NOTICE: list the serial-bus fixes that leave the bus released

UNLSN ended through iec_set_idle, which pulls CLK, so the C64 kept
holding CLK after UNLISTEN. UNTLK waited for the talker to pull CLK,
which no device does after UNTALK, timed out, and left CLK pulled with
carry set. And a serial routine that found no device also left CLK
pulled. A fastloader that waits for the drive to pull CLK after the
KERNAL's UNLSN (Turrican's) then went on too early.

The contract (Michael Steil, "Commodore Peripheral Bus: Part 4",
https://www.pagetable.com/?p=1135): at the end of a transmission the
sender releases CLK and the receivers release DATA, so the bus is idle
again, and the controller releases ATN after each command; after
UNTALK "the current talker will then release the bus" (Part 2,
https://www.pagetable.com/?p=1031). UNLSN and UNTLK now share their end
and release CLK and DATA; the device-not-present exit releases ATN, CLK,
and DATA. Commodore's ROMs leave the same lines in the same scenarios,
as differential tests in arnitech show.

Not reported upstream (https://github.com/MEGA65/open-roms).

Makefile

@@ -137,6 +137,7 @@ GEN_ZMAC_H     = $(DIR_ZMAC_TMP)/doc.inl
 # List of build directories
 
 DIR_C64        = build/target_c64
+DIR_MC64       = build/target_mc64
 DIR_CUS        = build/target_custom
 DIR_GEN        = build/target_generic
 DIR_GENCRT     = build/target_generic_crt
@@ -151,6 +152,7 @@ DIR_X16        = build/target_cx16
 
 CFG_CUS        = src/,,config_custom.s 
 CFG_C64        = src/,,config_c64.s
+CFG_MC64       = src/,,config_mc64.s
 CFG_GEN        = src/,,config_generic.s
 CFG_GENCRT     = src/,,config_generic_crt.s
 CFG_TST38      = src/,,config_testing_38K.s
@@ -164,6 +166,7 @@ CFG_U64CRT     = src/,,config_ultimate64_crt.s
 
 GEN_STR_CUS    = $(DIR_CUS)/,generated/,packed_strings.s
 GEN_STR_C64    = $(DIR_C64)/,generated/,packed_strings.s
+GEN_STR_MC64   = $(DIR_MC64)/,generated/,packed_strings.s
 GEN_STR_GEN    = $(DIR_GEN)/,generated/,packed_strings.s
 GEN_STR_GENCRT = $(DIR_GENCRT)/,generated/,packed_strings.s
 GEN_STR_TST38  = $(DIR_TST38)/,generated/,packed_strings.s
@@ -212,6 +215,7 @@ TARGET_CHR_PXL     = build/chargen_pxlfont.rom
 
 TARGET_CUS_B       = build/basic_custom.rom
 TARGET_C64_B       = build/basic_c64.rom
+TARGET_MC64_B      = build/basic_mc64.rom
 TARGET_GEN_B       = build/basic_generic.rom
 TARGET_GENCRT_B    = build/basic_generic_crt.rom
 TARGET_TST38_B     = build/basic_testing_38K.rom
@@ -221,6 +225,7 @@ TARGET_U64CRT_B    = build/basic_ultimate64_crt.rom
 
 TARGET_CUS_K       = build/kernal_custom.rom
 TARGET_C64_K       = build/kernal_c64.rom
+TARGET_MC64_K      = build/kernal_mc64.rom
 TARGET_GEN_K       = build/kernal_generic.rom
 TARGET_GENCRT_K    = build/kernal_generic_crt.rom
 TARGET_TST38_K     = build/kernal_testing_38K.rom
@@ -237,6 +242,7 @@ TARGET_X16_x       = build/cx16-dummy.rom
 
