800 lines of assembler code basically means just 800 assembler (CPU) instructions - for a complete FAT32 filesystem driver? I really hardly doubt that unless seen. :-)

Any way to opensource that or anything else to comment on your shockinly few 800 assembler instructions? ;-)

Not to detract from the overall point but a small nitpick. Lines of assembler don't always translate directly to CPU instructions. Some assemblers provide high level assembler instructions for commonly used series of instructions. It will be one assembler instruction that would translate to a couple CPU instructions.

Depending on the amount of abstraction involved, it doesn't sound impossible to do. Given sane and direct access to the controller of the underlying device, and a reasonably simple underlying device (e.g. a simple Flash device), raw reads and writes can be written in maybe two dozen instructions each. Assuming userbinator was using a reasonable macro assembler, so that the 800 lines didn't also include string comparisons and all that, 800 lines doesn't look so little. I'm not sufficiently familiar with FAT32 but given a sane enough architecture, it doesn't sound that impossible.

Depends what you mean by "complete", but being able to create/read/write/delete files and directories is pretty complete to me. It doesn't take very many instructions to traverse a linked list or index an array, the two primary structures in FAT32.

Sorry I can't opensource it (being for a commercial product) but this is the approximate breakdown of that ~800 lines (I didn't count exactly, there are comments and whitespace included here too):

    70  initialisation
    140 cluster chain operations
    300 directory operations
    240 higher-level module interface
    50  data area (buffers, states, etc.)

Given sane and direct access to the controller of the underlying device, and a reasonably simple underlying device (e.g. a simple Flash device), raw reads and writes can be written in maybe two dozen instructions each.

It calls into another driver for accessing the block device, so all block read/write operations take the ~6 instructions needed for a call.

Assuming userbinator was using a reasonable macro assembler, so that the 800 lines didn't also include string comparisons and all that, 800 lines doesn't look so little.

I didn't use a macroassembler. It's for an 8-bit MCU; the final firmware image (including all the other functions of the product) is roughly 20KB.

Aye, it's an MCU I was thinking of in the first place, where read/write operations are done through a reasonably simple Flash controller and can be abstracted in a separate driver. I have written FS drivers of fairly similar complexity, in pretty much similar space constraints. That's why I wasn't incredulous.