diff --git a/src/chkmat.s b/src/chkmat.s
index 1eddccf..51360c5 100644
--- a/src/chkmat.s
+++ b/src/chkmat.s
@@ -18,26 +18,40 @@ DONE:   .LOGOUT 2,                     ;Exit from the program
 ;Subroutine that checks a byte array, where one byte is a 4-bit value,
 ;for duplicate values.
 ;The algorithm works as follows:
-;The original word is walked through in for bit steps. Each 4-bit value
+;The original word is walked through in 4-bit steps. Each 4-bit value
 ;is read, and its value is used as a bit pointer to the 'hashing'-register.
 ;If a number is found, the bit in the hashing register is set. If the bit
-;is already set, the number has already been found the in original set.
+;is already set, the number has already been found the in original data.
 ;To make the walk through easy, we use a special feature of the PDP-10:
 ;We access the data register, as if it was a memory address.
 ;The following registers are used:
 ;A: Accumulator used to store the values that are read from the data field.
 ;B: Used to check if a bit is set, without destroying the contents of I.
 ;C: Contains the data field itself: 36 bits, in 4-bit segments.
+;D: Contains the result, 0 if successfull, else >0.
 ;E: Used as a table to store if a number was found in the field.
 ;X: The byte pointer to the data field.
 CHKMAT: PUSH  P,A                      ;Save the register A
+        PUSH  P,B                      ;Save the B-register
         PUSH  P,C                      ;Save the data register
         PUSH  P,E                      ;Save the E register
         PUSH  P,X                      ;Save the index register
-        SET   E                        ;Set the hash register to 0.
+        MOVEI D,11                     ;Load D with 16.
+        SETZ  E                        ;Set the hash register to 0.
         MOVE  X,[440400,,C]            ;Store a byte pointer to C
-LOOP:   ILDB  A,X                      ;Store a byte in A
-        MOVEI B,E                      ;Copy the table.
-        LSH   B,A                      ;Shift out the bit to check.
-        ANDI  B,1                      ;Check if the first bit is set.
+CKLOOP: SOJE  D,CHKEXT                 ;All bits set?
+        ILDB  A,X                      ;Store a byte in A
+        MOVEI B,1                      ;Set B to 1
+        LSH   B,A                      ;Set the Ath bit in the mask
+        TRNE  E,B                      ;Test, if a bit is already set
+         JRST CHKEXT                   ;Double value found, exit.
+        IORI  E,B                      ;Set the Ath bit in the code
+        JRST  LOOP                     ;Back to the loop
+CHKEXT: POP   P,X                      ;Restore X
+        POP   P,E                      ;Restore E
+        POP   P,C                      ;Restore C
+        POP   P,B                      ;Restore B
+        POP   P,A                      ;Restore A
+        POPJ  P,                       ;Return from function 
+