 TARGET_LIST_CUS    = $(TARGET_CUS_B) $(TARGET_CUS_K)
 TARGET_LIST_C64    = $(TARGET_C64_B) $(TARGET_C64_K)
+TARGET_LIST_MC64   = $(TARGET_MC64_B) $(TARGET_MC64_K)
 TARGET_LIST_GEN    = $(TARGET_GEN_B) $(TARGET_GEN_K)
 TARGET_LIST_GENCRT = $(TARGET_GENCRT_B) $(TARGET_GENCRT_K) $(TARGET_GENCRT_X)
 TARGET_LIST_TST38  = $(TARGET_TST38_B) $(TARGET_TST38_K)
@@ -249,6 +255,7 @@ TARGET_Z80TEST     = ./build/cpm/Z80TEST.COM
 TARGET_LIST        = $(TARGET_CHR_ORF)     \
                      $(TARGET_LIST_CUS)    \
                      $(TARGET_LIST_C64)    \
+                     $(TARGET_LIST_MC64)   \
                      $(TARGET_LIST_GEN)    \
                      $(TARGET_LIST_GENCRT) \
                      $(TARGET_LIST_TST38)  \
@@ -385,6 +392,8 @@ $(DIR_CUS)/OUTB_x.BIN $(DIR_CUS)/BASIC_combined.vs: \
     $(TOOL_ASSEMBLER) $(TOOL_BUILD_SEGMENT) $(DEP_BASIC) $(CFG_CUS) $(GEN_STR_CUS) $(DIR_CUS)/KERNAL_combined.sym
 $(DIR_C64)/OUTB_x.BIN $(DIR_C64)/BASIC_combined.vs: \
     $(TOOL_ASSEMBLER) $(TOOL_BUILD_SEGMENT) $(DEP_BASIC) $(CFG_C64) $(GEN_STR_C64) $(DIR_C64)/KERNAL_combined.sym
+$(DIR_MC64)/OUTB_x.BIN $(DIR_MC64)/BASIC_combined.vs: \
+    $(TOOL_ASSEMBLER) $(TOOL_BUILD_SEGMENT) $(DEP_BASIC) $(CFG_MC64) $(GEN_STR_MC64) $(DIR_MC64)/KERNAL_combined.sym
 $(DIR_GEN)/OUTB_x.BIN $(DIR_GEN)/BASIC_combined.vs: \
     $(TOOL_ASSEMBLER) $(TOOL_BUILD_SEGMENT) $(DEP_BASIC) $(CFG_GEN) $(GEN_STR_GEN) $(DIR_GEN)/KERNAL_combined.sym
 $(DIR_TST38)/OUTB_x.BIN $(DIR_TST38)/BASIC_combined.vs: \
@@ -398,6 +407,8 @@ $(DIR_CUS)/OUTK_x.BIN $(DIR_CUS)/KERNAL_combined.vs $(DIR_CUS)/KERNAL_combined.s
     $(TOOL_ASSEMBLER) $(TOOL_BUILD_SEGMENT) $(DEP_KERNAL) $(CFG_CUS) $(GEN_STR_CUS)
 $(DIR_C64)/OUTK_x.BIN $(DIR_C64)/KERNAL_combined.vs $(DIR_C64)/KERNAL_combined.sym: \
     $(TOOL_ASSEMBLER) $(TOOL_BUILD_SEGMENT) $(DEP_KERNAL) $(CFG_C64) $(GEN_STR_C64)
+$(DIR_MC64)/OUTK_x.BIN $(DIR_MC64)/KERNAL_combined.vs $(DIR_MC64)/KERNAL_combined.sym: \
+    $(TOOL_ASSEMBLER) $(TOOL_BUILD_SEGMENT) $(DEP_KERNAL) $(CFG_MC64) $(GEN_STR_MC64)
 $(DIR_GEN)/OUTK_x.BIN $(DIR_GEN)/KERNAL_combined.vs $(DIR_GEN)/KERNAL_combined.sym: \
     $(TOOL_ASSEMBLER) $(TOOL_BUILD_SEGMENT) $(DEP_KERNAL) $(CFG_GEN) $(GEN_STR_GEN)
 $(DIR_TST38)/OUTK_x.BIN $(DIR_TST38)/KERNAL_combined.vs $(DIR_TST38)/KERNAL_combined.sym: \
@@ -457,6 +468,7 @@ $(DIR_X16)/kernal.seg_1 $(DIR_X16)/KERNAL_1_combined.vs $(DIR_X16)/KERNAL_1_comb
 
