;**************************  POPD.ASM  ************************************
;
; 	POPD.ASM version 1.2
;
;	Usage:
;		POPD
;		 Will change to the most recently stored directory on 
;		 the stack.
;		POPD +n
;		 Will delete the directory stored in position n from
;		 the bottom of the stack, starting at one on the bottom.
;	Purpose: This program will either change to the topmost directory
;		on the stack created by PUSHD v. 1.1 (first usage), OR
;		will delete from the stack the directory stored at position
;		n, where 1 is the bottom of the stack.  In either case, 
;		the program will compress the stack, to remove any empty
;		storage space, and will update the position for the next
;		storage accordingly.
;	Preconditions: 
;		1) PUSHD must be hooked to interrupt 13.
;		2) PUSHD must have the same commands from beginning
;		 to after definition of push6d, so that it knows how
;		 to access the resident code of PUSHD.  (This includes
;		 the fact that there are 6 locations for directory storage
;		 which are 67 bytes in size.)
;		3) PUSHD must have the signature "PUSHD VERSION 1.1".
;		4) PUSHD must look for 7788 in ax and 7789 in bx for
;		 a call to int 13.
;		5) PUSHD must put the value of 7789 into ax and 7788
;		 into bx upon calling int 13 by DIRS, if PUSHD is installed.
;		6) PUSHD will set the data segment to be equal to the data
;		 segment that PUSHD used to install itself.
;		7) DS:[nextpush] (ds of PUSHD installation) must hold
;		 the location of the next empty directory stack location.
;	Postconditions:
;		1) An appropriate error message will be printed out on
;		 standard error, OR
;		2) DS:[nextpush] will be decremented by one directory
;		 storage unit (67 bytes) AND EITHER:
;		2a) the current directory will become the directory on
;		 top of the stack (usage #1) OR
;		2b) the current directory will remain the same, but the
;		 targeted directory (1-6) will be deleted with the 
;		 directories on the stack above shifted down one storage
;		 unit.
;		3) An error code will be generated.
;	Error Codes:
;		0) Successful completion.
;	      249) Error popping directory.
;	      250) Directory Stack is empty.
;	      251) Command Line parameter exceeds Directory Stack depth.
;	      252) Illegal Characters on the Command Line.
;	      253) Command Line parameter not in valid directory range (1-6).
;	      254) Invalid Command Line delimiter.
;	      255) PUSHD is not installed.
;	Version changes:
;		POPD v. 1.2:	July 23, 1990.
;			enhanced code using suggestions from David
;			 Kirschbaum (Toad Hall).
;			"cleaned out" directory stack characters that 
;			 were left after a POPD, which might not be over-
;			 written on next PUSHD.
;		POPD v. 1.1:	July 14, 1990.
;			added "+n" option.
;			changed hooked interrupt to 13 from 16.
;			added error codes for abnormal termination.
;			changed writes from vector 9 to vector 40h of 
;			 int 21h.
;			changed program termination from int 20h to vector
;			 4Cxxh int 21h, where xx represents the error code
;			 generated.
;			removed stack wrap around.
;		POPDIR v. 1.0: Copyright 1986, Ziff-Davis Publishing Co.
;			From PC Magazine, May 27, 1986
;			No changes.
;	Future Enhancements:
;		1) Change the "n" parameters so that it refers to the
;		 top of the stack, not the bottom (RELATIVE from the top
;		 NOT ABSOLUTE from the bottom).
;		2) Make program an .EXE file.
;		3) Correct error caused by 3 or more blanks on the
;		 end of the command line tail.
;		4) Add a help option, ?, from the command line.
;		5) Add -c option to clear ALL directories on stack.
;		6) Add -n option to clear n topmost directories on stack.
;
;	Created by:
;	  (from ideas based in UNIX and from PC Magazine's POPDIR.COM)
;		William P. Sarra
;			CIS:71041,347
;			arpa: sarra@TOPAZ.RUTGERS.EDU
;	uucp: ...{ames,cbosgd, harvard, moss}!rutgers!topaz.rutgers.edu!sarra
;
;*****************************************************************************

