Subtraction:

. ;r67 is first 16b value r6=msB r7=lsB
. ;r45 is second 16b value r6=msB r7=lsB
. ;r23 is used to stoe results
.
. clr c ;carry is used as borrow flag
. mov a,r7 ;start with two lsB
. subb a,r5 ;subtract with cleared borrow
. ;careful - the carry holds a partial result, don't change it
. mov r3,a ;store lsB result in r3
. mov r1,a ;store as partial zero-test
. mov a,r6 ;do the two msB
. subb a,r4 ;subtract with carry
. mov r2,a ;msB result stored
. orl a,r1 ;OR combine with r1 for zero test
.
. ; carry: 0 means r67>=r45, 1 means r67<r45
. ; accumulator: if zero, r67=r45
. ; r23 holds the difference.
.
ADD COMPARISON CODE HERE

-Jay C. Box