	c_installed
	MOV	AH,1
	MOV	CS:[_s_curs],AH
	@print_r <OFFSET cursonmsg>
	JMP	next
c_installed:
	MOV	AH,CS:[id]
	MOV	AL,2
	MOV	DH,prog_id
	MOV	DL,'J'
	INT	2Fh
	CMP	AH,1
	JE	curs_on
	@print_r <OFFSET cursoffmsg>
	@exit	0
curs_on:
	@print_r <OFFSET cursonmsg>
	@exit	0

unload:
	CALL	checkinst
	CMP	AH,1
	JNE	not_installed
	MOV	AH,CS:[id]
	MOV	AL,1
	MOV	DH,prog_id
	MOV	DL,'J'
	INT	2Fh
	CMP	AH,1
	JE	success
	@print	"Cannot unload program."
	@exit	2
success:
	@print	"Program unloaded."
	@exit	0
not_installed:
	@print	"Program is not loaded in memory."
	@exit	1

installed:
	@print	"Already loaded in memory. Use /u to unload."
	MOV	AH,CS:[id]
	MOV	AL,3
	MOV	DH,prog_id
	MOV	DL,'J'
	INT	2Fh
	CMP	AH,1
	JNE	curs_off
	@print_r <OFFSET cursonmsg>
	@exit	1
curs_off:
	@print_r <OFFSET cursoffmsg>
	@exit	1
begin:
	CALL	checkinst
	CMP	AH,1
	JE	installed
	PUSH	CS
	POP	ES
	ASSUME	ES:code
	LEA	SI,_data
	LEA	DI,data
	MOV	CX,(OFFSET _enddata - OFFSET _data + 1) / 2
	REP	MOVSW
	MOV	CS:[psp],CS
	MOV	AX,CS:[env]
	MOV	ES,AX
	MOV	AH,49h
	INT	21h

	MOV	AH,34h
	INT	21h
	MOV	CS:WORD PTR [reentr],BX
	MOV	CS:WORD PTR [reentr+2],ES

	@get_vec 2Fh
	MOV	CS:WORD PTR [old_2f+2],ES
	MOV	CS:WORD PTR [old_2f],BX
	LEA	DX,int_2f
	CLI
	@set_vec 2Fh
	STI
	@get_vec 28h
	MOV	CS:WORD PTR [old_28+2],ES
	MOV	CS:WORD PTR [old_28],BX
	LEA	DX,int_28
	CLI
	@set_vec 28h
	STI
	@get_vec 05h
	MOV	CS:WORD PTR [old_5+2],ES
	MOV	CS:WORD PTR [old_5],BX
	LEA	DX,int_5
	CLI
	@set_vec 05h
	STI
	@print	"Loaded in memory."
	MOV	AX,OFFSET main + 1
	MOV	CL,4
	SHR	AX,CL
	INC	AX
	MOV	DX,AX
	MOV	AX,3100h
	INT	21h


checkinst PROC	NEAR
	MOV	AH,CS:[id]
	XOR	AL,AL
	MOV	DH,prog_id
	MOV	DL,'J'
	INT	2Fh
	CMP	AH,1
	JNE	not_inst
	CMP	DL,'j'
	JNE	not_inst
	RET
not_inst:
	XOR	AH,AH
	RET
checkinst ENDP

jstr	DB	0Dh,0Ah,prog_name, "  Copyright (c) 1992-1993 by Jogy.",0Dh,0Ah,0Dh,0Ah,'$'
helpmsg DB	"Options:",0Dh,0Ah
	DB	"  /h - this help.",0Dh, 0Ah
	DB	"  /c - enable/disable cursor saving",0Dh,0Ah
	DB	"  /t - errorlevel 1 if installed in memory, 0 otherwise",0Dh,0Ah
	DB	"  /u - unload from memory.",0Dh,0Ah,'$'
cursonmsg DB	"Cursor saving on.",0Dh,0Ah,'$'
cursoffmsg DB	"Cursor saving off.",0Dh,0Ah,'$'

_data	DW	?
	DD	?
	DD	?
	DD	?
	DD	?
	DB	0
	DB	0
_s_curs DB	0
	DB	'File name: '
	DB	len DUP(0)
_enddata LABEL	BYTE

	.ERRNZ (OFFSET _enddata - OFFSET _data) - (OFFSET enddata - OFFSET data)

code	ENDS
	END	start

; end of file TRAPSCR.ASM

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