diff --git a/src/rz/factor.27 b/src/rz/factor.28
similarity index 90%
rename from src/rz/factor.27
rename to src/rz/factor.28
index e5837d5e..5b43c9e7 100644
--- a/src/rz/factor.27
+++ b/src/rz/factor.28
@@ -20,15 +20,20 @@ INFOC==3
 
 PDLL==100
 
+;The inner loop of the trial division code.
+
+;On pre-KL10 machines, it's faster to run from the ACs.
 IFE KL10,	LOC 7
-;This is the inner loop of the trial division code.
+;On the KL10, we can't TRC/HRRI the ADDI at 101, but we have a free AC instead.
+IFN KL10,	INCR=11
+
 ;CD is the number being tested. DV is the candidate factor.
-;The TRC/ADDI self-modifying code means that it alternates between
-;adding 2 and 4 to DV on each iteration, so the factors it tries are
-;5, 7, 11, 13, 17, 19, 23, 25... i.e. it skips multiples of 2 and 3.
-;(See <https://oeis.org/A007310> for more about this sequence.)
-TEST:	TRC INCR,6		;Complement the 2 and 4 bits in ADDI below
-INCR:	ADDI DV,2		;Next candidate; modified by TEST/NO3
+;The TRC means that it alternates between adding 2 and 4 to DV on each
+;iteration, so the factors it tries are 5, 7, 11, 13, 17, 19, 23, 25...
+;i.e. it skips multiples of 2 and 3. (See <https://oeis.org/A007310>.)
+TEST:	TRC INCR,6		;Complement the 2 and 4 bits in the step size
+IFE KL10,INCR:	ADDI DV,2	;Next candidate; modified by TEST/NO3
+IFN KL10,	ADD DV,INCR	;Next candidate
 RETEST:	MOVE Z,C
 	MOVE B,D
 	IDIV Z,DV		;Z := Z div DV; A := Z mod DV
@@ -36,11 +41,12 @@ RETEST:	MOVE Z,C
 	CAMLE DV,A
 	 JUMPE Z,PRIME		;No more factors to try, so CD is prime
 	JUMPN B,TEST		;Not divisible by E; try the next candidate
-;That's the end of the registers; the instruction below is at 20.
+;If we started at 7, the instruction below is at 20 (after the ACs).
 	JRST FOUND		;Found a factor (but CD may still be composite)
 
-;Entry point
 IFE KL10,	LOC 100
+
+;Entry point
 GO:	MOVE P,[-PDLL,,PDL]
 	.OPEN TYIC,[.UAI,,'TTY]
 	 JRST KILL