diff --git a/Source/HBIOS/hbios.asm b/Source/HBIOS/hbios.asm
index 8e49d713..654d3739 100644
--- a/Source/HBIOS/hbios.asm
+++ b/Source/HBIOS/hbios.asm
@@ -69,6 +69,8 @@
 ;
 #INCLUDE "std.asm"
 ;
+HBIOS_MUTEX	.EQU	TRUE
+;
 ; MAKE SURE EXACTLY ONE OF ROMBOOT, APPBOOT, IMGBOOT IS DEFINED.
 ;
 MODCNT	.EQU	0
@@ -269,7 +271,21 @@ HBX_IDENT:
 ; ACCOMMODATE FREERTOS.  HBX_BUF IS ONLY USED AS A BOUNCE BUFFER, SO IT'S
 ; USE WILL NEVER OVERLAP WITH BELOW.
 ;
+; HBX_INVOKE IS NOT RE-ENTRANT!  HB_INVBNK CAN BE USED GLOBALLY TO DETERMINE
+; IF HBIOS IS ALREADY ACTIVE.  HB_INVBNK WILL HAVE A VALUE != $FF WHEN HBIOS
+; IS ACTIVE.  ON RETURN, HB_INVBNK IS SET TO $FF TO INDICATE HBIOS IS NOT
+; ACTIVE.
+;
 HBX_INVOKE:
+
+#IF (HBIOS_MUTEX == TRUE)
+	PUSH	HL			; SAVE HL
+	LD	HL,HB_LOCK		; POINT TO LOCK
+	SRA	(HL)			; TEST/ACQUIRE MUTEX LOCK
+	JR	C,$-2			; KEEP TRYING ON FAILURE
+	POP	HL			; RESTORE HL
+#ENDIF
+
 	LD	(HBX_INVSP),SP		; SAVE ORIGINAL STACK FRAME
 	LD	A,(HB_CURBNK)		; GET CURRENT BANK
 	LD	(HB_INVBNK),A		; SAVE INVOCATION BANK
@@ -290,6 +306,13 @@ HBX_INVOKE:
 	LD	SP,0			; RESTORE ORIGINAL STACK FRAME
 HBX_INVSP	.EQU	$ - 2
 
+#IF (HBIOS_MUTEX == TRUE)
+	PUSH	HL			; SAVE HL
+	LD	HL,HB_LOCK		; POINT TO LOCK
+	LD	(HL),$FE		; RELEASE MUTEX LOCK
+	POP	HL			; RESTORE HL
+#ENDIF
+
 	RET				; RETURN TO CALLER
 ;
 ;;::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
@@ -383,7 +406,7 @@ HBX_BNKSEL1:
 ; Copy Data - Possibly between banks.  This resembles CP/M 3, but
 ;  usage of the HL and DE registers is reversed.
 ; Caller MUST preset HBX_SRCBNK and HBX_DSTBNK.
-; Caller MUST disable ints if IM1 active
+; IM1/IM2 interrupts are disabled during HBX_BNKCPY.
 ; Enter:
 ;	 HL = Source Address
 ;	 DE = Destination Address
@@ -394,6 +417,11 @@ HBX_BNKSEL1:
 ;::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
 ;
 HBX_BNKCPY:
+#IF (INTMODE > 0)
+	LD	A,I
+	DI
+	PUSH	AF
+#ENDIF
 	LD	(HBX_BC_SP),SP		; PUT STACK
 	LD	SP,HBX_TMPSTK		; ... IN HI MEM
 
@@ -423,6 +451,11 @@ HBX_BC_LAST:
 
 	LD	SP,$FFFF		; RESTORE STACK
 HBX_BC_SP	.EQU	$ - 2		; ... TO ORIGINAL VALUE
+#IF (INTMODE > 0)
+	POP	AF
+	JP	PO,$+4
+	EI
+#ENDIF
 	RET
 ;
 HBX_BC_ITER:
@@ -472,11 +505,21 @@ HBX_TGTADR	.EQU	$ + 1
 ;
 ; PEEK & POKE ROUTINES
 ;   ADDRESS IN HL, BANK IN D, VALUE IN/OUT IN E, A IS TRASHED
-;   CALLER MUST DISABLE INTS IF IM1 AND ACCESSING PAGE W/O IM1 INT VECTOR
+;
+; THESE ROUTINES ARE NOT INTENDED TO BE CALLED DIRECTLY -- THEY ARE
+; HELPERS FOR THE HBIOS API AND ARE CALLED BY HBIOS BANK CODE.  THE
+; HBIOS BANK CODE BRACKETS THE USE OF THESE ROUTINES WITH DI/EI IF
+; NECESSARY FOR THE CURRENT INTERRUPT MODE.
