Revert "removed faulty JiffyDOS optimization (compatibility problems)"

This reverts commit 02863c5cf5.

c64/kernal/iec/iec_tx_command.s

@@ -16,22 +16,6 @@ iec_tx_command:
 	phx_trash_a
 	phy_trash_a
 
-#if CONFIG_IEC_JIFFYDOS
-
-	// IMHO detecting JiffDOS would be sufficient for TALK and LISTEN commands,
-	// this would also make the protocol slightly faster. But, for some reason
-	// (this is clearly visible in the logs) the original JiffyDOS Kernal tries
-	// to detect the JiffyDOS drive during each and every command - and trying
-	// to limit the detection to TALK/LISTEN only was causing transmission
-	// errors (at least with 1541 JiffyDOS ROM)
-
-	lda #$FF // force JiffyDOS detection
-	sta IECPROTO
-
-	// FALLTROUGH
-
-#endif // CONFIG_IEC_JIFFYDOS
-
 	// Notify all devices that we are going to send a byte
 	// and it is going to be a command (pulled ATN)
 	jsr iec_pull_atn_clk_release_data

c64/kernal/iec/talk.s

@@ -32,6 +32,18 @@ common_talk_listen: // common part of TALK and LISTEN
 
 	sta TBTCNT
 
+#if CONFIG_IEC_JIFFYDOS
+
+	// It seems that only TALK/LISTEN is used to detect the JiffyDOS support
+
+	lda #$FF // unknown protocol - check what is supported
+	sta IECPROTO
+
+	// FALLTROUGH
+
+#endif // CONFIG_IEC_JIFFYDOS
+
+
 	jmp iec_tx_command
 
 #else

c64/kernal/iec/unlsn.s

@@ -18,6 +18,13 @@ UNLSN:
 	// According to serial-bus.pdf (page 15) this routine flushes the IEC out buffer
 	jsr iec_tx_flush
 
+#if CONFIG_IEC_JIFFYDOS
+
+	lda #$00 // switch back to standard protocol
+	sta IECPROTO
+
+#endif
+
 	// Buffer empty, send the command
 	lda #$3F
 

c64/kernal/iec/untlk.s

@@ -18,6 +18,13 @@ UNTLK:
 	// According to serial-bus.pdf (page 15) this routine flushes the IEC out buffer
 	jsr iec_tx_flush
 
+#if CONFIG_IEC_JIFFYDOS
+
+	lda #$00 // switch back to standard protocol
+	sta IECPROTO
+
+#endif
+
 	// Buffer empty, send the command
 	lda #$5F