Maximum File Size on a Standard Commodore 128 Disk

The maximum size of a single file on a standard Commodore 128 disk depends on the specific disk drive format used, generally capping at around 170 KB for single-sided disks. This guide details the block structure of CBM DOS, the capacity differences between 1541 and 1571 drives, and the exact byte calculations for maximum file storage.

The Commodore 128 computer does not have a built-in disk drive, so storage limits are defined by the external floppy drive connected to the system, most commonly the 1541 or the 1571. These drives utilize the CBM DOS file system, which organizes data into blocks rather than a continuous stream. Each block on a standard 5.25-inch floppy disk holds 256 bytes, but 2 bytes are reserved for block linking pointers, leaving 254 bytes of usable data per block.

For the ubiquitous 1541 drive format, which is single-sided and the most compatible standard across the Commodore 8-bit line, there are 664 available blocks for data storage. By multiplying the available blocks by the usable bytes per block, the maximum theoretical file size is 168,656 bytes. In practical terms, this is often referred to as 165 KB or 170 KB depending on whether binary or decimal kilobyte measurements are used. A single file can occupy every available data block on the disk, meaning the maximum file size is effectively equal to the total usable capacity of the formatted disk.

Users utilizing the Commodore 1571 drive, which was frequently bundled with the Commodore 128, have access to higher capacities. When formatted in native 1571 double-sided mode, the disk offers 1,316 usable blocks. This doubles the storage potential, allowing for a maximum single file size of approximately 334,264 bytes, or roughly 340 KB. However, many users formatted 1571 disks in 1541 mode to maintain compatibility with Commodore 64 software, which reverted the limit back to the single-sided 170 KB standard. Ultimately, while the Commodore 128 CPU can address much larger memory spaces, the bottleneck for file size remains the physical constraints of the floppy disk media and the CBM DOS block allocation system.