Physical Boot Process: Floppy vs Hard Drive

While DOS Boot covers the logical software handoff (from Kickstart to dos.library to the Startup-Sequence), it's equally important to understand the physical hardware mechanisms involved when an Amiga boots from a storage medium.

The physical boot sequence dictates how raw data is pulled from magnetic media into Chip RAM before the CPU ever sees a DOS\0 signature or an executable bootblock. This is especially relevant when analyzing Custom Trackloaders which bypass the OS and interact with the hardware directly.

1. Booting from Floppy (DF0:)

The Amiga's floppy drive interface is entirely "dumb." Unlike a modern SATA drive with its own onboard microcontroller, the Amiga CPU and custom chips must manually control the floppy drive's motors and decode its raw magnetic data.

1.1 The Hardware Handoff

When the Kickstart ROM's strap module decides to boot from DF0:, it invokes trackdisk.device. This device driver issues commands directly to the CIA (Complex Interface Adapter) and Paula chips.

1.2 Step-by-Step Physical Process

sequenceDiagram
    participant CPU (trackdisk.device)
    participant CIA (8520)
    participant Floppy Drive
    participant Paula (8364)

    CPU->>CIA: Set Motor On (CIA-B Port B)
    CIA->>Floppy Drive: Spin up spindle motor
    CPU->>CIA: Assert /STEP until /TRK0 is low
    CIA->>Floppy Drive: Move read head to Cylinder 0
    CPU->>Paula: Write $4489 to DSKSYNC
    CPU->>Paula: Write buffer address to DSKPTH
    CPU->>Paula: Write $8000+size to DSKLEN
    Paula->>Floppy Drive: Start reading flux transitions
    Floppy Drive-->>Paula: Raw MFM bitstream
    Paula-->>Paula: Wait for $4489 Sync Word
    Paula->>Chip RAM: DMA stream data
    CPU->>CPU: Decode MFM to binary (Sectors 0 & 1)

Spin Up: The CPU toggles a bit on CIA-B Port B ($BFD100) to turn on the floppy drive motor. It waits a few milliseconds for the spindle to reach a stable 300 RPM.
Seek Track 0: The CPU pulses the /STEP line while monitoring the /TRK0 sensor. The drive clicks as the read/write head physically moves to the outermost cylinder (Track 0).
MFM DMA Read: The CPU configures Paula for a disk read:
- Sets the sync word in DSKSYNC ($4489).
- Points DSKPTH (Disk Pointer) to an empty buffer in Chip RAM.
- Writes to DSKLEN to start the DMA transfer.
Data Streaming: Paula watches the raw magnetic flux stream coming from the drive. The moment it sees the magnetic pattern for $4489, it starts streaming the subsequent MFM data straight into Chip RAM via DMA.
Software Decode: trackdisk.device reads the MFM data from RAM and runs an EOR/Bit-shift loop to convert the MFM bits back into standard binary bytes. It extracts the first two sectors (the 1024-byte bootblock).
Execution: If the bootblock is executable, the CPU jumps to it. At this exact moment, Custom Trackloaders (used by games) will typically execute Forbid() and Disable(), permanently locking out trackdisk.device and manually repeating steps 2-5 for the rest of the game's data.

2. Booting from Hard Drive (IDE / SCSI)

Unlike floppies, Hard Disk Drives (HDDs) are "smart." They have onboard controllers that handle physical head stepping, sector addressing (LBA or CHS), and error correction automatically. The Amiga simply asks the drive for a specific block of data.

However, the Amiga's architecture means it doesn't intrinsically know how to read an HDD upon power-up.

2.1 The AutoConfig Phase

Before an HDD can boot, the Amiga must discover its controller.

Zorro / AutoConfig: During early startup, Kickstart probes the expansion bus.
ROM Mapping: The HDD controller board (e.g., A2091 SCSI or A1200 internal IDE) responds. Kickstart maps the controller's onboard ROM into the Amiga's memory space.
Driver Initialization: The controller's ROM contains the device driver (e.g., scsi.device). Kickstart runs the initialization code in this ROM, which binds the driver to the OS.