+;
+; NOTE: THE SIZE OF HBX_TMPSTK (TYPICALLY 20 BYTES) IS INSUFFICIENT FOR
+; FREERTOS IF AN INTERRUPT STRIKES WHILE THE TEMPORARY STACK IS ACTIVE.
+; BELOW, HBX_BUF HAS BEEN USURPED TO PROVIDE A LARGER TEMP STACK TO
+; ACCOMMODATE FREERTOS.  HBX_BUF IS ONLY USED AS A BOUNCE BUFFER, SO IT'S
+; USE WILL NEVER OVERLAP WITH BELOW.
 ;
 HBX_PEEK:
 	LD	(HBX_PPSP),SP		; SAVE ORIGINAL STACK FRAME
-	LD	SP,HBX_TMPSTK		; USE SMALL TEMP STACK FRAME IN HI MEM
+	LD	SP,HBX_BUF_END		; BORROW HBX_BUF FOR TEMP STACK
 	LD	A,(HB_CURBNK)
 	PUSH	AF
 	LD	A,D
@@ -486,7 +529,7 @@ HBX_PEEK:
 ;
 HBX_POKE:
 	LD	(HBX_PPSP),SP		; SAVE ORIGINAL STACK FRAME
-	LD	SP,HBX_TMPSTK		; USE SMALL TEMP STACK FRAME IN HI MEM
+	LD	SP,HBX_BUF_END		; BORROW HBX_BUF FOR TEMP STACK
 	LD	A,(HB_CURBNK)
 	PUSH	AF
 	LD	A,D
@@ -646,23 +689,24 @@ HBX_BUF_END	.EQU	$
 ; HBIOS PROXY MGMT BLOCK (TOP 32 BYTES)
 ;
 #IFDEF ROMBOOT
-	.DB	BID_BOOT		; CURRENTLY ACTIVE LOW MEMORY BANK ID
+	.DB	BID_BOOT		; HB_CURBNK: CURRENTLY ACTIVE LOW MEMORY BANK ID
 #ELSE
-	.DB	BID_USR			; CURRENTLY ACTIVE LOW MEMORY BANK ID
-#ENDIF
-	.DB	0			; BANK ACTIVE AT TIME OF HBIOS CALL INVOCATION
-	.DW	0			; BNKCPY SOURCE ADDRESS
-	.DB	BID_USR			; BNKCPY SOURCE BANK ID
-	.DW	0			; BNKCPY DESTINATION ADDRESS
-	.DB	BID_USR			; BNKCPY DESTINATION BANK ID
-	.DW	0			; BNKCPY LENGTH
-	.FILL	6,0			; FILLER, RESERVED FOR FUTURE HBIOS USE
-	JP	HBX_INVOKE		; FIXED ADR ENTRY FOR HBX_INVOKE (ALT FOR RST 08)
-	JP	HBX_BNKSEL		; FIXED ADR ENTRY FOR HBX_BNKSEL
-	JP	HBX_BNKCPY		; FIXED ADR ENTRY FOR HBX_BNKCPY
-	JP	HBX_BNKCALL		; FIXED ADR ENTRY FOR HBX_BNKCALL
+	.DB	BID_USR			; HB_CURBNK: CURRENTLY ACTIVE LOW MEMORY BANK ID
+#ENDIF
+	.DB	$FF			; HB_INVBNK: BANK ACTIVE AT TIME OF HBIOS CALL INVOCATION
+	.DW	0			; HB_SRCADR: BNKCPY SOURCE ADDRESS
+	.DB	BID_USR			; HB_SRCBNK: BNKCPY SOURCE BANK ID
+	.DW	0			; HB_DSTADR: BNKCPY DESTINATION ADDRESS
+	.DB	BID_USR			; HB_DSTBNK: BNKCPY DESTINATION BANK ID
+	.DW	0			; HB_CPYLEN: BNKCPY LENGTH
+	.FILL	5,0			; FILLER, RESERVED FOR FUTURE HBIOS USE
+	.DB	$FE			; HB_LOCK: HBIOS MUTEX LOCK
+	JP	HBX_INVOKE		; HB_INVOKE: FIXED ADR ENTRY FOR HBX_INVOKE (ALT FOR RST 08)
+	JP	HBX_BNKSEL		; HB_BNKSEL: FIXED ADR ENTRY FOR HBX_BNKSEL
+	JP	HBX_BNKCPY		; HB_BNKCPY: FIXED ADR ENTRY FOR HBX_BNKCPY