main	group	CSEG
CSEG	segment	public	para	'code'
	ASSUME	CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN

org	80h
cmd_line	label byte

org	100h				;.COM file

BEGIN:	jmp	Start			;program starts here

signature	db	'PUSHD VERSION 1.1'
SIG_LEN		equ	$-signature

savedint13	dd	?		;used to be identical to pushd

nextpush	dw	offset MAIN:push1d	;next place to save a dir

push1d	db	67 dup (0)
push2d	db	67 dup (0)
push3d	db	67 dup (0)
push4d	db	67 dup (0)
push5d	db	67 dup (0)
push6d	db	67 dup (0)

;up to here must be EXACTLY identical in PUSHD, POPD and DIRS.

;****************************************************************
;
;	Data Section.
;
;****************************************************************

not_inst	db	'Must run PUSHD.COM before POPD.COM'
		db	' will do anything.',13,10,10
NOT_INST_LN	equ	$-not_inst
errpop1		db	'Error popping the current directory',13,10,10
ERRPOP1_LN	equ	$-errpop1
empty_stack	db	'No directories on the stack.',13,10
EMPTY_STACK_LN	equ	$-empty_stack

BLANK		equ	20h		;' '
DELIMITER	equ	2Bh		;'+'

LOW_LIMIT	equ	31h		;'1'
UP_LIMIT	equ	36h		;'6'

bad_delim	db	'Invalid Delimiter.',13,10
BAD_DELIM_LN	equ	$-bad_delim
in_dir_msg	db	'Valid directories range between 1 and 6.',13,10
IN_DIR_MSG_LN	equ	$-in_dir_msg
bs_online	db	'Invalid Command Line Parameters.',13,10
BS_ONLINE_LN	equ	$-bs_online
not_pushed	db	'Parameter exceeds Directory Stack depth.',13,10
NOT_PUSHED_LN	equ	$-not_pushed

params		db	?		;storage for command line tail

targ_dir	dw	offset main:push1d	;address storage

;*******************************************************************
;
;	Code Starts.
;
;*******************************************************************

START:

	;get and store command line tail as it may get destroyed

	cld

;v1.2	I entirely rewrote this section.  What a kludge!

	mov	si,offset cmd_line		;command line tail
	lodsb					;snarf length byte, bump SI

	;SI -> first char (probably a space!)

	xor	ah,ah				;clear msb
	mov	cx,ax				;into CX
	push	ax				;save length on stack for later
	jcxz	INST_CHK			;CX is 0, branch

	;We have a non-null command line

	mov	di,offset MAIN:params		;store since it may get
	rep	movsb				;destroyed

INST_CHK:

	;is PUSHD already installed ?

	mov	ax,7788h			;signature request
	mov	bx,7789h			;signature request
	mov	si,offset MAIN:signature	;point ds:si to signature
	int	13h				;is it installed ?
	
	ASSUME	DS:CSEG			;assume TSR's DS	v1.2
	
	cmp	bx,7788h			;were ax and bx switched ?
	jne	NOTINSTALLED			;no
	cmp	ax,7789h			;were ax and bx switched ?
	je	ISINSTALLED		;v1.2

NOTINSTALLED:

	;PUSHD is not installed, terminate with an error message.

	mov	dx,offset MAIN:not_inst		;error message
	mov	cx,NOT_INST_LN
	mov	al,0FFH			;ERRORLEVEL		v1.2
	jmp	Msg_Term		;display, terminate	v1.2

ISINSTALLED:

	; check for command line parameters

	pop	cx			;get saved cx
	push	DS			;store current ds

	mov	ax,CS			;v1.2
	mov	DS,ax			;v1.2
	ASSUME	DS:MAIN			;reminder			v1.2

	or	cx,cx			;check for no parameters
	jnz	ERR1
	jmp	CHK_STACK

ERR1:	mov	di,offset MAIN:params
	mov	al,BLANK		;remove leading blanks
	repe	scasb
	dec	di
	inc	cx
	mov	si,di			;save position
	lodsb				;snarf first char, bump SI	v1.2
	cmp	al,BLANK		;space?				v1.2

	jne	ERR2
	jmp	CHK_STACK		;only blanks on tail
ERR2:
	cmp	al,DELIMITER		;is it '+' delimiter?		v1.2
	je	CHK_VALUE

