I changed the routine from Erik somewhat in order to be able to use it for the P89LPC935.
Here it is:

<pre>
NAME NOTOUCH935


PGM_MTP EQU 0FF03H

?PR?no_touch935?NOTOUCH935 SEGMENT CODE
PUBLIC no_touch935
; void no_touch935(){

RSEG ?PR?no_touch935?NOTOUCH935
no_touch935:
jbc EA,restore_EA ; restore EA afterwards

call do_it ; EA was not set
RET

restore_EA:
call do_it
setb EA
RET

do_it:
mov R0, #0FFH ;IAP authorization key first
mov @R0,#96H

mov A, #02
mov R5,#01 ;write BOOTSTAT with 01H
mov R7,#03
lcall PGM_MTP
RET

; }

; END OF no_touch935

END
<pre>

In C use:

<pre>
extern void no_touch935(void);

if (condition) no_touch935();
<pre>

and that's it!

Lex.