diff --git a/src/apicode.a b/src/apicode.a
index d65be58..eb0b125 100644
--- a/src/apicode.a
+++ b/src/apicode.a
@@ -49,6 +49,10 @@
                           ; compile-time flag, no way to change at runtime
 
 FIRSTFILTER
+;gIsDavidson
+         !byte FALSE      ; 0=true, 1=false
+                          ; reset before each operation
+                          ; set after reading T22,S00
 ;gIsPanglosDOS
          !byte FALSE      ; 0=true, 1=false
                           ; reset before each operation
diff --git a/src/apidefs.a b/src/apidefs.a
index b333123..3e75e6e 100644
--- a/src/apidefs.a
+++ b/src/apidefs.a
@@ -89,9 +89,10 @@ gPolarwareTamperCheck         = gIsTrillium-$01                   ; byte
 gForceDiskVol                 = gPolarwareTamperCheck-$01         ; byte
 gIsAdvent                     = gForceDiskVol-$01                 ; byte
 gIsPanglosDOS                 = gIsAdvent-$01                     ; byte
+gIsDavidson                   = gIsPanglosDOS-$01                 ; byte
 ;LASTFILTER ; add new gIs* above this line
 
-gOnAClearDayYouCanReadForever = gIsPanglosDOS-$01                 ; byte
+gOnAClearDayYouCanReadForever = gIsDavidson-$01                   ; byte
 gUsingRAMDisk                 = gOnAClearDayYouCanReadForever-$01 ; byte
 gRAMDiskRef                   = gUsingRAMDisk-$01                 ; byte
 gDisplayBytes                 = gRAMDiskRef-$0A                   ; 10 bytes
@@ -152,13 +153,14 @@ ConstructStandardDelivery = jConstructStandardDelivery
 !warn "gIsLaureate=",gIsLaureate
 !warn "gIsDatasoft=",gIsDatasoft
 !warn "gIsSierra=",gIsSierra
-!warn "gIsSierra13=",gIsSierra13
+!warn "gissierra13=",gissierra13
 !warn "gIsF7F6=",gIsF7F6
 !warn "gIsTrillium=",gIsTrillium
 !warn "gPolarwareTamperCheck=",gPolarwareTamperCheck
 !warn "gForceDiskVol=",gIsForceDiskVol
 !warn "gIsAdvent=",gIsAdvent
 !warn "gIsPanglosDOS=",gIsPanglosDOS
+!warn "gIsDavidson=",gIsDavidson
 !warn "gOnAClearDayYouCanReadForever=",gOnAClearDayYouCanReadForever
 !warn "gUsingRAMDisk=",gUsingRAMDisk
 !warn "gRAMDiskRef=",gRAMDiskRef
diff --git a/src/id/davidson.a b/src/id/davidson.a
new file mode 100644
index 0000000..e715253
--- /dev/null
+++ b/src/id/davidson.a
@@ -0,0 +1,34 @@
+;-------------------------------
+; IDDavidson
+; identify Davidson disk after failing to read T22,S00
+;
+; in:  track buffer contains track $22
+; out: C clear if Davidson disk found
+;      C set otherwise
+;      all registers clobbered
+;      all other flags clobbered
+;-------------------------------
+!zone {
+IDDavidson
+         lda   gIsProDOS  ; ProDOS?
+         bne   .no        ; no, give up
+         lda   gTrack     ; track $22?
+         cmp   #$22
+         bne   .no        ; no, give up
+         lda   gSector    ; sector $00?
+         bne   .no        ; no, give up
+         lda   #$00       ; try re-reading the sector but
+         sta   $B92E      ; ignore data field checksum failure
+         jsr   ReadSector
+         lda   #$13
+         sta   $B92E
+         bcs   .no        ; didn't work, give up
+         lda   #s_davidson
+         jsr   PrintByID
+         lda   #TRUE
+         sta   gIsDavidson
+         clc
+         !byte $24
+.no      sec
+         rts
+}
diff --git a/src/passport.a b/src/passport.a
index 76f7f3c..23e0f2a 100755
--- a/src/passport.a
+++ b/src/passport.a
@@ -149,6 +149,7 @@ FirstMover
          !source "id/dinkeydos.a"
          !source "id/advent.a"
          !source "id/panglosdos.a"
+         !source "id/davidson.a"
          !source "print.a"
          !source "compare.a"
          !source "modify.a"
@@ -440,10 +441,14 @@ checksector
          lda   gIsDOS32   ; is this a DOS 3.2 disk?
          beq   .fatal     ; yes, so read error is fatal
          lda   gTriedUniv ; have we tried the universal RWTS?
-         beq   .fatal     ; yes, so read error is fatal
+         beq   .maybedavidson ; yes, but check one last thing
          jsr   SwitchToUniv ; no, switch it in now
          jmp   .read      ; and re-read this sector
 
