diff --git a/Source/HBIOS/hbios.asm b/Source/HBIOS/hbios.asm
index 081a67cd..6c0e6036 100644
--- a/Source/HBIOS/hbios.asm
+++ b/Source/HBIOS/hbios.asm
@@ -1588,6 +1588,7 @@ HB_SPDTST:
 #ENDIF
 ;
 INITSYS3:
+	DIAG(0)
 ;
 	CALL	PRTSUM			; PRINT UNIT/DEVICE SUMMARY TABLE
 ;
@@ -1949,19 +1950,36 @@ DIO_TBL		.FILL	DIO_SIZ,0	; SPACE FOR ENTRIES
 ;
 ; TOS=READ FN ADR
 ; HL=BUF ADR
-; C=SEC COUNT
+; E=SEC COUNT
 ; D=BUF BANK ID
 ;	
 HB_DSKREAD:
+;
+	; THE ACTUAL SECTOR READ FUNCTION ADDRESS IS ON TOS, SAVE IT
+	EX	(SP),HL			; SAVE HL TO TOS, HL := READ FN ADR
+	LD	(HB_DSKFNADR),HL	; IMBED IN CALL OP BELOW
+	POP	HL			; RECOVER HL
+;
+#IF (DIAGENABLE)
+	; SAVE DISK UNIT NUMBER BIT MASK
+	LD	A,C			; GET DISK UNIT NUMBER
+	LD	B,A			; PUT IN B FOR LOOP COUNTER
+	INC	B			; LOOP ONE EXTRA TIME TO HANDLE UNIT=0
+	XOR	A			; START WITH ACCUM ZERO
+	SCF				; ... AND CF SET
+HB_DSKREAD0:
+	RLA				; ROTATE BIT
+	DJNZ	HB_DSKREAD0		; ... UNTIL IN PROPER LOCATION
+	LD	(HB_DSKBIT),A		; SAVE IT FOR DIAGNOSTICS
+#ENDIF
 ;
 #IF 1
-	; CHECK TO SEE IF INTER-BANK I/O NEEDED.  IF NOT
-	; WE JUST ABORT BACK TO DISK DRIVER TO HANDLE.
+	; CHECK TO SEE IF INTER-BANK I/O NEEDED.
 	BIT	7,H			; TGT BUF IN UPPER 32K?
-	RET	NZ			; IF SO, NOTHING TO DO
+	JP	NZ,HB_DSKIO		; IF SO, NON-BANKED
 	LD	A,D			; GET TGT BANK
 	CP	BID_BIOS		; BIOS BANK?
-	RET	Z			; IF SO, NOTHING TO DO
+	JP	Z,HB_DSKIO		; IF SO, NON-BANKED
 #ENDIF
 ;
 #IF 1
@@ -1975,23 +1993,16 @@ HB_DSKREAD:
 	LD	(HB_IOBUF),HL
 	LD	A,D
 	LD	(HB_IOBNK),A
-;
-	; SETUP READ FN ADR
-	POP	HL			; READ FN ADR IS AT TOS
-	LD	(HB_DSKREADFN),HL	; IMBED IN CALL OP BELOW
 ;
 	; SETUP READ AND LOOP COUNT
 	LD	B,E			; SEC LOOP COUNTER
 	LD	C,0			; SEC COMPLETE COUNTER
 ;
 HB_DSKREAD1:
+	LD	HL,HB_WRKBUF		; USE WORK BUF REAL I/O
+;
 	; CALL READ FN
-	PUSH	BC			; SAVE COUNTERS
-	LD	E,1			; ONE SECTOR
-	LD	HL,HB_WRKBUF		; USE WORK BUF FOR DATA
-HB_DSKREADFN	.EQU	$+1
-	CALL	PANIC			; READ ONE SECTOR
-	POP	BC			; RESTORE COUNTERS
+	CALL	HB_DSKFN		; READ ONE SECTOR
 ;
 	; IF FAIL, RETURN ERR
 	JR	NZ,HB_DSKREADX		; BAIL OUT ON ERROR
@@ -2016,9 +2027,7 @@ HB_DSKREADFN	.EQU	$+1
 ;
 HB_DSKREADX:
 	LD	HL,(HB_IOBUF)		; NEXT BUF ADR
-	LD	E,C			; READ COUNT TO E
-	OR	A			; SET RESULT FLAGS
-	RET				; DONE
+	JR	HB_DSKIOX		; DONE
 ;
 ;==================================================================================================
 ; DISK WRITE HELPER
@@ -2029,19 +2038,36 @@ HB_DSKREADX:
 ;
 ; TOS=WRITE FN ADR
 ; HL=BUF ADR
-; C=SEC COUNT
+; E=SEC COUNT
 ; D=BUF BANK ID
 ;	
 HB_DSKWRITE:
+;
+	; THE ACTUAL SECTOR READ FUNCTION ADDRESS IS ON TOS, SAVE IT
+	EX	(SP),HL			; SAVE HL TO TOS, HL := READ FN ADR
+	LD	(HB_DSKFNADR),HL	; IMBED IN CALL OP BELOW
+	POP	HL			; RECOVER HL
+;
+#IF (DIAGENABLE)
+	; SAVE DISK UNIT NUMBER BIT MASK
+	LD	A,C			; GET DISK UNIT NUMBER
+	LD	B,A			; PUT IN B FOR LOOP COUNTER
+	INC	B			; LOOP ONE EXTRA TIME TO HANDLE UNIT=0
+	XOR	A			; START WITH ACCUM ZERO
+	SCF				; ... AND CF SET
+HB_DSKWRITE0:
+	RLA				; ROTATE BIT
+	DJNZ	HB_DSKWRITE0		; ... UNTIL IN PROPER LOCATION
+	LD	(HB_DSKBIT),A		; SAVE IT FOR DIAGNOSTICS
+#ENDIF
 ;
 #IF 1
-	; CHECK TO SEE IF INTER-BANK I/O NEEDED.  IF NOT
-	; WE JUST ABORT BACK TO DISK DRIVER TO HANDLE.
