long integer division on pdp11's

[scs at mit-math]

Anyone who uses any version of unix or one of its derivatives on
pdp11's should be aware of a machine dependency in the c run-time
long integer division routines.  Although the result of the pdp11
div instruction is officially undefined after an error, these
routines (ldiv, lrem, aldiv, and alrem) all use the result at one
point.  The problem crops up at a point where the quotient just
computed is expressible in 16 bits, but is greater than 32767 and
so not a proper two's complement number.  Apparently most 11's
put the divisor in the first destination register and leave the
second alone, but the 11/23 clears it.  A fix is to put a

	mov	r2, r1

before the second div instruction in each of the four routines
(ldiv.s, lrem.s, aldiv.s, and alrem.s).  They are usually kept in
/usr/src/libc/crt .  Of course they will have to be re-assembled
and put in /lib/libc.a to take advantage of the fix.

I haven't tested this fully, but it definitely helps.  It would
be safest to re-set r0 as well, but I haven't figured out what
the algorithm assumes.
					Steve Summit