 $(DIR_CUS)/OUTx_x.BIN:     $(DIR_CUS)/OUTB_x.BIN     $(DIR_CUS)/OUTK_x.BIN
 $(DIR_C64)/OUTx_x.BIN:     $(DIR_C64)/OUTB_x.BIN     $(DIR_C64)/OUTK_x.BIN
+$(DIR_MC64)/OUTx_x.BIN:    $(DIR_MC64)/OUTB_x.BIN    $(DIR_MC64)/OUTK_x.BIN
 $(DIR_GEN)/OUTx_x.BIN:     $(DIR_GEN)/OUTB_x.BIN     $(DIR_GEN)/OUTK_x.BIN
 $(DIR_GENCRT)/OUTx_0.BIN:  $(DIR_GENCRT)/OUTB_0.BIN  $(DIR_GENCRT)/OUTK_0.BIN
 $(DIR_TST38)/OUTx_x.BIN:   $(DIR_TST38)/OUTB_x.BIN   $(DIR_TST38)/OUTK_x.BIN
@@ -468,6 +480,7 @@ $(DIR_X16)/OUTx_0.BIN:     $(DIR_X16)/OUTB_0.BIN     $(DIR_X16)/OUTK_0.BIN
 
 $(TARGET_CUS_B):    $(DIR_CUS)/OUTx_x.BIN
 $(TARGET_C64_B):    $(DIR_C64)/OUTx_x.BIN
+$(TARGET_MC64_B):   $(DIR_MC64)/OUTx_x.BIN
 $(TARGET_GEN_B):    $(DIR_GEN)/OUTx_x.BIN
 $(TARGET_TST38_B):  $(DIR_TST38)/OUTx_x.BIN
 $(TARGET_TST60_B):  $(DIR_TST60)/OUTx_x.BIN
@@ -475,6 +488,7 @@ $(TARGET_U64_B):    $(DIR_U64)/OUTx_x.BIN
 
 $(TARGET_CUS_K):    $(DIR_CUS)/OUTx_x.BIN
 $(TARGET_C64_K):    $(DIR_C64)/OUTx_x.BIN
+$(TARGET_MC64_K):   $(DIR_MC64)/OUTx_x.BIN
 $(TARGET_GEN_K):    $(DIR_GEN)/OUTx_x.BIN
 $(TARGET_TST38_K):  $(DIR_TST38)/OUTx_x.BIN
 $(TARGET_TST60_K):  $(DIR_TST60)/OUTx_x.BIN
@@ -482,6 +496,7 @@ $(TARGET_U64_K):    $(DIR_U64)/OUTx_x.BIN
 
 build/symbols_custom.vs:          $(DIR_CUS)/BASIC_combined.vs       $(DIR_CUS)/KERNAL_combined.vs
 build/symbols_c64.vs:             $(DIR_C64)/BASIC_combined.vs       $(DIR_C64)/KERNAL_combined.vs
+build/symbols_mc64.vs:            $(DIR_MC64)/BASIC_combined.vs      $(DIR_MC64)/KERNAL_combined.vs
 build/symbols_generic.vs:         $(DIR_GEN)/BASIC_combined.vs       $(DIR_GEN)/KERNAL_combined.vs
 build/symbols_generic_crt.vs:     $(DIR_GENCRT)/BASIC_0_combined.vs  $(DIR_GENCRT)/KERNAL_0_combined.vs
 build/symbols_testing_38K.vs:     $(DIR_TST38)/BASIC_combined.vs     $(DIR_TST38)/KERNAL_combined.vs
@@ -499,6 +514,10 @@ $(GEN_STR_C64): $(TOOL_GENERATE_STRINGS) $(CFG_C64)
 	@mkdir -p $(DIR_C64)/,generated
 	$(TOOL_GENERATE_STRINGS) -o $@ -c $(CFG_C64)
 
+$(GEN_STR_MC64): $(TOOL_GENERATE_STRINGS) $(CFG_MC64)
+	@mkdir -p $(DIR_MC64)/,generated
+	$(TOOL_GENERATE_STRINGS) -o $@ -c $(CFG_MC64)
+
 $(GEN_STR_GEN): $(TOOL_GENERATE_STRINGS) $(CFG_GEN)
 	@mkdir -p $(DIR_GEN)/,generated
 	$(TOOL_GENERATE_STRINGS) -o $@ -c $(CFG_GEN)
@@ -541,6 +560,7 @@ build/,generated/,z80_tables.s: $(TOOL_GENERATE_Z80_TABLES)
 
 GEN_STR_custom         = $(GEN_STR_CUS)
 GEN_STR_c64            = $(GEN_STR_C64)
+GEN_STR_mc64           = $(GEN_STR_MC64)
 GEN_STR_generic        = $(GEN_STR_GEN)
 GEN_STR_generic_crt    = $(GEN_STR_GENCRT)
 GEN_STR_testing_38K    = $(GEN_STR_TST38)

