The floppy controller emulates an
Intel 82077AA CHMOS
single-chip floppy disk controller. It connects to the storage stack
through
Disk
— the same backend abstraction used by NVMe and SCSI. Data transfers
use ISA DMA channel 2; interrupts use IRQ 6.
Two variants exist:
FloppyDiskController— full emulator with disk I/O.StubFloppyDiskController— reports "no drives" for PCAT BIOS compatibility when no floppy is configured.
The controller auto-detects the floppy format from the disk image byte size. See Wikipedia's list of floppy disk formats for background on these formats.
| Format | Capacity | Sectors/track | Notes |
|---|---|---|---|
| Low density (SS) | 360 KB | 9 | Single-sided (one head) |
| Low density | 720 KB | 9 | |
| Medium density | 1.2 MB | 15 | |
| High density | 1.44 MB | 18 | Most common format |
| DMF | 1.68 MB | 21 | Microsoft Distribution Media Format |
| XDF | 1.72 MB | 23 | Extended density (fixed 23 SPT variant) |
All formats use 512-byte sectors, 80 cylinders, CHS addressing. The controller rejects images that don't match a known format size.
Register offsets from base (typically 0x3F0):
| Offset | Read | Write | Purpose |
|---|---|---|---|
| +0 | STATUS_A | — | Fixed 0xFF (not emulated) |
| +1 | STATUS_B | — | Fixed 0xFC (no tape drives) |
| +2 | DOR | DOR | Motor control, drive select, DMA gate, reset |
| +4 | MSR | DSR | Main status (busy, direction, RQM) / data rate select |
| +5 | DATA | DATA | Command/parameter/result FIFO (16-byte) |
| +7 | DIR | CCR | Disk change signal / config control |
The controller claims port 0x3F7 for DIR/CCR separately from the 6-byte base region, because 0x3F6 is shared with the IDE controller's alternate status register.
The real 82077AA supports four drives; OpenVMM supports one. The emulator implements a pragmatic subset of the command set — enough for MS-DOS, Windows, and Linux floppy drivers to detect the controller, identify media, and perform read/write/format operations. Commands that interact with physical media timing (perpendicular recording mode, power management) are accepted but largely no-op'd.
Key differences from real hardware:
- No multi-drive support (real hardware supports drives 0–3).
- Physical media timing (step rate, head load/unload from SPECIFY) is accepted but doesn't affect I/O timing.
- CHS-to-LBA translation is straightforward — the controller doesn't emulate track-level interleave or skew.
- STATUS_A and STATUS_B registers return fixed values rather than reflecting physical drive state.
| Crate | Purpose | Rustdoc |
|---|---|---|
floppy |
Full 82077AA emulator | rustdoc |
floppy_pcat_stub |
Stub controller (no drives) | rustdoc |
floppy_resources |
Config types (Resource-based instantiation not yet implemented) | rustdoc |
The storage pipeline page covers how the floppy controller connects to the broader disk backend abstraction.