;
;	Printer Filter - will watch for print sequences and convert
;			 them into new sequences
;
;	Setup so that an NEC P5XL will more closely emulate an Epson
;	FX-80 or JX-80.  Note that a P5XL is Epson LQ compatible
;
;	This program installs itself as a TSR and monitors int 17h which
;	is the bios printer support. If the requested operation is an
;	output byte for printer LPT1, the program is activated.
;
;	The printer control codes are intercepted as follows:
;
;		ESC % (select ROM/Downloaded Character set) is ignored.
;
;		ESC & (define download character set) is ignored.
;
;		ESC m (can not remember why I did this - had a program
;			that required it) is ignored.
;
;		ESC A (set line spacing in n/72nds of an inch) - converted
;			to setting in n/180ths of an inch by using the
;			formula n * 5 / 2.
;
;		ESC 3 (set line spacing in n/216ths of an inch) - converted
;			to setting in n/180ths of an inch by using the
;			formula n * 5 / 6.
;
;	For both the spacing setting conversions, they can not be exact in
;	all cases. If n is odd in ESC A, the error is 1/360th of an inch
;	per line. In the ESC 3 case, the max error is 5/1080 of an inch per
;	line - .306 in per 66 lines.
;
;	If you really have an NEC P5 and are sure of what is being sent
;	to it, the following changes will make the emulation more exact:
;
;		Change the labeled lines as shown here
;
;		esca:	mov	esc_seq,1
;
;		esc3:	mov	esc_seq,3
;
;		out_cd:	mov	al,1ch			;get FS
;
;	This uses an P5 mode to output in 360ths of an inch but is limited
;	to a max line spacing of 255/360 (approx .711 in). This should be
;	okay for most graphics related software - but I had one package
;	(SAMNA Word III) that was trying to set spacing to 1 inch and had
;	problems.  For ESC A, the result is exact. For ESC 3, the possible
;	error is reduced to a max of 2/1080 of an inch per line or .122 in.
;
;	Installation
;
;	Assemble and create com file as follows: (you can use the copy
;		of prnfiltr.com with this archive if no changes are needed).
;
;		masm prnfiltr;
;		link prnfiltr;
;		exe2bin prnfiltr
;		del prnfiltr.exe
;		ren prnfiltr.bin prnfiltr.com
;
;	Add prnfiltr as a line in your autoexec.bat file
;
;	Comments
;
;	This program produced by:
;
;		Tom Frank
;		6474 Skyward Ct.
;		Columbia, Md 21045
;
;	I can be reached on either of these two local BBS systems.
;
;		Computer Connection (202) 547-2008
;		Baltimore Washington PC Exchange (301) 381-6411
;	or
;		Compuserve 73200,1162.
;
;	Comments or enhancements are welcome.  I wrote this originally
;	so that my P5XL would work properly with several graphics /
;	word processors which did not support it directly.  As always,
;	use at your own risk.
;
code_seg	segment
		assume	cs:code_seg, ds:code_seg

		org	100h

first:		jmp	load_it

;	data storage

esc_seq		db	0			;non-zero if doing replacement
last_esc	db	0			;non-zero if last char was ESC
skip_cnt	dw	0			;non-zero-count of chars to ignore
lc_set		db	0			;non-zero if in download char set command
dwn_c1		db	0			;1st char in download sequence
five		db	5			;for mpy
twelve		db	0Ch			;for mpy

;	symbolic equates

old_int		equ	60h

routine		proc	far

		push	ds
		push	cs
		pop	ds

		cmp	dl,0			;lpt 1?
		jne	std_int			;no-do std code

		cmp	ah,0			;output byte function?
		jne	std_int			;no

		test	lc_set,0ffh		;in download?
		jz	go_on_1			;no
		jmp	download		;yes
go_on_1:	test	skip_cnt,0ffh		;skipping chars?
		jz	go_on_2			;no
		jmp	skip_chars		;yes		
go_on_2:	test	esc_seq,0ffh		;doing an esc sequence?
		jnz	do_seq			;yes

		test	last_esc,0ffh		;last char an escape?
		jz	see_esc			;no - see if this one is

;	last char was an escape - is this something we need to do?

		mov	last_esc,0		;reset escape seen flag
		cmp	al,41h			;is the char 'A'?
		je	esca			;yes
		cmp	al,33h			;is it '3'?
		je	esc3			;yes
		cmp	al,6Dh			;is the char 'm'?
		je	escm			;yes
		cmp	al,26h			;is the char '&'
		je	esc_and			;yes
		cmp	al,25h			;is the char '%'?
		je	esc_perc

