Quick explanation: How GEOS handles disk drives on the C64 + why CONFIGURE version matters

Hey everyone,

Just a quick note for anyone new to GEOS on the Commodore 64 (especially version 2.0, which most people find and run today):

GEOS doesn't use the regular Commodore KERNAL routines for disk support. Instead, it loads its own custom disk drivers (like drv1541, drv1571, drv1581) directly into memory. These drivers talk to the drive hardware in a special way and include diskTurbo — Berkeley Softworks' built-in fast loader that massively speeds up the stock 1541.

Because of this custom handling:
- GEOS sees drives as "A:", "B:", "C:" (not device #8, #9, etc.)
- It supports multiple drives, but the exact types and combos depend on what drivers are built into your GEOS kernel + what CONFIGURE lets you select.
- On a side note you never want to VALIDATE, scratch files from BASIC, or use non-GEOS tools on GEOS disks — it'll corrupt the special file structure in GEOS (GEOS uses VLIR for many files, which looks weird to normal dir listings).

To set up or change drives (e.g., adding a second 1541, a 1571, 1581, RAM disk from REU, etc.), you run the CONFIGURE program from the deskTop. It lets you assign drive types to A/B/C slots and saves the config back to the system disk (usually as part of the kernel or a config file).

Why the version of CONFIGURE is super important:
- Early GEOS versions (1.x) had very limited Configure — basically just 1541 support, maybe 1571 depending on the version of Configure you find.
- GEOS 2.0 brought official support for 1581, better REU RAM disks, and more flexible drive mixing.
- Later patches / third-party versions (e.g., CMD's Configure 2.1 patch for Creative Micro Designs hardware like RAMLink, FD-2000/4000, HD) added drivers for non-standard drives or bigger REUs (like recognizing full 2MB REU as a drive).
- If you use the wrong Configure version for your GEOS kernel/hardware, you might get:
     - No second/third drive support
     - RAM disk not appearing or wrong size
     - Crashes when accessing certain drives
     - Inability to use 1581 partitions or CMD-style devices properly

Bottom line: Match your CONFIGURE to your GEOS version and hardware. Most people running stock-ish setups stick with plain GEOS 2.0 Configure, but if you're on Megapatch, Wheels, gateWay, or CMD gear, grab the patched 2.1 or equivalent — it makes a huge difference and I'll be posting about these in these future.

Hope that clears some things up for anyone fighting drive configs!

-Sutehk