usbdrv/usbdrvasm165.inc:

replaced comparison in USB ISR to save program cycles: was program counter comparison, now a magic word comparison

main.c:
adds magic word as first pushes onto stack, clears magic word before jumping to app

bootloader runs and enumerates with PC, haven't tested with app yet

firmware/main.c

@@ -279,6 +279,10 @@ static inline __attribute__((noreturn)) void leaveBootloader(void)
 #endif
 
 #ifdef TINY85MODE
+    // clear magic word from bottom of stack before jumping to the app
+    *(uint8_t*)(RAMEND) = 0x00;
+    *(uint8_t*)(RAMEND-1) = 0x00;
+
     // jump to application reset vector at end of flash
     asm volatile ("rjmp __vectors - 4");
 #else
@@ -498,6 +502,21 @@ uchar usbFunctionRead(uchar *data, uchar len)
 
 /* ------------------------------------------------------------------------ */
 
+#ifdef TINY85MODE
+void PushMagicWord (void) __attribute__ ((naked)) __attribute__ ((section (".init3")));
+
+// put the word "B007" at the bottom of the stack (RAMEND - RAMEND-1)
+void PushMagicWord (void)
+{
+    asm volatile("ldi r16, 0xB0"::);
+    asm volatile("push r16"::);
+    asm volatile("ldi r16, 0x07"::);
+    asm volatile("push r16"::);
+}
+#endif
+
+/* ------------------------------------------------------------------------ */
+
 static inline void initForUsbConnectivity(void)
 {
     usbInit();

firmware/usbdrv/usbdrvasm165.inc

@@ -46,28 +46,28 @@ of CPU cycles, but even an exact number of cycles!
 USB_INTR_VECTOR:
 ;order of registers pushed: YL, SREG [sofError], r0, YH, shift, x1, x2, x3, x4, cnt
     push    YL              ; push only what is necessary to sync with edge ASAP
-    in      YL, SREG				; [-36]
-    push    YL						; [-34]
+    in      YL, SREG				; 
+    push    YL						; 
 
 #ifdef TINY85MODE
-	push YH									; [-32]
-	in	YL, SPL								; [-30]
-	in	YH, SPH								; [-28]
-	adiw YL, 4								; [-26]
-	ld YL, Y 								; [-25] YL now contains the high byte of the program counter (word address)
-	cpi YL, ((BOOTLOADER_ADDRESS/256)/2) 	; [-23]
-	brge cleanupBootloaderIntStack 			; [-22] branch if high byte of program counter >= start of boot loader
+	lds YL, RAMEND
+	cpi YL, 0xB0
+	brne cleanupAndJumpToApp
 
+	lds YL, RAMEND-1
+	cpi YL, 0x07
+	breq cleanupBootloaderIntStack
+
+cleanupAndJumpToApp:
 	; program counter was in application space, put registers back to normal and jump to app
-	pop YH
 	pop YL
 	out      SREG, YL
 	pop YL
 	
 	rjmp __vectors - TINYVECTOR_USBPLUS_OFFSET
 
 cleanupBootloaderIntStack:
-	pop YH					; [-20]
+	CLR YL
 #endif
 ;----------------------------------------------------------------------------
 ; Synchronize with sync pattern: