diff --git a/Source/HBIOS/hbios.asm b/Source/HBIOS/hbios.asm
index 80376b43..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
@@ -275,10 +277,18 @@ HBX_IDENT:
 ; 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
-	
-	LD	(HBX_INVSP),SP		; SAVE ORIGINAL STACK FRAME
 
 	LD	SP,HBX_BUF_END		; BORROW HBX_BUF FOR TEMP STACK
 	LD	A,BID_BIOS		; HBIOS BANK
@@ -296,10 +306,12 @@ HBX_INVOKE:
 	LD	SP,0			; RESTORE ORIGINAL STACK FRAME
 HBX_INVSP	.EQU	$ - 2
 
-	PUSH	AF			; SAVE AF
-	OR	$FF			; CLEAR INVBNK TO INACTIVE VALUE
-	LD	(HB_INVBNK),A		; SAVE IT
-	POP	AF			; RESTORE AF
+#IF (HBIOS_MUTEX == TRUE)
+	PUSH	HL			; SAVE HL
+	LD	HL,HB_LOCK		; POINT TO LOCK
+	LD	(HL),$FE		; RELEASE MUTEX LOCK
+	POP	HL			; RESTORE HL
+#ENDIF
 
 	RET				; RETURN TO CALLER
 ;
@@ -493,13 +505,21 @@ HBX_TGTADR	.EQU	$ + 1
 ;
 ; PEEK & POKE ROUTINES
 ;   ADDRESS IN HL, BANK IN D, VALUE IN/OUT IN E, A IS TRASHED
-;   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.
+;
+; 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
@@ -509,7 +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
@@ -679,7 +699,8 @@ HBX_BUF_END	.EQU	$
 	.DW	0			; HB_DSTADR: BNKCPY DESTINATION ADDRESS
 	.DB	BID_USR			; HB_DSTBNK: BNKCPY DESTINATION BANK ID
 	.DW	0			; HB_CPYLEN: BNKCPY LENGTH
-	.FILL	6,0			; FILLER, RESERVED FOR FUTURE HBIOS USE
+	.FILL	5,0			; FILLER, RESERVED FOR FUTURE HBIOS USE
+	.DB	$FE			; HB_LOCK: HBIOS MUTEX LOCK
 	JP	HBX_INVOKE		; HB_INVOKE: FIXED ADR ENTRY FOR HBX_INVOKE (ALT FOR RST 08)
 	JP	HBX_BNKSEL		; HB_BNKSEL: FIXED ADR ENTRY FOR HBX_BNKSEL
 	JP	HBX_BNKCPY		; HB_BNKCPY: FIXED ADR ENTRY FOR HBX_BNKCPY
@@ -2452,14 +2473,18 @@ SYS_SETSECS:
 ; ENTRY: D=BANK ID, HL=ADDRESS
 ; RETURN: E=BYTE VALUE
 ;
+; IF WE ARE USING INTERRUPT MODE 1, WE NEED TO PREVENT INTERRUPTS
+; BECAUSE THE LOW MEMORY BANK CONTAINING THE IM1 VECTOR WILL PROBABLY
+; GET BANKED OUT DURING THE PEEK PROCESSING.
+;
 SYS_PEEK:
-#IF (INTMODE > 0)
+#IF (INTMODE == 1)
 	LD	A,I
 	DI
 	PUSH	AF
 #ENDIF
 	CALL	HBX_PEEK		; IMPLEMENTED IN PROXY
-#IF (INTMODE > 0)
+#IF (INTMODE == 1)
 	POP	AF
 	JP	PO,$+4
 	EI
@@ -2470,14 +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