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07_dos: - file_io.md: 108→240+ lines — buffered I/O (FRead/FWrite/SetVBuf), access mode comparison, FileHandle struct with offsets, standard handles, Printf %ld warning, FileInfoBlock, practical patterns (copy file, get size, load to RAM), error code table - filesystem.md: 91→270+ lines — full disk geometry (ADF/HDF), all 8 DOS\x filesystem IDs, root block byte-level layout, file header layout with reverse-order pointer quirk, OFS vs FFS data blocks with efficiency numbers, bitmap blocks, extension blocks, checksum algorithm, Python ADF reader - locks_examine.md: 113→270+ lines — lock semantics diagram, FileLock struct with handler discovery, ExAll bulk scan, practical patterns (atomic write, path resolution, volume info), 4 antipatterns (leaked locks, exclusive too long, unchecked IoErr, DupLock), pattern matching 10_devices: - audio.md: 73→240+ lines — hardware architecture diagram, channel registers with offsets, period/frequency table, priority allocation, double-buffering, audio interrupts, AM/PM modulation, direct HW - timer.md: 80→230+ lines — CIA timer hardware, all 5 units with decision flowchart, non-blocking delays, signal-based waiting, time arithmetic, ReadEClock, periodic game loop pattern, pitfalls - trackdisk.md: 82→210+ lines — MFM encoding, track format, disk geometry, read/write/motor, change notification, track caching, direct hardware access, FPGA timing implications - keyboard.md: 58→220+ lines — CIA-A serial handshake protocol with sequence diagram, bit rotation quirk, complete key code map, key matrix bitmap, reset sequence, FPGA notes
178 lines
5.4 KiB
Markdown
178 lines
5.4 KiB
Markdown
[← Home](../README.md) · [Devices](README.md)
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# trackdisk.device — Floppy Disk DMA Controller
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## Overview
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`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).
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---
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## Hardware Architecture
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```mermaid
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graph LR
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subgraph "Custom Chips"
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DSKBYTR["DSKBYTR<br/>$DFF01A<br/>Disk Data Byte"]
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DSKLEN["DSKLEN<br/>$DFF024<br/>DMA Length"]
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DSKPT["DSKPT<br/>$DFF020<br/>DMA Pointer"]
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end
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subgraph "CIA-B"
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CIAPRB["PRA/PRB<br/>$BFD100<br/>Motor, Side, Step"]
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end
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DISK["3.5 DD Disk"] -->|"MFM bitstream"| DSKBYTR
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CIAPRB -->|"Motor/Step/Side"| DISK
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DSKPT -->|"DMA to/from"| CHIPRAM["Chip RAM Buffer<br/>(~13 KB/track)"]
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```
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### Disk Geometry
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| Parameter | Value |
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|---|---|
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| Tracks | 80 (0–79) |
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| Sides | 2 (0=upper, 1=lower) |
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| Sectors per track | 11 (DD), 22 (HD) |
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| Bytes per sector | 512 |
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| Total capacity | 880 KB (DD), 1,760 KB (HD) |
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| Rotation speed | 300 RPM (1 revolution = 200 ms) |
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| Transfer rate | ~250 kbit/s (DD raw MFM) |
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| Track-to-track seek | ~3 ms |
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### MFM Encoding
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The disk stores data in **Modified Frequency Modulation** format. Each byte becomes 16 bits on disk (clock + data interleaved):
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```
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Data bit: 1 0 1 1 0 0 0 1
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MFM: 01 10 01 01 10 10 10 01
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↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑
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c/d pairs (clock bit inserted before each data bit)
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```
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A raw track is ~12,668 bytes of MFM data (including gaps, sync words, and sector headers).