NOTICE

@@ -0,0 +1,33 @@
+This is a modified version of MEGA65 Open ROMs (https://github.com/MEGA65/open-roms),
+forked at upstream commit ad178dbe4d48cd6a317737a8e0e7e662f7e33d32.
+
+Modified by Christian Werner, 2026-10-14 and 2026-10-15, for the mc64 computers of
+arnitech, a set of Minecraft mods (https://git.susdog.de/chris/arnitech). The fork lives
+at https://git.susdog.de/chris/open-roms, branch mc64.
+
+Changes:
+
+- src/,,config_mc64.s: a new build configuration, copied from src/,,config_c64.s, with its
+  own config ID ($6D), the brand "MC64", and BCD_SAFE_INTERRUPTS turned off.
+- Makefile: targets for that configuration, build/kernal_mc64.rom and build/basic_mc64.rom,
+  made the same way as the c64 ones.
+- src/kernal/iec/ciout.s (2026-10-15): CIOUT buffers its byte in TBTCNT, where the send
+  routine takes it from, not in BSOUR (upstream PR #113).
+- src/kernal/iec/close_iec.s (2026-10-15): CLOSE sends LISTEN, $E0 plus the secondary
+  address, and UNLISTEN; it sent a command ORed with zero page $E0 and no LISTEN.
+- src/kernal/iec/chkin_iec.s, ckout_iec.s (2026-10-15): CHKIN and CHKOUT take the secondary
+  address from SAT, not from LAT, the logical file numbers.
+- src/kernal/iec/tksa.s (2026-10-15): TKSA sends the secondary address it is given, without
+  ORing $90 into it (upstream issue #116).
+- src/kernal/iec/iec_tx_byte.s (2026-10-15): a data byte that no listener holds DATA for
+  sets ST to $80 (device not present) and returns, instead of waiting forever.
+- src/kernal/iec/close_iec.s (2026-10-15): a CLOSE that finds no device sets ST to $80
+  (device not present), and still frees the file number.
+- src/kernal/iec/unlsn.s, untlk.s (2026-10-15): UNLSN and UNTLK end with CLK, DATA, and ATN
+  released; UNLSN left CLK pulled, and UNTLK waited for a talker to pull CLK, which after
+  UNTALK none does, and then left CLK pulled.
+- src/kernal/iec/iec_return.s (2026-10-15): a serial routine that finds no device releases
+  CLK, DATA, and ATN; it left CLK pulled.
+- NOTICE: this file.
+
+The licence terms are unchanged; see LICENSE, COPYING, and COPYING.LESSER.

