trackdisk.device — Floppy Disk DMA Controller

Overview

trackdisk.device interfaces with the Amiga's floppy disk controller — a custom DMA engine that reads and writes raw MFM-encoded data from double-density 3.5" disks. It provides block-level access (512 bytes/sector, 11 sectors/track, 80 tracks × 2 sides = 1,760 sectors = 880 KB per disk).

Hardware Architecture

graph LR
    subgraph "Custom Chips"
        DSKBYTR["DSKBYTR<br/>$DFF01A<br/>Disk Data Byte"]
        DSKLEN["DSKLEN<br/>$DFF024<br/>DMA Length"]
        DSKPT["DSKPT<br/>$DFF020<br/>DMA Pointer"]
    end

    subgraph "CIA-B"
        CIAPRB["PRA/PRB<br/>$BFD100<br/>Motor, Side, Step"]
    end

    DISK["3.5 DD Disk"] -->|"MFM bitstream"| DSKBYTR
    CIAPRB -->|"Motor/Step/Side"| DISK
    DSKPT -->|"DMA to/from"| CHIPRAM["Chip RAM Buffer<br/>(~13 KB/track)"]

Disk Geometry

Parameter	Value
Tracks	80 (0–79)
Sides	2 (0=upper, 1=lower)
Sectors per track	11 (DD), 22 (HD)
Bytes per sector	512
Total capacity	880 KB (DD), 1,760 KB (HD)
Rotation speed	300 RPM (1 revolution = 200 ms)
Transfer rate	~250 kbit/s (DD raw MFM)
Track-to-track seek	~3 ms

MFM Encoding

The disk stores data in Modified Frequency Modulation format. Each byte becomes 16 bits on disk (clock + data interleaved):

Data bit:  1   0   1   1   0   0   0   1
MFM:      01  10  01  01  10  10  10  01
           ↑   ↑   ↑   ↑   ↑   ↑   ↑   ↑
           c/d pairs (clock bit inserted before each data bit)

A raw track is ~12,668 bytes of MFM data (including gaps, sync words, and sector headers).

Track Format (AmigaDOS)

Track = 11 sectors, each containing:

  Sync:     $4489 $4489         (2 words — MFM-encoded $A1 $A1)
  Header:   format, track, sector, sectors_to_gap (MFM-encoded)
  Header checksum: XOR of header longs
  Data:     512 bytes of payload (MFM-encoded = 1024 bytes on disk)
  Data checksum: XOR of data longs

Gaps between sectors: variable-length padding

Using trackdisk.device

Opening

struct MsgPort *diskPort = CreateMsgPort();
struct IOExtTD *diskReq = (struct IOExtTD *)
    CreateIORequest(diskPort, sizeof(struct IOExtTD));

/* Unit numbers: DF0:=0, DF1:=1, DF2:=2, DF3:=3 */
BYTE err = OpenDevice("trackdisk.device", 0,
                       (struct IORequest *)diskReq, 0);

Reading Sectors

UBYTE *buf = AllocMem(512, MEMF_CHIP);  /* MUST be Chip RAM */

diskReq->iotd_Req.io_Command = CMD_READ;
diskReq->iotd_Req.io_Data    = buf;
diskReq->iotd_Req.io_Length  = 512;       /* bytes to read */
diskReq->iotd_Req.io_Offset  = 0;        /* byte offset on disk */
/* offset = (track * 2 + side) * 11 * 512 + sector * 512 */
DoIO((struct IORequest *)diskReq);

Writing + Updating (Motor Control)

/* Write a sector: */
diskReq->iotd_Req.io_Command = CMD_WRITE;
diskReq->iotd_Req.io_Data    = buf;
diskReq->iotd_Req.io_Length  = 512;
diskReq->iotd_Req.io_Offset  = 512 * 10;  /* sector 10 */
DoIO((struct IORequest *)diskReq);

/* Flush write buffer to disk: */
diskReq->iotd_Req.io_Command = CMD_UPDATE;
DoIO((struct IORequest *)diskReq);

/* Turn off motor when done: */
diskReq->iotd_Req.io_Command = TD_MOTOR;
diskReq->iotd_Req.io_Length  = 0;  /* 0=off, 1=on */
DoIO((struct IORequest *)diskReq);

Disk Change Notification

/* Wait for disk insertion/removal: */
diskReq->iotd_Req.io_Command = TD_CHANGENUM;
DoIO((struct IORequest *)diskReq);
ULONG changeCount = diskReq->iotd_Req.io_Actual;

/* Async notification: */
diskReq->iotd_Req.io_Command = TD_ADDCHANGEINT;
diskReq->iotd_Req.io_Data    = (APTR)&myInterrupt;
SendIO((struct IORequest *)diskReq);
/* myInterrupt is signaled on disk change */

Track Caching

trackdisk.device reads an entire track (11 sectors) into an internal buffer on each access. Subsequent reads of other sectors on the same track are served from cache:

Read sector 0 → DMA reads track 0 (11 sectors) → cache hit for sectors 1–10
Read sector 11 → new track → DMA reads track 1
Read sector 5 → cache hit (still in track 0 buffer)

FPGA implication: the MiSTer core must emulate this whole-track DMA behavior for correct timing. Games that measure seek+read latency will behave incorrectly if only single sectors are transferred.

Direct Hardware Access (Games/Demos)

Games often bypass trackdisk.device for speed and copy protection:

; Direct floppy read — raw track DMA:
    LEA     $DFF000, A5              ; custom base
    MOVE.L  #TrackBuffer, $20(A5)    ; DSKPT — DMA pointer (Chip RAM)
    MOVE.W  #$8210, $96(A5)          ; DMACON — enable disk DMA

    ; Select drive, side, seek to track:
    MOVE.B  #$F7, $BFD100            ; CIA-B PRB — select DF0, motor on
    ; ... step head to desired track ...

    ; Start reading one track:
    MOVE.W  #$8000|6300, $24(A5)     ; DSKLEN — enable, ~6300 words
    MOVE.W  #$8000|6300, $24(A5)     ; write twice to start DMA

    ; Wait for DMA complete (DSKBLK interrupt):
    BTST    #1, $DFF01F              ; INTREQR — DSKBLK bit
    BEQ.S   .-4

References

NDK39: devices/trackdisk.h, resources/disk.h
HRM: Amiga Hardware Reference Manual — Disk Controller chapter
ADCD 2.1: trackdisk.device autodocs
See also: filesystem.md — FFS/OFS block format on top of trackdisk

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