	;Current character is not valid -- Print error message and exit

	mov	dx,offset MAIN:bad_delim	;bad delimiter message
	mov	cx,BAD_DELIM_LN
	mov	al,0FEH			;ERRORLEVEL		v1.2
	jmp	Msg_Term		;display, terminate	v1.2

CHK_VALUE:

	;check that parameter values are within specified limits

	dec	cx

;v1.2	Recoded
	lodsb				;snarf digit after '+', bump SI
	cmp	al,UP_LIMIT		;above '6'?
	ja	INVALID_DIR		;yep, bogus
	cmp	al,LOW_LIMIT		;below '1'?'
	jnb	FINISH_CHK		;nope, in range ['1'..'6']

INVALID_DIR:
	
	;invalid directory number -- print message and exit

	mov	dx,offset MAIN:in_dir_msg	;error message
	mov	cx,IN_DIR_MSG_LN
	mov	al,0FDH				;ERRORLEVEL	v1.2
	jmp	Msg_Term			;display, terminate	v1.2

FINISH_CHK:

	;continue checking for invalid characters on the command line
	; after getting valid characters.

	dec	cx			;decr cmdline count for digit	v1.2
	jcxz	VALID_DIR

	mov	ah,al			;save digit			v1.2
	mov	al,BLANK		;check for following nonblank
	repne	scasb			;characters
	mov	al,ah			;replace digit			v1.2

	jcxz	VALID_DIR

	;Invalid characters on the command line -- print message and
	; exit

	mov	dx,offset MAIN:bs_online	;error message
	mov	cx,BS_ONLINE_LN
	mov	al,0FCH				;ERRORLEVEL		v1.2
	jmp	Msg_Term			;display, terminate	v1.2

VALID_DIR:

	;get stack location of targeted directory

	xor	ah,ah			;clear ah	v1.2
	mov	cx,67			;multiplier	v1.2
	sub	al,'1'			;'0'..'5' range
	mul	cx			;directory offset	v1.2
	add	ax,offset MAIN:push1d	;base directory buffer	v1.2
	mov	targ_dir,ax		;save it	v1.2

CHK_DIR:

	;check that the directory stack depth is at least to
	; the targeted directory location

	pop	DS				;restore ds
	ASSUME DS:CSEG		;reminder	v1.2

	mov	bp,DS:[nextpush]		;get location of last
	sub	bp,67				;pushed directory
	cmp	bp,CS:[targ_dir]		;make sure directory stack
	jge	PROCESS_DIRS			;is deep enough for params

	;the parameter number "n" exceeds the depth of the directory stack
	; print a message and exit with an error

	mov	dx,offset MAIN:not_pushed	;error message
	mov	cx,NOT_PUSHED_LN
	mov	al,0FBH				;ERRORLEVEL	v1.2
	jmp	Msg_Term			;display, terminate	v1.2	

PROCESS_DIRS:

	;stack is deep enough, if the targeted directory location is
	; at the top of the stack, simply decrement ds:[nextpush],
	; effectively erasing the top directory and exit successfully.

	jne	NOT_TOP			;check on popping top of stack, set
	mov	DS:[nextpush],bp	;nextpush to be top of current stack
	jmp	SHORT EXITING

NOT_TOP:

	;targeted directory is in the middle of stack, so decrement
	; DS:[nextpush] to reflect stack deletion, set the current location
	; to be the targeted stack location, and...

	mov	DS:[nextpush],bp	;set nextpush to top of stack
	mov	bp,CS:[targ_dir]	;set bp to be targeted directory
	mov	ax,DS			;v1.2
	mov	ES,ax			;v1.2
	ASSUME	ES:CSEG			;reminder 	v1.2
	
;handy constants	v1.2
	mov	dx,DS:[nextpush]	;v1.2
	mov	ax,67			;v1.2
	mov	bx,bp			;v1.2