+.maybedavidson
+         jsr   IDDavidson
+         bcc   .optional
+
 .fatal   pla              ; if we get to here, we've
          jmp   FatalError ; decided the read error is fatal
 
@@ -780,6 +785,8 @@ _applyToAll
          !source "patchers/f7f6.a"              ; gIsF7F6 only
          !source "patchers/trillium.a"          ; gIsTrillium only
          !source "patchers/advent.a"            ; gIsAdvent only
+         !source "patchers/davidsonforth.a"     ; gIsDavidson only
+         !source "patchers/davidsonasm.a"       ; gIsDavidson only
 
          lda   gPatchCount
          beq   .nopatches
diff --git a/src/patchers/davidsonasm.a b/src/patchers/davidsonasm.a
new file mode 100644
index 0000000..34dad8e
--- /dev/null
+++ b/src/patchers/davidsonasm.a
@@ -0,0 +1,36 @@
+;-------------------------------
+; #DAVIDSONASM
+; bad block check of T22,S00
+; implemented in assembly
+;
+; tested on
+; - Alge-Blaster Plus 1.0
+; - Math Blaster Mystery 1.0
+; - Math Blaster Mystery 1.3
+; - Reading and Me
+; - Spell It Plus
+; - Word Attack Plus 1.2
+; - Word Attack Plus French 1.2
+; - Word Attack Plus Spanish 1.2
+;-------------------------------
+!zone {
+         lda   gIsDavidson
+         bne   .exit
+
+         lda   #$0F
+         sta   .sector+1
+.sector  lda   #$FF       ; modified at runtime
+         ldx   #$00
+         ldy   #$06
+         jsr   compare
+         !byte $20,$D2,$25; JSR $25D2
+         !byte $4C,$74,$08; JMP $0874
+         bcs   +
+         ldx   #$04
+         ldy   #$01
+         jsr   modify
+         !byte $79        ; JMP to $0879 instead
++        dec   .sector+1
+         bpl   .sector
+.exit
+}
diff --git a/src/patchers/davidsonforth.a b/src/patchers/davidsonforth.a
new file mode 100644
index 0000000..247e9a9
--- /dev/null
+++ b/src/patchers/davidsonforth.a
@@ -0,0 +1,26 @@
+;-------------------------------
+; #DAVIDSONFORTH
+; bad block check of T22,S00
+; implemented in FORTH
+;
+; tested on
+; - Math Blaster Plus 1.6
+; - Read N Roll 1.0
+; - Read N Roll 1.1
+; - Math and Me 1.0
+;-------------------------------
+!zone {
+         lda   gIsDavidson
+         bne   .exit
+
+         ldy   #$08
+         jsr   SearchTrack
+         !byte $08,$80,$00,$FD,$1C,$D1,$0D,$EC
+         bcs   .exit
+         sta   gDisplayBytes
+         inx
+         ldy   #$01
+         jsr   modify
+         !byte $CC        ; MLI CLOSE command (instead of BLOCKREAD)
+.exit
+}
diff --git a/src/strings/en.a b/src/strings/en.a
index 0435585..189fccf 100755
--- a/src/strings/en.a
+++ b/src/strings/en.a
@@ -118,6 +118,7 @@ StringTable
          !word .microfun
          !word .advent
          !word .gathering
+         !word .davidson
 ;
 ; Text can contain substitution strings, which
 ; are replaced by current values at runtime. Each
@@ -143,7 +144,7 @@ StringTable
 ; can be set directly before calling PrintByID.
 ;
 .header
-         !text "Passport by 4am               2018-06-08",$00
+         !text "Passport by 4am               2018-06-16",$00
 .mainmenu
          !text "________________________________________",$8D,$8D,$8D,$8D,$8D,$8D,$8D
          !text "              "
@@ -442,5 +443,7 @@ StringTable
 .advent
          !text "T%t,S%0 Found Interplay bootloader",$8D,$00
 .gathering
-         !text "Gathering per-file encryption keys",$8D,00
+         !text "Gathering per-file encryption keys",$8D,$00
+.davidson
+         !text "T%t Found Davidson & Associates disk",$8D,$00
 }
diff --git a/src/strings/enid.a b/src/strings/enid.a
index f5c39cd..f102aec 100644
--- a/src/strings/enid.a
+++ b/src/strings/enid.a
@@ -105,4 +105,5 @@ s_tamper =     $63
 s_microfun =   $64
 s_advent =     $65
 s_gathering  = $66
-STRINGCOUNT =  $67
+s_davidson =   $67
+STRINGCOUNT =  $68