diff --git a/mach/i80/libmon/mon.s b/mach/i80/libmon/mon.s
new file mode 100644
index 000000000..28f908972
--- /dev/null
+++ b/mach/i80/libmon/mon.s
@@ -0,0 +1,105 @@
+.define .mon
+
+! Monitor call
+! Expects on stack:	monitor call number
+!			parameters
+! Implemented are the following monitor calls:
+! number  1:	exit
+! number  3:	read
+! number  4:	write
+! number  5:	open
+! number  6:	close
+! number 54:	ioctl
+! If called with a number of a call that is not implemented,
+! a trap is generated.
+
+.mon:	pop h
+	shld .retadr
+	mov h,b
+	mov l,c
+	shld .bcreg
+
+	pop h		! monitor call number
+	mov a,l
+	cpi 1
+	jz monexit	! is it an exit?
+	cpi 3
+	jz monread	! is it a read?
+	cpi 4
+	jz monwrite	! is it a write?
+	cpi 5
+	jz monopen	! is it an open?
+	cpi 6
+	jz monclose	! is it a close?
+	cpi 54
+	jz monioctl	! is it an ioctl?
+	jmp ebadmon	! trap
+
+monexit:
+	rst 4
+
+monread:
+	pop h		! file-descriptor, not used
+	pop h		! hl = pointer to output buffer
+	pop d		! de = number of bytes to be read
+	lxi b,0		! bc will contain the number of bytes actually read
+1:	mov a,d
+	ora e
+	jz 2f
+	call getchar
+	push psw
+	call putchar	! echo character
+	pop psw
+	mov m,a
+	inx h
+	inx b
+	dcx d
+	cpi 0x0A	! is it a newline?
+	jnz 1b
+2:	push b
+	lxi h,0
+	push h
+	jmp monret
+
+monwrite:
+	pop h		! file-descriptor, not used
+	pop h		! hl = pointer to output buffer
+	pop d		! de = number of bytes
+	push d
+1:	mov a,e
+	ora d
+	jz 2f
+	mov a,m
+	call putchar
+	inx h
+	dcx d
+	jmp 1b
+
+2:	push d		! no error
+	jmp monret
+
+monopen:
+	pop h		! pointer to string
+	lxi h,0
+	xthl		! pop  flag and push file descriptor
+	push h		! error code
+	jmp monret
+
+monclose:
+	lxi h,0
+	xthl		! pop file descriptor and push error code
+	jmp monret
+
+monioctl:
+	pop h		! file descriptor
+	pop h		! request
+	lxi h,0
+	xthl		! remove argp and push error code
+	jmp monret
+	
+monret:
+	lhld .bcreg
+	mov b,h
+	mov c,l
+	lhld .retadr
+	pchl