DesTest MAX-Switch Software
See below for the DesTest MAX-Switch and DesTest MAX-Switch-Select cartridges that make all this magic work!
The MAX-mode of the C64 expansion-port is truly a marvel since it allows the program on the cartridge to run right from the get-go bypassing the kernal and its assumptions about working memory. This convenience, however, comes at a cost: All but the bottom 4K of memory plus the kernal, BASIC and character ROMs are not available.
That's why DesTestMAX only tests 4K! That's all it can test.
The DesTest MAX-Switch (or just the 'Switch) solution aims to fix this limitation!
The custom DesTest MAX-Switch and Switch-Select cartridges (see below) add the capability to switch (hence the name) between the MAX-mode and regular cartridge-mode under program control. The new DesTest MAX-Switch Software allows the following features:
- Full 64K memory test using the March-B algorithm.
- No dependence on the ROMs, CIAs or SID: they can be removed.
- ROM checksum verification (if installed).
- CIA TOD consistency verification.
Want to see a DesTest MAX-Switch in action? We've got you covered! Watch these videos that feature the DesTest MAX-Switch being put through its paces.
DesTest MAX-Switch Software as it boots.
The DesTest MAX-Switch Cartridge
If you want a simple cartridge to run the DesTest MAX-Switch software without too many bells and whistles then the 'Switch cartridge may be for you. Despite the simplicity, up to 4 switch-mode or /GAME (Max-mode) images may be installed into the PROM and they may be switched between by changing the A14 and A15 jumpers on the board.
The DesTest MAX-Switch hardware includes provisions for the following:
- Power LED. Most of the time when diagnosing or repairing a C64, the keyboard is removed along with the Power LED. The DesTest MAX-Switch cartridge includes a simple power LED to avoid mistakes like removing or inserting ICs with the power on.
- Reset Switch and LED. A faulty reset-circuit in a C64 can prevent it starting-up properly. A reset-switch can temporarily bypass a faulty reset-circuit and the LED gives a visual indication if something is wrong!
If that's not a meaty-enough cartridge for you, then how about the 'Switch-Select?
A close-up of a MAX-Switch board made by PCBWay with red solder-mask and ENIG finishing.
The DesTest MAX Switch-Select Cartridge
The DesTest MAX-Switch Select cartridge expands upon the regular 'Switch cartridge by adding the capability to select between up-to 6 diagnostics programs. You may program the 28F512 PROM with up to 2 16K 'Switch images, 2 8K /GAME (MAX) images and 2 8K /EXROM images. You can use the two slide-switches to select between the six available images at your whim. This cartridge is your one-stop-shop for all your diagnostic program needs.
The DesTest MAX-Switch-Select hardware provides the following:
- Power LED. Most of the time when diagnosing or repairing a C64, the keyboard is removed along with the Power LED. The DesTest MAX-Switch cartridge includes a simple power LED to avoid mistakes like removing or inserting ICs with the power on.
- Reset Switch and LED. A faulty reset-circuit in a C64 can prevent it starting-up properly. A reset-switch can temporarily bypass a faulty reset-circuit and the LED gives a visual indication if something is wrong!
- Mode Slide Switch. The switch on the left-hand side of the cartridge selects what kind of diagnostic-image will run at power-up. Use it to pick between a 16K 'Switch image, an 8K /GAME (MAX) image or an 8K /EXROM image.
- Bank Slide Switch. Each of the three kinds of images (switch, game and exrom) can have two images installed. The right switch selects between these two images.
A close-up of a MAX-Switch-Select board made by PCBWay with red solder-mask and ENIG finishing.
Several ways to get one
Building a DesTest MAX-Switch or i is an ideal beginner project! The soldering is all through-hole and doesn't require anything but the most basic of skill. The required parts are readily available and are low-cost. The Programmable Read Only Memory (PROM) is the trickiest part of this build as there are a few different technologies to pick from (EPROM, EEPROM or Flash) and the devices needs to be programmed.
- You can buy a pre-made Switch or Switch-Select from one of the vendors listed on the Where To Buy page. Be aware, fraudulent sellers are trying their luck on ebay and similar sites. Please only buy from the sellers listed
- Have PCBWay make your Switch or Switch-Select boards and send them directly to your door. Visit my PCBWay Shared Projects gateway for reviews and links to get the PCBs fabricated.
- I hope to be able to offer either PCB-sales or perhaps gerber-sales, possibly both in the near future.
Building a DesTest MAX-Switch or 'Switch-Select make ideal beginner projects! The soldering is all through-hole and doesn't require anything but the most basic of skill. The required parts are readily available and are low-cost. The Programmable Read Only Memory (PROM) is the trickiest part of this build as the devices needs to be programmed.
Download the DesTest MAX-Switch and Switch-Select distribution-archives here.
While the archive does include the necessary Bill of Materials and Assembly Instructions, you may also view them at the DesTest MAX-Switch Resources page.
Latest version: v0.3, 14 Feb 2026. Includes additional detection of faulty U13/U25 multiplexers that can prevent the VIC from displaying properly.
DesTest MAX-Switch Output Gallery