src/,,config_mc64.s

@@ -0,0 +1,130 @@
+
+;; #CONFIG# ID $6D ; should be 1 byte, different for each config file!
+
+;
+; Please read CONFIG.md before modifying this file!
+;
+
+; Idea:
+; The Firmware of arnitech's mc64 computers, a Minecraft mod
+; Same as the 'c64' configuration, except:
+; - the banner names mc64
+; - BCD_SAFE_INTERRUPTS is off, for IRQ entry timing closer to the original
+
+
+
+; --- Hardware platform and brand
+
+;; #CONFIG# PLATFORM_COMMODORE_64      YES
+;; #CONFIG# BRAND_CUSTOM               "MC64"
+
+
+; --- Processor instruction set
+
+;; #CONFIG# CPU_MOS_6502               YES
+;; #CONFIG# CPU_DTV_6502               NO
+;; #CONFIG# CPU_RCW_65C02              NO
+;; #CONFIG# CPU_WDC_65C02              NO
+;; #CONFIG# CPU_WDC_65816              NO
+;; #CONFIG# CPU_CSG_65CE02             NO
+
+
+; --- Memory model
+
+;; #CONFIG# MEMORY_MODEL_38K           YES
+;; #CONFIG# MEMORY_MODEL_46K           NO
+;; #CONFIG# MEMORY_MODEL_50K           NO
+;; #CONFIG# MEMORY_MODEL_60K           NO
+
+
+; --- IEC bus configuration
+
+;; #CONFIG# IEC                        YES
+;; #CONFIG# IEC_DOLPHINDOS             NO
+;; #CONFIG# IEC_DOLPHINDOS_FAST        NO
+;; #CONFIG# IEC_JIFFYDOS               NO
+;; #CONFIG# IEC_JIFFYDOS_BLANK         NO
+
+
+; --- Tape deck configuration
+
+;; #CONFIG# TAPE_NORMAL                YES
+;; #CONFIG# TAPE_TURBO                 NO
+;; #CONFIG# TAPE_AUTODETECT            NO
+;; #CONFIG# TAPE_NO_KEY_SENSE          NO
+;; #CONFIG# TAPE_NO_MOTOR_CONTROL      NO
+
+
+; --- Sound support
+
+;; #CONFIG# SID_2ND_ADDRESS            NO
+;; #CONFIG# SID_3RD_ADDRESS            NO
+;; #CONFIG# SID_D4XX                   NO
+;; #CONFIG# SID_D5XX                   NO
+;; #CONFIG# SID_D6XX                   NO
+;; #CONFIG# SID_D7XX                   NO
+
+
+; --- Keyboard settings
+
+;; #CONFIG# KEYBOARD_C128              NO
+;; #CONFIG# KEYBOARD_C128_CAPS_LOCK    NO
+;; #CONFIG# KEY_REPEAT_DEFAULT         NO
+;; #CONFIG# KEY_REPEAT_ALWAYS          NO
+;; #CONFIG# KEY_FAST_SCAN              YES
+;; #CONFIG# JOY1_CURSOR                YES
+;; #CONFIG# JOY2_CURSOR                YES
+
+;; #CONFIG# PROGRAMMABLE_KEYS          YES
+
+;; #CONFIG# KEYCMD_RUN                 "\5FL"
+
+;; #CONFIG# KEYCMD_F1                  "@8$"
+;; #CONFIG# KEYCMD_F2                  "LOAD"
+;; #CONFIG# KEYCMD_F3                  "@9$"
+;; #CONFIG# KEYCMD_F4                  "RUN:"
+;; #CONFIG# KEYCMD_F5                  "@10$"
+;; #CONFIG# KEYCMD_F6                  NO
+;; #CONFIG# KEYCMD_F7                  "@11$"
+;; #CONFIG# KEYCMD_F8                  NO
+
+;; #CONFIG# KEYCMD_HELP                "LIST"
+
+
+; --- Screen editor
+
+;; #CONFIG# EDIT_STOPQUOTE             YES
+
+
+; --- Software features
+
+;; #CONFIG# PANIC_SCREEN               YES
+;; #CONFIG# DOS_WEDGE                  NO
+;; #CONFIG# TAPE_WEDGE                 NO
+;; #CONFIG# TAPE_HEAD_ALIGN            NO
+;; #CONFIG# BCD_SAFE_INTERRUPTS        NO
+
+
+; --- BASIC features
+
+;; #CONFIG# ORIGINAL_UNPATCHED_BASIC   YES
+;; #CONFIG# TRANSCENDENTAL_FUNCTIONS   YES
+
+
+; --- Eye candy
+
+;; #CONFIG# COLORS_BRAND               NO
+;; #CONFIG# BANNER_SIMPLE              YES
+;; #CONFIG# BANNER_FANCY               NO
+;; #CONFIG# SHOW_FEATURES              YES
+
+
+; --- Debug options
+
+;; #CONFIG# DBG_STUBS_BRK              NO
+;; #CONFIG# DBG_PRINTF                 NO
+
+
+; --- Other
+
+;; #CONFIG# COMPRESSION_LVL_2          NO

src/kernal/iec/chkin_iec.s

@@ -14,7 +14,7 @@ chkin_iec:
 
 	; Fail if the file is open for writing (secondary address 1)
 	
-	lda LAT, Y
+	lda SAT, Y
 	cmp #$01
 	+beq chkin_file_not_input
 
@@ -24,7 +24,7 @@ chkin_iec:
 	jsr TALK
 	+bcs chkinout_device_not_present
 
-	lda LAT, Y
+	lda SAT, Y
 	ora #$60
 	jsr TKSA
 	+bcs chkinout_device_not_present

src/kernal/iec/ciout.s

@@ -28,9 +28,10 @@ CIOUT:
 	
 ciout_store_in_buffer:
 
-	; Store in the buffer data to be sent next, return
+	; Store in the buffer data to be sent next, return; iec_tx_byte
+	; sends the byte from TBTCNT, so that is our buffer, not BSOUR
 	pla
-	sta BSOUR
+	sta TBTCNT
 	rts
 	
 ciout_send_byte:

src/kernal/iec/ckout_iec.s

@@ -14,7 +14,7 @@ ckout_iec:
 
