added full protection against illegal IEC device numbers

c64/kernal/iec_devnum_check.s

@@ -0,0 +1,24 @@
+
+;;
+;; Checks whether device numbe (in .A) is a correct IEC device number
+;;
+;; Preserves .A, .X and .Y
+;;
+
+iec_devnum_check:
+	;; 0, 1, 2, or 3 are illegal - reserved for non-IEC devices on Commodore
+	cmp #$03
+	bpl +
+	bcs +
+	;; FALLTROUGH
+iec_devnum_check_failed:
+	sec
+	rts
+*
+	;; 30 and above are illegal too, due to IEC command encoding scheme,
+	;; see https://www.pagetable.com/?p=1031
+	cmp #$1E
+	bpl iec_devnum_check_failed
+	bcs iec_devnum_check_failed
+	clc
+	rts

c64/kernal/listen.s

@@ -12,7 +12,10 @@
 
 listen:
 
-	and #$1F ; make sure bits encoding the command are cleared out
+	;; First check whether device number is correct
+	jsr iec_devnum_check
+	bcs kernalerror_DEVICE_NOT_FOUND
+	;; Encode and execute the command
 	ora #$20
 	sta BSOUR
 	jmp iec_tx_command

c64/kernal/load.s

@@ -38,6 +38,18 @@ load:
 	;; Allow platform-specific routine to takeover the flow
 	`PLATFORM_HOOK_LOAD
 
+	;; Check whether we support the requested device
+	lda current_device_number
+	and #$FC
+	beq lvs_illegal_device_number ; device number below 4, not an IEC device
+	
+	;; Device numbers above 30 are also illegal (see https://www.pagetable.com/?p=1031),
+	;; as the protocol combines device number with command code int one byte,
+	;; above 30 it is no longer possible (UNLISTEN and UNTALK codes prevent using
+	;; number 31), yet original ROMs show ILLEGAL DEVICE NUMBER error only
+	;; for devices 0-3; in our implementation LISTEN / TALK will cause device
+	;; not present detected for numbers above 30
+
 	;; Display SEARCHING FOR + filename
 	jsr lvs_display_searching_for
 

c64/kernal/lvs_helpers.s

@@ -114,6 +114,11 @@ lvs_device_not_found_error:
 	jsr kernalstatus_DEVICE_NOT_FOUND
 	jmp kernalerror_DEVICE_NOT_FOUND
 
+lvs_illegal_device_number:
+
+	jsr kernalstatus_DEVICE_NOT_FOUND
+	jmp kernalerror_ILLEGAL_DEVICE_NUMBER
+
 .alias lvs_file_not_found_error kernalerror_FILE_NOT_FOUND
 
 lvs_load_verify_error:

c64/kernal/talk.s

@@ -14,7 +14,10 @@
 
 talk:
 
-	and #$1F ; make sure bits encoding the command are cleared out
+	;; First check whether device number is correct
+	jsr iec_devnum_check
+	bcs kernalerror_DEVICE_NOT_FOUND
+	;; Encode and execute the command
 	ora #$40
 	sta BSOUR
 	jmp iec_tx_command