DIR_LOOP:

	;...cycle through the stack copying the contents of the directory
	; above the current location into the current location...

	add	bp,ax			;go to next pushed directory	v1.2
	cmp	bp,dx			;DS:[nextpush]	v1.2
	je	LAST_DIR		;found the last directory
	mov	si,bp			;copy contents to
	mov	di,bx			;targ_dir	v1.2
	mov	cx,ax			;67	v1.2
	rep	movsb
	add	bx,ax			;targ_dir,67	v1.2
	jmp	SHORT DIR_LOOP		;continue

LAST_DIR:

	;...until the last directory is copied into the nearest lower
	; storage location

	mov	si,bp			;one more copy
	mov	di,bx			;targ_dir v1.2
	mov	cx,ax			;67	v1.2
	rep	movsb
	jmp	SHORT EXITING		;exit successfully

CHK_STACK:

	;handle parameterless command

	;get the location of the directory most recently saved by pushd
	; (top of stack)

	pop	DS				;restore ds
	ASSUME DS:CSEG			;reminder	v1.2

	mov	bp,DS:[nextpush]		;get the next push location
	sub	bp,67				;back up one to the last push
	cmp	DS:[nextpush],offset MAIN:push1d	;empty stack?
	jne	NOTEMPTY			;no

	;directory stack is empty (ds:[nextpush] = location of first
	; storage location) -- print message and exit with error

	mov	dx,offset MAIN:empty_stack	;error message
	mov	cx,EMPTY_STACK_LN
	mov	al,0FAH				;ERRORLEVEL	v1.2
	jmp	Msg_Term			;display terminate	v1.2

NOTEMPTY:

	;directory stack is not empty -- change to directory on top of 
	; stack

	;set the current directory 

	mov	dx,bp			;load DS:dx with directory to set
	mov	ah,3bh			;dos function number
	int	21h			;set current dir back
	jc	ERRPOP			;branch on error

	;set the current drive if previous commands are successful  

	mov	al,DS:[bp]			;get drive letter from path
	sub	al,'A'				;convert to binary (0=A, 1=B)
	mov	dl,al
	mov	ah,0eh				;dos function number
	int	21h				;set drive
	jc	ERRPOP

	mov	DS:[nextpush],bp	;update [nextpush] if successful

EXITING:

	;clear up previous directory storage	v1.2

	mov	ax,DS				;v1.2
	mov	ES,ax				;v1.2
	ASSUME	ES:CSEG				;reminder	v1.2

	mov	cx,67				;v1.2
	mov	di,DS:[nextpush]		;v1.2
	mov	al,BLANK			;v1.2
	rep	stosb				;v1.2

	;exit successfully with no message

	mov	ax,4c00h
	int	21h 				;exit

ERRPOP:

	;An error occurred while attempting to change to targeted
	; directory -- print error message and exit with error

	mov	dx,offset MAIN:errpop1		;error message
	mov	cx,ERRPOP1_LN
	mov	al,0F9H			;ERRORLEVEL	v1.2
;	jmp	Msg_Term		;display, terminate	v1.2

;*********************************************************************
;
;	PROCEDURE:	Msg_Term
;
;	Purpose: This procedure will handle the printing of error messages
;		and termination with the correct errorlevel.
;	Preconditions: 
;		DX = error message address.
;		CX = error message length.
;		AL = ERRORLEVEL.
;	Postconditions: 
;		The program will be terminated with the appropriate
;		 error message printed on standard error and with the
;		 the errorlevel stored in al.
;
;***************************************************************************

Msg_Term	PROC	NEAR
;v1.2	Common routine to display error messages and terminate.

	push	CS			;insure this is so
	pop	DS
	ASSUME	DS:MAIN

	push	ax				;save ERRORLEVEL in AL
	mov	bx,2				;write to stderr
	mov	ah,40h				;dos function number
	int	21h				;show error message
	pop	ax				;restore ERRORLEVEL in AL
	mov	ah,4CH				;terminate process
	int	21h

Msg_Term	ENDP

	
CSEG	ends
end	BEGIN				;start execution at BEGIN