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### Track Format (AmigaDOS)
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```
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Track = 11 sectors, each containing:
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Sync: $4489 $4489 (2 words — MFM-encoded $A1 $A1)
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Header: format, track, sector, sectors_to_gap (MFM-encoded)
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Header checksum: XOR of header longs
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Data: 512 bytes of payload (MFM-encoded = 1024 bytes on disk)
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Data checksum: XOR of data longs
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Gaps between sectors: variable-length padding
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```
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---
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## Using trackdisk.device
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### Opening
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```c
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struct MsgPort *diskPort = CreateMsgPort();
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struct IOExtTD *diskReq = (struct IOExtTD *)
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CreateIORequest(diskPort, sizeof(struct IOExtTD));
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/* Unit numbers: DF0:=0, DF1:=1, DF2:=2, DF3:=3 */
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BYTE err = OpenDevice("trackdisk.device", 0,
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(struct IORequest *)diskReq, 0);
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```
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### Reading Sectors
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```c
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UBYTE *buf = AllocMem(512, MEMF_CHIP); /* MUST be Chip RAM */
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diskReq->iotd_Req.io_Command = CMD_READ;
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diskReq->iotd_Req.io_Data = buf;
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diskReq->iotd_Req.io_Length = 512; /* bytes to read */
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diskReq->iotd_Req.io_Offset = 0; /* byte offset on disk */
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/* offset = (track * 2 + side) * 11 * 512 + sector * 512 */
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DoIO((struct IORequest *)diskReq);
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```
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### Writing + Updating (Motor Control)
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```c
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/* Write a sector: */
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diskReq->iotd_Req.io_Command = CMD_WRITE;
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diskReq->iotd_Req.io_Data = buf;
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diskReq->iotd_Req.io_Length = 512;
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diskReq->iotd_Req.io_Offset = 512 * 10; /* sector 10 */
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DoIO((struct IORequest *)diskReq);
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/* Flush write buffer to disk: */
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diskReq->iotd_Req.io_Command = CMD_UPDATE;
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DoIO((struct IORequest *)diskReq);
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/* Turn off motor when done: */
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diskReq->iotd_Req.io_Command = TD_MOTOR;
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diskReq->iotd_Req.io_Length = 0; /* 0=off, 1=on */
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DoIO((struct IORequest *)diskReq);
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```
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### Disk Change Notification
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```c
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/* Wait for disk insertion/removal: */
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diskReq->iotd_Req.io_Command = TD_CHANGENUM;
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DoIO((struct IORequest *)diskReq);
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ULONG changeCount = diskReq->iotd_Req.io_Actual;
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/* Async notification: */
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diskReq->iotd_Req.io_Command = TD_ADDCHANGEINT;
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diskReq->iotd_Req.io_Data = (APTR)&myInterrupt;
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SendIO((struct IORequest *)diskReq);
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/* myInterrupt is signalled on disk change */
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```
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### Track Caching
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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:
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```
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Read sector 0 → DMA reads track 0 (11 sectors) → cache hit for sectors 1–10
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Read sector 11 → new track → DMA reads track 1
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Read sector 5 → cache hit (still in track 0 buffer)
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```
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> **FPGA implication**: the MiSTer core must emulate this whole-track DMA behaviour for correct timing. Games that measure seek+read latency will behave incorrectly if only single sectors are transferred.
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---
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## Direct Hardware Access (Games/Demos)
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Games often bypass trackdisk.device for speed and copy protection:
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```asm
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; Direct floppy read — raw track DMA:
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LEA $DFF000, A5 ; custom base
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MOVE.L #TrackBuffer, $20(A5) ; DSKPT — DMA pointer (Chip RAM)
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MOVE.W #$8210, $96(A5) ; DMACON — enable disk DMA
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; Select drive, side, seek to track:
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MOVE.B #$F7, $BFD100 ; CIA-B PRB — select DF0, motor on
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; ... step head to desired track ...
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; Start reading one track:
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MOVE.W #$8000|6300, $24(A5) ; DSKLEN — enable, ~6300 words
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MOVE.W #$8000|6300, $24(A5) ; write twice to start DMA
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; Wait for DMA complete (DSKBLK interrupt):
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BTST #1, $DFF01F ; INTREQR — DSKBLK bit
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BEQ.S .-4
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```
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---
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## References
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- NDK39: `devices/trackdisk.h`, `resources/disk.h`
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- HRM: *Amiga Hardware Reference Manual* — Disk Controller chapter
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- ADCD 2.1: trackdisk.device autodocs
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- See also: [filesystem.md](../07_dos/filesystem.md) — FFS/OFS block format on top of trackdisk
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