+	; CHECK TO SEE IF INTER-BANK I/O NEEDED.
 	BIT	7,H			; TGT BUF IN UPPER 32K?
-	RET	NZ			; IF SO, NOTHING TO DO
+	JP	NZ,HB_DSKIO		; IF SO, NON-BANKED
 	LD	A,D			; GET TGT BANK
 	CP	BID_BIOS		; BIOS BANK?
-	RET	Z			; IF SO, NOTHING TO DO
+	JP	Z,HB_DSKIO		; IF SO, NON-BANKED
 #ENDIF
 ;
 #IF 1
@@ -2055,10 +2081,6 @@ HB_DSKWRITE:
 	LD	(HB_IOBUF),HL
 	LD	A,D
 	LD	(HB_IOBNK),A
-;
-	; SETUP WRITE FN ADR
-	POP	HL			; WRITE FN ADR IS AT TOS
-	LD	(HB_DSKWRITEFN),HL	; IMBED IN CALL OP BELOW
 ;
 	; SETUP WRITE AND LOOP COUNT
 	LD	B,E			; SEC LOOP COUNTER
@@ -2079,12 +2101,8 @@ HB_DSKWRITE1:
 	POP	BC			; RESTORE COUNTERS
 ;
 	; CALL WRITE FN
-	PUSH	BC			; SAVE COUNTERS
-	LD	E,1			; ONE SECTOR
-	LD	HL,HB_WRKBUF		; USE WORK BUF FOR DATA
-HB_DSKWRITEFN	.EQU	$+1
-	CALL	PANIC			; READ ONE SECTOR
-	POP	BC			; RESTORE COUNTERS
+	LD	HL,HB_WRKBUF		; WRITE FROM WORK BUFFER
+	CALL	HB_DSKFN		; WRITE ONE SECTOR
 ;
 	; IF FAIL, RETURN ERR
 	JR	NZ,HB_DSKWRITEX		; BAIL OUT ON ERROR
@@ -2096,12 +2114,51 @@ HB_DSKWRITEFN	.EQU	$+1
 ;
 HB_DSKWRITEX:
 	LD	HL,(HB_IOBUF)		; NEXT BUF ADR
+	JR	HB_DSKIOX		; DONE
+;
+;==================================================================================================
+; NON-BANKED DISK READ/WRITE
+;==================================================================================================
+;
+HB_DSKIO:
+;
+	; SETUP LOOP COUNT
+	LD	B,E			; SEC LOOP COUNTER
+	LD	C,0			; SEC COMPLETE COUNTER
+;
+HB_DSKIO1:
+	; CALL READ/WRITE FN
+	CALL	HB_DSKFN		; READ/WRITE ONE SECTOR
+;
+	; IF FAIL, RETURN ERR
+	JR	NZ,HB_DSKIOX		; BAIL OUT ON ERROR
+;
+	; INC SECTOR COUNT
+	INC	C			; BUMP SEC READ/WRITE COUNT
+	DJNZ	HB_DSKIO1		; LOOP AS NEEDED
+	XOR	A			; SIGNAL SUCCESS
+;
+HB_DSKIOX:
 	LD	E,C			; WRITE COUNT TO E
 	OR	A			; SET RESULT FLAGS
 	RET				; DONE
 ;
-HB_IOBUF	.DW	0
-HB_IOBNK	.DB	0
+HB_DSKFN:
+	PUSH	BC			; SAVE COUNTERS
+#IF (DIAGENABLE)
+	LD	A,(HB_DSKBIT)		; LOAD UNIT DISK BIT MASK
+	OUT	(DIAGPORT),A		; DISPLAY ON DIAG LEDS
+#ENDIF
+	LD	E,1			; ONE SECTOR
+HB_DSKFNADR	.EQU	$+1
+	CALL	PANIC			; READ ONE SECTOR
+	DIAG(0)				; CLEAR DIAG DISPLAY
+	POP	BC			; RESTORE COUNTERS
+	RET				; RETURN
+;
+HB_DSKBIT	.DB	0		; ACTIVE DISK UNIT
+HB_IOBUF	.DW	0		; CURRENT IO BUFFER ADR
+HB_IOBNK	.DB	0		; CURRENT IO BUFFER BANK ID
 ;
 ;==================================================================================================
 ;   REAL TIME CLOCK DEVICE DISPATCHER
@@ -2723,10 +2780,12 @@ HB_DISPCALL:
 	JR	NC,HB_DISPERR		; HANDLE FN NUM OUT OF RANGE ERROR
 
 	; BUMP IY TO ACTUAL XXX_TBL ENTRY FOR INCOMING UNIT INDEX
+	PUSH	BC			; SAVE BC
 	LD	B,0			; MSB IS ALWAYS ZERO
 	RLC	C			; MULTIPLY UNIT INDEX
 	RLC	C			; ... BY 4 FOR TABLE ENTRY OFFSET
 	ADD	IY,BC			; SET IY TO ENTRY ADDRESS
+	POP	BC			; RESTORE BC
 
 	; DERIVE DRIVER FUNC ADR TO CALL
 	PUSH	HL			; SAVE INCOMING HL