diff --git a/src/id/d5d5f7.a b/src/id/d5d5f7.a
new file mode 100644
index 0000000..648e6ea
--- /dev/null
+++ b/src/id/d5d5f7.a
@@ -0,0 +1,34 @@
+;-------------------------------
+; IDD5D5F7
+; Identify specific version of DOS 3.3 bootloader
+; shared by all disks that use the D5D5F7 check.
+; Note: this detection pattern makes no logical
+; sense; it just happens to be a useful filter for
+; the D5D5F7 patcher to use later. It filters out
+; about 2/3 of disks on which I've never seen a
+; D5D5F7 protection check, so we can save time by
+; not doing full-track searches on those disks.
+;
+; in:  $0800..$08FF contains T00,S00
+;      IDDOS33 returned success
+; out: C clear if specific version of DOS 3.3 bootloader found
+;      C set otherwise
+;      X,Y preserved
+;      all other flags and registers clobbered
+;-------------------------------
+!zone {
+IDD5D5F7
+         lda   $084C
+         cmp   #$08
+         bne   .no
+         lda   $08B0
+         cmp   #$B6
+         bne   .no
+         lda   $08D0
+         cmp   #$20
+         bne   .no
+         clc
+         !byte $24
+.no      sec
+         rts
+}
diff --git a/src/id/inspect0.a b/src/id/inspect0.a
index 368e81d..de0ce4b 100755
--- a/src/id/inspect0.a
+++ b/src/id/inspect0.a
@@ -22,6 +22,7 @@ IDBootloader
          sta   gIsEA
          sta   gIsEEEF
          sta   gIsMECCFastloader
+         sta   gPossibleD5D5F7
          lda   #$00
          sta   gLastTrack
 ;
@@ -50,10 +51,14 @@ IDBootloader
 ; Exit via custom trace function if found.
 ;
          jsr   IDDOS33
-         bcs   +
+         bcs   .notdos33
          lda   #TRUE
          sta   gIsBoot0
-         jsr   IDDiversi
+         jsr   IDD5D5F7
+         bcs   +
+         lda   #TRUE
+         sta   gPossibleD5D5F7
++        jsr   IDDiversi
          bcc   .diversi
          jsr   IDPronto
          bcc   .pronto
@@ -71,7 +76,8 @@ IDBootloader
 ; early to munge the nibble tables used by the drive firmware.
 ; Exit via custom trace function if found.
 ;
-+        jsr   ID8b3
+.notdos33
+         jsr   ID8b3
          bcs   +
          lda   #s_jsr8b3
          jsr   PrintByID
diff --git a/src/passport.a b/src/passport.a
index 345f527..bcad9e9 100755
--- a/src/passport.a
+++ b/src/passport.a
@@ -157,6 +157,7 @@ FirstMover
          !source "id/quickdos.a"
          !source "id/diversidos.a"
          !source "id/prontodos.a"
+         !source "id/d5d5f7.a"
          !source "print.a"
          !source "compare.a"
          !source "modify.a"
@@ -806,6 +807,10 @@ gIsMECCFastloader
          !byte FALSE      ; 0=true, 1=false
                           ; reset before each operation
                           ; set in IDBootloader() after reading T00,S00
+gPossibleD5D5F7
+         !byte FALSE      ; 0=true, 1=false
+                          ; reset before each operation
+                          ; set in IDBootloader() after reading T00,S00
 gOnAClearDayYouCanReadForever
          !byte FALSE      ; 0=true, 1=false
                           ; retry reads with a captured RWTS forever,
diff --git a/src/patchers/d5d5f7.a b/src/patchers/d5d5f7.a
index 949a98c..f5ef80c 100755
--- a/src/patchers/d5d5f7.a
+++ b/src/patchers/d5d5f7.a
@@ -2,11 +2,37 @@
 ; #D5D5F7
 ; nibble count with weird bitstream
 ; involving $D5 and $F7 as delimiters
-; e.g. NoteCard Maker, many Mindplay titles
-; (Ace Detective, Robomath, Fraction-oids)
+;
+; Ace Detective
+; Cat 'N Mouse
+; Cotton Tales
+; Dyno-Quest
+; Easy Street
+; Fraction-oids
+; Math Magic
+; RoboMath
+; Sum Ducks
+; NoteCard Maker
 ;-------------------------------
 !zone {
-         ldy   #$20
+;
+; always run on Pascal disks
+;
+         lda   gIsPascal
+         beq   +
+;
+; if DOS 3.3-shaped bootloader, only run if we found
+; specific markers on T00,S00 earlier that are shared
+; by all the samples I have that use this protection.
+; (This filter is subject to revision if we find
+; additional samples.)
+;
+         lda   gIsBoot0
+         bne   .exit
+         lda   gPossibleD5D5F7
+         bne   .exit
+
++        ldy   #$20
          jsr   SearchTrack
          !byte $BD,$8C,$C0; LDA $C08C,X
          !byte $10,$FB    ; BPL -$FB