+	JP	HBX_BNKCALL		; HB_BNKCALL: FIXED ADR ENTRY FOR HBX_BNKCALL
 	.DW	HBX_IDENT		; ADDRESS OF HBIOS PROXY START (DEPRECATED)
-	.DW	HBX_IDENT		; ADDRESS OF HBIOS IDENT INFO DATA BLOCK
+	.DW	HBX_IDENT		; HB_IDENT: ADDRESS OF HBIOS IDENT INFO DATA BLOCK
 ;
 	.FILL	MEMTOP - $		; FILL TO END OF MEMORY (AS NEEDED)
 	.ORG	HBX_IMG + HBX_SIZ	; RESET ORG
@@ -2221,17 +2265,7 @@ SYS_BNKCPY:
 	LD	HL,(HB_CPYLEN)		; HL := COPY LEN (SAVED IN SETCPY)
 	EX	(SP),HL			; RESTORE HL & SET (SP) TO COPY LEN
 	POP	BC			; BC := COPY LEN
-#IF (INTMODE > 0)
-	LD	A,I
-	DI
-	PUSH	AF
-#ENDIF
 	CALL	HB_BNKCPY
-#IF (INTMODE > 0)
-	POP	AF
-	JP	PO,$+4
-	EI
-#ENDIF
 	XOR	A
 	RET
 ;
@@ -2439,14 +2473,18 @@ SYS_SETSECS:
 ; ENTRY: D=BANK ID, HL=ADDRESS
 ; RETURN: E=BYTE VALUE
 ;
+; IF WE ARE USING INTERRUPT MODE 1, WE NEED TO PREVENT INTERRUPTS
+; BECAUSE THE LOW MEMORY BANK CONTAINING THE IM1 VECTOR WILL PROBABLY
+; GET BANKED OUT DURING THE PEEK PROCESSING.
+;
 SYS_PEEK:
-#IF (INTMODE > 0)
+#IF (INTMODE == 1)
 	LD	A,I
 	DI
 	PUSH	AF
 #ENDIF
 	CALL	HBX_PEEK		; IMPLEMENTED IN PROXY
-#IF (INTMODE > 0)
+#IF (INTMODE == 1)
 	POP	AF
 	JP	PO,$+4
 	EI
@@ -2457,14 +2495,18 @@ SYS_PEEK:
 ; WRITE A BYTE OF MEMORY TO SPECIFIED BANK
 ; ENTRY: D=BANK ID, HL=ADDRESS IN HBIOS BANK, E=BYTE VALUE
 ;
+; IF WE ARE USING INTERRUPT MODE 1, WE NEED TO PREVENT INTERRUPTS
+; BECAUSE THE LOW MEMORY BANK CONTAINING THE IM1 VECTOR WILL PROBABLY
+; GET BANKED OUT DURING THE POKE PROCESSING.
+;
 SYS_POKE:
-#IF (INTMODE > 0)
+#IF (INTMODE == 1)
 	LD	A,I
 	DI
 	PUSH	AF
 #ENDIF
 	CALL	HBX_POKE		; IMPLEMENTED IN PROXY
-#IF (INTMODE > 0)
+#IF (INTMODE == 1)
 	POP	AF
 	JP	PO,$+4
 	EI
diff --git a/Source/HBIOS/hbios.inc b/Source/HBIOS/hbios.inc
index 441cfcc2..d47d53f9 100644
--- a/Source/HBIOS/hbios.inc
+++ b/Source/HBIOS/hbios.inc
@@ -180,6 +180,7 @@ HB_SRCBNK	.EQU	HBX_XFCDAT + 4		; BNKCPY: SOURCE BANK ID
 HB_DSTADR	.EQU	HBX_XFCDAT + 5		; BNKCPY: DESTINATION ADDRESS
 HB_DSTBNK	.EQU	HBX_XFCDAT + 7		; BNKCPY: SOURCE ADDRESS
 HB_CPYLEN	.EQU	HBX_XFCDAT + 8		; BNKCPY: COPY LENGTH
+HB_LOCK		.EQU	HBX_XFCDAT + 15		; HBIOS MUTEX LOCK
 ;
 HBX_XFCFNS	.EQU	HBX_XFC + $10		; JUMP TABLE PORTION OF HBIOS PROXY INTERFACE AREA
 HB_INVOKE	.EQU	HBX_XFCFNS + (0 * 3)	; INVOKE HBIOS FUNCTION