2.2 Reading the RDB

The Amiga does not use PC-style MBR (Master Boot Record) or GPT. It uses the Rigid Disk Block (RDB).

RDB Scan: scsi.device asks the drive to read Block 0. If it doesn't find the ASCII signature RDSK, it checks blocks 1 through 15 until it finds it.
Partition Discovery: The RDB contains a linked list of partitions (e.g., DH0:, DH1:).
Filesystem Loading: Crucially, the Amiga supports loadable filesystems. If DH0: uses a custom filesystem (like PFS3 or SFS) that isn't in Kickstart, the RDB actually contains the binary code for the filesystem itself. scsi.device loads the filesystem code from the RDB into RAM and initializes it.

2.3 Step-by-Step Physical Process

sequenceDiagram
    participant CPU (scsi.device)
    participant HDD Controller
    participant HDD
    participant Fast RAM

    CPU->>HDD Controller: Send READ command (LBA 0)
    HDD Controller->>HDD: Fetch RDB Sector
    HDD-->>HDD Controller: Sector Data
    HDD Controller->>Fast RAM: DMA Transfer
    CPU->>CPU: Parse RDB & Mount DH0:
    CPU->>CPU: Load Filesystem from RDB (if needed)
    CPU->>HDD Controller: Request Bootblock (LBA of DH0: root)
    HDD Controller->>HDD: Fetch Bootblock
    HDD-->>HDD Controller: Data
    HDD Controller->>Fast RAM: DMA Transfer
    CPU->>CPU: Execute Bootblock / Mount DOS

Unlike floppies, HDD DMA transfers can typically go directly into Fast RAM, bypassing the chipset entirely (depending on the controller). Once the partition's bootblock is loaded and executed, control passes to dos.library and the Startup-Sequence exactly as it does for a floppy.

3. What Can Go Wrong (Failure Modes)

Because physical boot sequences rely on moving parts and magnetic media, they are prone to specific physical failure states.

3.1 Floppy Failures

Track 0 Sensor Failure: If the drive's /TRK0 microswitch is dirty or broken, the drive will repeatedly slam the read head against the physical stop, producing a loud, rapid "grinding" or "machine gun" noise. It cannot find sector 0.
Alignment Drift: Over time, the stepper motor can drift. The drive's head will physically rest between two tracks, reading a garbled mix of flux transitions. Paula will never see the $4489 sync word. The system hangs or presents a "Not a DOS disk" error.
Dirty Read Heads: Dust on the magnetic head weakens the flux signal. The hardware reads the $4489 sync word, but random bits in the payload flip. The software MFM decode loop succeeds, but the Bootblock Checksum fails, kicking the user back to the Kickstart screen.

3.2 Hard Drive Failures

Spin-Up Timeout: Older SCSI drives take up to 15 seconds to reach operational RPM. If the Kickstart ROM's wait loop times out before the drive asserts the READY signal, the Amiga will boot to the Kickstart screen, ignoring the HDD entirely. (Fixed by pressing Ctrl-Amiga-Amiga for a warm reboot after the drive is spinning).
RDB Corruption: If the RDB sectors are overwritten, scsi.device will scan blocks 0-15, find nothing, and report the drive as unformatted/empty, even if all partition data is perfectly intact.
Filesystem Validation: If the Amiga was powered off during a write, the filesystem's bitmap (which tracks free space) is left in an inconsistent state. On the next boot, dos.library locks the partition as Read-Only and launches a background "Validator" process. The hard drive LED will blink rapidly for minutes while it rebuilds the bitmap. If validation fails, the drive remains Read-Only.

4. Summary

Feature	Floppy (DF0:)	Hard Drive (IDE/SCSI)
Hardware Control	CPU directly controls motors & head stepping	Onboard controller handles mechanics
Data Encoding	Raw MFM (decoded by CPU in software)	Hardware handles sector decoding
Boot Discovery	Hardcoded to Track 0, Sector 0	Scans blocks 0-15 for RDB (Rigid Disk Block)
DMA Target	Must go to Chip RAM	Usually goes to Fast RAM
OS Bypass	Trivial (Custom Trackloaders)	Rare (Requires full IDE/SCSI driver implementation)

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