OK, I had a quick look, and it seems to be a modified version of mboot.s: http://minnie.tuhs.org/cgi-bin/utree.pl?file=V6/usr/source/mdec/mboot.s I had a look through the rest of the likely files in 'mdec', and I didn't find a better match. I'm too lazy busy to do a complete dis-assembly, and work out exactly how it's different, though.. A few observations: 000: 000407 000606 000000 000000 000000 000000 000000 000001 An a.out header, with the 0407 'magic' here performing its original intended function - to branch past the header. 314: 105737 177560 002375 Some console I/O stuff - this two instruction loop waits for the input ready bit to be set. 326: 042700 177600 020027 000101 103405 020027 000132 101002 More character processing - the first instruction clears the high bits of R0, and the next two sets of two instructions compare the contents with two characters (0101 and 0132), and branch. 444: 000207 005000 021027 000407 001004 016020 460: 000020 020006 103774 012746 137000 005007 This seems like the code that checks to see if the thing is an a.out file (note the 'cmp *r0, $0407'), but the code is different from that code in mboot.s; in that, the instruction before the 'clr r0' (at 0446 here) is a 'jsr', whereas in this it's an 'rts pc'. And the code after the 'cmp r0, sp' and branch is different too. I love the '05007' - not very often you see _that_ instruction! 502: 012700 177350 012701 177342 012711 000003 105711 Clearly the code at 'taper:' (TC11 version). Noel