 	; Fail if the file is open for reading (secondary address 0)
 	
-	lda LAT, Y
+	lda SAT, Y
 	+beq ckout_file_not_output
 
 	; Send LISTEN + SECOND first
@@ -23,7 +23,7 @@ ckout_iec:
 	jsr LISTEN
 	+bcs chkinout_device_not_present
 
-	lda LAT, Y
+	lda SAT, Y
 	ora #$60
 	jsr SECOND
 	+bcs chkinout_device_not_present

src/kernal/iec/close_iec.s

@@ -23,11 +23,23 @@ close_iec:
 	jsr iec_close_load 
 	jmp close_remove_from_table
 @1:
-	ora $E0 ; CLOSE command
+	; LISTEN, CLOSE, UNLISTEN - see https://www.pagetable.com/?p=1031
+	lda FAT, y
+	jsr LISTEN
+	bcs close_iec_not_present
+	lda SAT, y
+	ora #$E0 ; CLOSE command
 	sta TBTCNT
 	jsr iec_tx_command
-	bcs close_iec_end
+	bcs close_iec_not_present
 	jsr iec_tx_command_finalize
+	jsr UNLSN
+	jmp close_remove_from_table
+
+close_iec_not_present:
+
+	; The file is closed all the same; READST says device not present
+	jsr kernalstatus_DEVICE_NOT_FOUND
 
 	; FALLTROUGH
 

src/kernal/iec/iec_return.s

@@ -36,8 +36,8 @@ iec_return_success:
 
 iec_return_DEVICE_NOT_FOUND:
 
-	; Set IEC line to normal idle state
+	; Nobody answers: release all lines, so the bus is idle - see https://www.pagetable.com/?p=1135
-	jsr iec_set_idle
+	jsr iec_release_atn_clk_data
 
 	; BSOUR is certainly not valid anymore
 	lda #$00

src/kernal/iec/iec_tx_byte.s

@@ -51,6 +51,17 @@ iec_tx_byte:
 	+phx_trash_a
 	+phy_trash_a
 
+	; A listener holds DATA until we release CLK; if nobody pulls it
+	; within 256us, no device is present - see https://www.pagetable.com/?p=1135
+	ldx #$18
+@0:
+	lda CIA2_PRA
+	bpl @1                             ; branch if DATA is pulled
+	dex
+	bne @0
+	jsr kernalstatus_DEVICE_NOT_FOUND
+	jmp iec_return_DEVICE_NOT_FOUND
+@1:
 	; Notify all devices that we are going to send a byte
 	; and it is going to be a data byte (released ATN)
 	jsr iec_release_atn_clk_data

src/kernal/iec/tksa.s

@@ -28,8 +28,6 @@ TKSA:
 	; - https://www.lemon64.com/forum/viewtopic.php?t=57694&sid=531ba3592bcffadef2ac8b9162f2e529
 	; - https://www.pagetable.com/?p=1031 (Kernal API description)
 
-	ora #$90
-
 	jmp common_untlk_tksa
 
 } else {

src/kernal/iec/unlsn.s

@@ -42,7 +42,15 @@ UNLSN:
 	; Buffer empty, send the command
 	lda #$3F
 
-	jmp common_open_close_unlsn_second
+common_unlsn_untlk: ; common part of UNLSN and UNTLK
+
+	jsr common_open_close_unlsn_second
+	bcs @1
+
+	; The transmission is over: release CLK and DATA too, so the bus is idle - see https://www.pagetable.com/?p=1135
+	jmp iec_release_clk_data
+@1:
+	rts
 
 } else {
 

src/kernal/iec/untlk.s

@@ -39,10 +39,11 @@ UNTLK:
 	sta IECPROTO
 }
 
-	; Buffer empty, send the command
+	; Buffer empty, send the command; the talker releases the bus - see https://www.pagetable.com/?p=1031
 	lda #$5F
+	jmp common_unlsn_untlk
 
-common_untlk_tksa: ; common part of UNTLK and TKSA
+common_untlk_tksa: ; the rest of TKSA
 
 	; XXX - for TKSA, is it really the right place to do a turnaround? I have some doubts
 	; (it forces us to send the TKSA command 'manually' sometimes), but