Editing Jump-less Dual-Kernal Switcher

{{Infobox hardware
| name           = Jump-less Dual-Kernal Switcher
| image          = [[File:Switchless-Kernal-SKS64.jpg|250px]]
| caption        = A switch-less SKS64 board installed inside an early ''Assy 250407'' C64 (EPROM not fitted)
| developer      = Community-developed (original concept & firmware by ''bwack''; later boards by PCBWay Open Source Project, Byte-Munchers, Retrowhiz et al.)
| type           = Internal, switch-less multi-KERNAL ROM adapter
| first_released = 2018 (v0.9 proof-of-concept)
| latest_release = SKS64 v1.1 / Switchless-27C010 PCB (2022)
| operating_voltage = +5 V DC (from C64 mainboard)
| application    = Commodore 64 / 64C / SX-64 ‹long- & short-boards›
}}

The '''Jump-less Dual-Kernal Switcher''' (often called the ''switch-less'' or ''jumper-less'' KERNAL board) is an open-hardware add-on that lets a Commodore 64 toggle between two —or up to sixteen—KERNAL ROM images '''without any external toggle-switch, jumper wire, or case mod'''.  Selection is performed entirely from the keyboard or by a special reset-pulse sequence, leaving the computer’s case stock-looking and the user free from “which switch is which?” confusion.
== Why replace / switch KERNALs? ==
* Run a fast-loading DOS such as '''JiffyDOS''' while still retaining the stock Commodore KERNAL for compatibility.  
* Alternate between region ROMs (PAL vs NTSC) or diagnostic ROMs (Dead-Test, Diag64).  
* Test self-built or patched KERNALs without repeatedly desoldering chips.  

== Design overview ==
{| class="wikitable" style="width:70%; text-align:center;"
! colspan="3" | Major functional blocks (SKS64 reference design)
|-
| style="width:33%;" | '''EPROM / Flash'''<br />27C256 (4 × 8 KB)<br />27C512 (8 × 8 KB)<br />27C010 (16 × 8 KB)
| style="width:33%;" | '''ATtiny13A μC'''<br />Decodes reset / RESTORE pulses,<br />latches two address bits to pick the bank
| style="width:33%;" | '''74xx logic'''<br />HCT08 & HCT02 gate ROM CS<br />and isolate the bus during bank switch
|-
| colspan="3" | The board plugs into the KERNAL socket (U4 on long-boards, U4/U32 on 250469) and '''requires no cuts or flying leads'''.
|}

The micro-controller runs at 9.6 MHz (internal RC) and samples '''~60 Hz VSYNC''' from the VIC-II to time-out key-presses reliably over PAL & NTSC.  Two GPIO lines drive A15/A16 of the EPROM through 10 k pull-ups, giving up to 4/8/16 selectable images depending on device size.

=== Switch-less selection methods ===
* '''Short double-reset''' – press the C64 reset button twice within < 0.9 s.  
* '''Long RESTORE hold''' – hold the RESTORE key ≥ 1 s while tapping RUN/STOP.  
The μC counts either event and advances an internal bank counter; the chosen bank number is flashed back on the C64 power LED (1-long, 2-long, …).  A cold-reset is then issued.

== Board variants ==
{| class="wikitable sortable"
! PCB / Project !! EPROM !! Max banks !! Notes
|-
| '''SKS64 v1.1''' || 27C256/512/010 || 4-16 || Most popular; plated-through-hole; Gerbers on GitHub.
|-
| '''Bwack 27C256 adapter''' || 27C256 || 4 || Original proof-of-concept; SMD ATtiny13A; published 2018.
|-
| '''Switchless-27C020''' || 27C020 || 32 || Experimental; needs extra 74HC139 for A17 gating.
|-
| '''SaRuMan-64 KS''' || 29F040 flash || 64 || Adds USB-ASP header for in-system reflashing.
|}

All share the same firmware with a single compile-time constant (``#define BANKS``).

== Compatibility ==
* Long-boards: 326298, 250407, 250425, 250466 – direct plug-in.  
* Short-board 250469-(xx): requires '''/HIRAM jumper''' wired to A14 because the 16 KB ROM is mapped as a single chip; most switcher PCBs include a solder-bridge.  
* SX-64 & VIC-20 CR: electrically identical 2364 pinout, confirmed working with minor socket shims.  

No effect on cartridges—the KERNAL is banked only after reset, so memory configuration is stable during normal operation.

== Building & flashing ==
# '''Program the EPROM''' with your chosen images in contiguous 8 KB blocks.  (Popular layout: C= KERNAL bank 0, JiffyDOS bank 1, Diagnostic bank 2, SpeedDOS bank 3 …).  
# '''Flash the ATtiny13A''' with ``sks13.hex`` via ISP (AVR-ISP mkII, USBasp, etc.).  
# Solder in sockets and logic ICs; fit 330 Ω LED-sense resistor if you wish to flash the front-panel LED.  
# Plug the assembly into U4, minding orientation; no other wiring needed on long-boards.  

== Using the switcher ==
{| class="wikitable" style="width:80%; text-align:center;"
! Action !! Key / button sequence !! Result
|-
| Next bank || Double-tap reset || Cycles 0 → 1 → 2…
|-
| Previous bank || Hold RESTORE ≥1 s, then tap RUN/STOP || Cycles backwards (firmware ≥1.1)
|-
| Identify bank || Hold RESTORE ≥3 s || LED blinks current bank count
|-
| Force cold-boot || Hold RESTORE while power-on || Bypasses autoboot carts
|}

== Advantages ==
* '''No holes or toggles''' – perfect for collector-grade cases.  
* '''One-hand operation''' – useful when the rear panel is obstructed.  
* '''Multiple ROM sizes supported''' – simply fit a larger EPROM for more images.  
* Works in parallel with cartridge port DOS accelerators (e.g., Kung Fu Flash).

== Limitations ==
* Cannot change bank while code is executing; must reset.  
* Requires stable +5 V (excessive ripple may brown-out the ATtiny).  
* Firmware presently uses a fixed LED blink protocol—may clash with aftermarket LED dimmers.

== See also ==
* [[JiffyDOS Kernal Upgrade]] – the most common alternative ROM paired with this adapter  
* [[Commodore 64 Power-Supply Protector (C64 Saver)|C64 Saver]] – advisable PSU-protector when fitting any internal mod  

[[Category:Commodore 64 Modifications and Enhancements]]
[[Category:Commodore Systems]]