diff --git a/libsrc/atari/lseek.s b/libsrc/atari/lseek.s
index 27bc1ad65..259d89ab2 100644
--- a/libsrc/atari/lseek.s
+++ b/libsrc/atari/lseek.s
@@ -1,5 +1,5 @@
 ;
-; Christian Groessler, August 2000
+; Christian Groessler, May 2002
 ;
 ; this file provides the lseek() function
 ;
@@ -8,19 +8,21 @@
 
 
  	.export		_lseek
-	.import		incsp6,__errno,__oserror
-	.importzp	sreg
+	.import		incsp6,__oserror
+	.import		__inviocb,ldax0sp,ldaxysp,fdtoiocb
+	.import		__seterrno
+	.import		fd_table
+	.importzp	sreg,ptr1,ptr2,ptr3,ptr4
+	.importzp	tmp1,tmp2,tmp3
 	.include	"atari.inc"
 	.include	"errno.inc"
+	.include	"fd.inc"
 
-.proc	_lseek
-
-; dummy implementation, return -1 and ENOSYS errno value
-	jsr	incsp6
+; seeking not supported, return -1 and ENOSYS errno value
+no_supp:jsr	incsp6
 	lda	#<ENOSYS
-	sta	__errno
-	lda	#>ENOSYS
-	sta	__errno+1
+	ldx	#>ENOSYS
+	jsr	__seterrno
 	ldx	#0
 	stx	__oserror
 	dex
@@ -29,4 +31,184 @@
 	sta	sreg+1
 	rts
 
+parmerr:
+iocberr:jsr	incsp6
+	ldx	#255
+	stx	sreg
+	stx	sreg+1
+	jmp	__inviocb
+
+
+; lseek() entry point
+
+.proc	_lseek
+
+	cpx	#0		; sanity check whence parameter
+	bne	parmerr
+	cmp	#3		; valid values are 0..2
+	bcs	parmerr
+	sta	tmp1		; remember whence
+
+	ldy	#5
+	jsr	ldaxysp		; get fd
+	jsr	fdtoiocb	; convert to iocb in A, fd_table index in X
+	bmi	iocberr
+	sta	tmp3		; remember iocb
+
+	jsr	chk_supp	; check, whether seeking is supported by DOS/Disk
+	bcc	no_supp
+
+	ldx	tmp1		; whence
+	beq	cur
+	dex
+	beq	end
+
+; SEEK_SET
+	dex
+	stx	ptr3
+	stx	ptr3+1
+	stx	ptr4
+	stx	ptr4+1
+	beq	cont
+
+; SEEK_CUR
+cur:	ldx	tmp3
+	lda	#38		; NOTE
+	sta	ICCOM,x
+	jsr	CIOV		; read it
+	bmi	xxerr
+l01:	lda	ICAX3,x		; low byte of position
+	sta	ptr3
+	lda	ICAX4,x		; med byte of position
+	sta	ptr3+1
+	lda	ICAX5,x		; high byte of position
+	sta	ptr4
+	lda	#0
+	sta	ptr4+1
+	beq	cont
+
+; SEEK_END
+end:	ldx	tmp3
+	lda	#39		; get file size
+	sta	ICCOM,x
+	jsr	CIOV
+	bpl	l01
+
+; error returned from CIO
+xxerr:	sty	__oserror
+	bmi	iocberr
+
+; check for offset 0, SEEK_CUR (get current position)
+cont:	ldy	#3
+	jsr	ldaxysp		; get upper word
+	sta	ptr1
+	stx	ptr1+1
+	jsr	ldax0sp		; get lower word
+	stx	tmp2
+	ora	tmp2
+	ora	ptr1
+	ora	ptr1+1
+	bne	seek
+	lda	tmp1		; whence (0 = SEEK_CUR)
+	bne	seek
+
+; offset 0, SEEK_CUR: return current fp
+ret:	jsr	incsp6
+
+	lda	ptr4+1
+	sta	sreg+1
+	lda	ptr4
+	sta	sreg
+	ldx	ptr3+1
+	lda	ptr3
+
+.if 0
+	; return exactly the position DOS has
+	ldx	tmp3
+	lda	#38		; NOTE
+	sta	ICCOM,x
+	jsr	CIOV		; read it
+	bmi	xxerr
+	lda	#0
+	sta	sreg+1
+	lda	ICAX5,x		; high byte of position
+	sta	sreg
+	lda	ICAX3,x		; low byte of position
+	pha
+	lda	ICAX4,x		; med byte of position
+	tax
+	pla
+.endif
+
+	rts
+
+parmerr1: jmp	parmerr
+
+; we have to call POINT
+seek:	jsr	ldax0sp		; get lower word of new offset
+	clc
+	adc	ptr3
+	sta	ptr3
+	txa
+	adc	ptr3+1
+	sta	ptr3+1
+	lda	ptr1
+	adc	ptr4
+	sta	ptr4
+	lda	ptr1+1
+	adc	ptr4+1
+	sta	ptr4+1
+	bne	parmerr1	; resulting value too large
+
+	ldx	tmp3		; IOCB
+	lda	ptr3
+	sta	ICAX3,x
+	lda	ptr3+1
+	sta	ICAX4,x
+	lda	ptr4
+	sta	ICAX5,x
+	lda	#37		;POINT
+	sta	ICCOM,x
+	jsr	CIOV
+	bpl	ret
+	bmi	xxerr
+
 .endproc
+
+; check, whether seeking is supported
+; tmp3:		iocb
+; X:		index into fd_table
+;
+; We check, whether CIO function 39 (get file size) returns OK.
+; If yes, NOTE and POINT work with real file offset. @@@ check out more DOS versions @@@
+; If not, NOTE and POINT work with the standard ScNum/Offset values.
+; We remember a successful check in fd_table.ft_flag, bit 3.
+
+chk_supp:
+	lda	fd_table+ft_flag,x
+	and	#8
+	bne	supp
+
+; do the test
+unkn:	txa
+	pha
+	ldx	tmp3		; iocb to use
+	lda	#39		; get file size
+	sta	ICCOM,x
+	jsr	CIOV
+	bmi	notsupp		; error code ? should be 168 (invalid command)
+
+; seeking is supported on this DOS/Disk combination
+
+	pla
+	tax
+	lda	#8
+	ora	fd_table+ft_flag,x
+	sta	fd_table+ft_flag,x
+supp:	sec
+	rts
+
+notsupp:pla
+	clc
+	rts
+