;	not a sequence we change - output an escape and then this char

		push	ax
		mov	al,1bh			;get an escape
		int	old_int
		mov	dx,0
		pop	ax
		int	old_int

leave:		pop	ds
		iret

std_int:	int	old_int
		jmp	leave

see_esc:	cmp	al,1bh			;is char ESC?
		je	see_esc1		;yes
		jmp	chk_csub		;no-check for char substitution
see_esc1:	mov	last_esc,0ffh		;yes-set flag
		jmp	leave

esca:		mov	esc_seq,2		;get divisor-set flag
		jmp	leave

esc3:		mov	esc_seq,6
		jmp	leave

escm:		mov	esc_seq,0ffh
		jmp	leave

esc_and:	mov	lc_set,1		;set saying just starting
		jmp	leave

esc_perc:	mov	skip_cnt,2		;skip next 2 chars
		jmp	leave

do_seq:		push	ax			;save argumet to sequence
		mov	al,esc_seq		;get sequence code
		cmp	al,0ffh			;special one to skip?
		je	skip_it			;yes
out_cd:		mov	al,1bh			;get ESC
		int	old_int
		mov	dx,0
		mov	ah,0
		mov	al,33h			;get '3'
		int	old_int
		pop	ax			;get arg back
		mul	five			;times multiplier
		div	esc_seq			;divide by argument
						;result is in 180ths of an inch
		mov	dx,0
		mov	ah,0
		int	old_int
		mov	esc_seq,0
		jmp	leave

skip_it:	pop	ax
		mov	esc_seq,0
		jmp	leave

skip_chars:	dec	skip_cnt		;reduce count by 1
		jmp	leave

download:	cmp	lc_set,1		;only seen ESC & so far?
		jne	down_1			;no-seen more
		mov	lc_set,2		;yes-prepare for 1st char id
		jmp	leave
down_1:		cmp	lc_set,2		;is this the 1st char?
		jne	down_2			;no
		mov	dwn_c1,al		;yes-save it
		mov	lc_set,3		;say looking for 2nd char
		jmp	leave
down_2:		sub	al,dwn_c1		;get #chars to define
		inc	al			;normalize it
		mul	twelve			;times chars to skip
		mov	skip_cnt,ax		;save as chars to skip
		mov	lc_set,0		;clear download seq flag
		jmp	leave

chk_csub:	cmp	al,7Fh
		ja	chk_1			;jump if user defined
		jmp	std_int			;no-just output it
chk_1:		cmp	al,0B2h			;do we want to sub it?
		jne	chk_2			;no
		mov	al,0DAh
		jmp	std_int
chk_2:		cmp	al,0B5h			;do we want to sub it?
		jne	chk_3			;no
		mov	al,0C4h
		jmp	std_int
chk_3:		cmp	al,0B0h			;do we want to sub it?
		jne	chk_4			;no
		mov	al,0C0h
		jmp	std_int
chk_4:		cmp	al,0B1h			;do we want to sub it?
		jne	chk_5			;no
		mov	al,0B3h
		jmp	std_int
chk_5:		cmp	al,0B4h			;do we want to sub it?
		jne	chk_6			;no
		mov	al,0D9h
		jmp	std_int
chk_6:		cmp	al,0B7h			;do we want to sub it?
		jne	chk_7			;no
		mov	al,0BFh
		jmp	std_int
chk_7:		cmp	al,0B3h			;do we want to sub it?
		jne	chk_8			;no
		mov	al,0C3h
		jmp	std_int
chk_8:		cmp	al,0B6h			;do we want to sub it?
		jne	chk_9			;no
		mov	al,0C1h
		jmp	std_int
chk_9:		cmp	al,0B8h			;do we want to sub it?
		jne	chk_A			;no
		mov	al,0B4h
		jmp	std_int
chk_A:		cmp	al,0B9h			;do we want to sub it?
		jne	chk_B			;no
		mov	al,0C2h
		jmp	std_int
chk_B:		cmp	al,0BAh			;do we want to sub it?
		jne	chk_C			;no
		mov	al,0C5h
		jmp	std_int
chk_C:		jmp	std_int

routine		endp

load_it		proc	near
		mov	ah,35h			;get int vector
		mov	al,17h			;bios printer spt
		int	21h
		push	es
		pop	ds			;get segment
		mov	dx,bx			;get offset
		mov	ah,25h			;set int vector
		mov	al,old_int		;for old as this one
		int	21h

		push	cs
		pop	ds			;get our address
		mov	dx,offset routine
		mov	ah,25h
		mov	al,17h
		int	21h

		mov	dx,offset load_it
		int	27h

load_it		endp

code_seg	ends

		end	first