diff --git a/doc/apple2.sgml b/doc/apple2.sgml
index 7b4bd7226..16ea35dd7 100644
--- a/doc/apple2.sgml
+++ b/doc/apple2.sgml
@@ -433,13 +433,6 @@ BASIC.SYSTEM) there are some limitations for DOS 3.3:
 </descrip><p>
 
 
-<sect1>DIO<p>
-
-Although <htmlurl url="dio.html" name="DIO"> generally works with all ProDOS 8
-devices, the function <htmlurl url="dio-3.html" name="dio_query_sectcount()">
-simply always return 280 (which is only correct for a 140 KB disk).
-
-
 <sect1>Direct console I/O<p>
 
 <descrip>
@@ -493,15 +486,27 @@ name="assembler manual">.
 
 <sect1>DIO<p>
 
-The function <htmlurl url="dio-1.html" name="dio_open()"> has the single parameter
-<tt/drive_id/ to identify the drive to be opened. Therefore an Apple&nbsp;II
-slot and drive pair is mapped to that <tt/drive_id/ according to the formula
+<descrip>
 
-<verb>
+  <tag/Drive ID/
+  The function <htmlurl url="dio-1.html" name="dio_open()"> has the single
+  parameter <tt/drive_id/ to identify the drive to be opened. Therefore an
+  Apple&nbsp;II slot and drive pair is mapped to that <tt/drive_id/ according
+  to the formula
+
+  <verb>
     drive_id = slot * 16 + (drive - 1) * 128
-</verb>
+  </verb>
 
-so that for example slot 6 drive 2 is mapped to <tt/drive_id/ 224.
+  so that for example slot 6 drive 2 is mapped to <tt/drive_id/ 224.
+
+  <tag/Sector count/
+  The function <htmlurl url="dio-3.html" name="dio_query_sectcount()"> returns
+  the correct sector count for all ProDOS 8 disks. However for any non-ProDOS 8
+  disk it simply always return 280 (which is only correct for a 140 KB disk).
+  This condition is indicated by the _oserror value 82.
+
+</descrip><p>
 
 
 
diff --git a/doc/apple2enh.sgml b/doc/apple2enh.sgml
index 70fdcf4e7..9286005f5 100644
--- a/doc/apple2enh.sgml
+++ b/doc/apple2enh.sgml
@@ -434,13 +434,6 @@ BASIC.SYSTEM) there are some limitations for DOS 3.3:
 </descrip><p>
 
 
-<sect1>DIO<p>
-
-Although <htmlurl url="dio.html" name="DIO"> generally works with all ProDOS 8
-devices, the function <htmlurl url="dio-3.html" name="dio_query_sectcount()">
-simply always return 280 (which is only correct for a 140 KB disk).
-
-
 <sect1>Direct console I/O<p>
 
 <descrip>
@@ -499,15 +492,27 @@ name="assembler manual">.
 
 <sect1>DIO<p>
 
-The function <htmlurl url="dio-1.html" name="dio_open()"> has the single parameter
-<tt/drive_id/ to identify the drive to be opened. Therefore an Apple&nbsp;II
-slot and drive pair is mapped to that <tt/drive_id/ according to the formula
+<descrip>
 
-<verb>
+  <tag/Drive ID/
+  The function <htmlurl url="dio-1.html" name="dio_open()"> has the single
+  parameter <tt/drive_id/ to identify the drive to be opened. Therefore an
+  Apple&nbsp;II slot and drive pair is mapped to that <tt/drive_id/ according
+  to the formula
+
+  <verb>
     drive_id = slot * 16 + (drive - 1) * 128
-</verb>
+  </verb>
 
-so that for example slot 6 drive 2 is mapped to <tt/drive_id/ 224.
+  so that for example slot 6 drive 2 is mapped to <tt/drive_id/ 224.
+
+  <tag/Sector count/
+  The function <htmlurl url="dio-3.html" name="dio_query_sectcount()"> returns
+  the correct sector count for all ProDOS 8 disks. However for any non-ProDOS 8
+  disk it simply always return 280 (which is only correct for a 140 KB disk).
+  This condition is indicated by the _oserror value 82.
+
+</descrip><p>
 
 
 
diff --git a/libsrc/apple2/diosectcount.s b/libsrc/apple2/diosectcount.s
index 749248232..41bcb3045 100644
--- a/libsrc/apple2/diosectcount.s
+++ b/libsrc/apple2/diosectcount.s
@@ -5,8 +5,91 @@
 ;
 
         .export 	_dio_query_sectcount
+        .import 	_dio_query_sectsize, _malloc, _free
+
+        .include	"zeropage.inc"
+        .include	"errno.inc"
+        .include	"mli.inc"
 
 _dio_query_sectcount:
-        lda	#<280
-        ldx	#>280
+
+        ; Set handle
+        sta	mliparam + MLI::ON_LINE::UNIT_NUM
+
+        ; Get ProDOS 8 block size (clears __oserror)
+        jsr	_dio_query_sectsize
+
+        ; Alloc buffer
+        jsr	_malloc
+        sta	ptr4
+        stx	ptr4+1
+
+        ; Set buffer
+        sta	mliparam + MLI::ON_LINE::DATA_BUFFER
+        stx	mliparam + MLI::ON_LINE::DATA_BUFFER+1
+
+        ; Check buffer (hibyte is enough)
+        txa
+        beq	nomem
+
+        ; Get device state
+        lda	#ON_LINE_CALL
+        ldx	#ON_LINE_COUNT
+        jsr	callmli
+        bcs	check
+
+        ; UNIT_NUM already set
+        .assert MLI::RW_BLOCK::UNIT_NUM = MLI::ON_LINE::UNIT_NUM, error
+
+        ; DATA_BUFFER already set
+        .assert MLI::RW_BLOCK::DATA_BUFFER = MLI::ON_LINE::DATA_BUFFER, error
+
+        ; Read volume directory key block
+        tax			; A = 0
+        lda	#$02
+        sta	mliparam + MLI::RW_BLOCK::BLOCK_NUM
+        stx	mliparam + MLI::RW_BLOCK::BLOCK_NUM+1
+        lda	#READ_BLOCK_CALL
+        ldx	#RW_BLOCK_COUNT
+        jsr	callmli
+        bcs	oserr
+
+        ; Get and save total blocks from volume directory header
+        ldy	#$29
+        lda	(ptr4),y
+        pha
+        iny
+        lda	(ptr4),y
+        pha
+
+        ; Cleanup buffer
+done:   lda	ptr4
+        ldx	ptr4+1
+        jsr	_free
+
+        ; Restore total blocks and return
+        pla
+        tax
+        pla
         rts
+
+nomem:  lda	#$FF		; Error code for sure not used by MLI
+oserr:  sta	__oserror
+
+        ; Save total blocks for failure
+        lda	#$00
+        pha
+        pha
+        beq	done		; Branch always
+
+        ; Check for non-ProDOS disk
+check:  cmp	#$52		; "Not a ProDOS volume"
+        bne	oserr
+        sta	__oserror
+
+        ; Save total blocks for a 16-sector disk
+        lda	#<280
+        pha
+        lda	#>280
+        pha
+        bne	done		; Branch always