amiga-bootcamp/10_devices/scsi.md

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scsi.device — Hard Disk, CD-ROM, and Mass Storage I/O

Overview

SCSI and IDE hard disks on the Amiga are accessed through scsi.device or compatible device drivers. The API is consistent across implementations — the same IORequest structure and commands work regardless of whether the underlying hardware is Commodore's native Gayle IDE, a Zorro SCSI card, or an accelerator-integrated controller.

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Disk Interfaces by Amiga Model

Model	Interface	Controller	Device Name	Bandwidth Limit
A500	None (stock)	—	—	Requires external SCSI card
A500+	None (stock)	—	—	Requires external SCSI card
A600	IDE (44-pin)	Gayle	scsi.device	~1.5 MB/s (PIO, 16-bit CIA)
A1000	None (stock)	—	—	Requires SCSI sidecar
A1200	IDE (44-pin)	Gayle	scsi.device	~1.5 MB/s (PIO)
A2000	None (stock)	—	—	Zorro II SCSI cards
A3000	SCSI (50-pin)	WD33C93	scsi.device	~3 MB/s (DMA)
A4000	IDE (40-pin)	A4000 IDE	scsi.device	~2 MB/s (PIO)
A4000T	SCSI (50-pin) + IDE	NCR 53C710 + IDE	2nd.scsi.device / scsi.device	~10 MB/s (SCSI DMA)
CD32	IDE (internal CD)	Akiko	scsi.device	~1.5 MB/s
CDTV	SCSI (internal CD)	DMAC + WD33C93	scsi.device	~150 KB/s (1× CD)

Why Native Bandwidth Is Limited

The A600/A1200 Gayle IDE interface is the most common and the most constrained:

flowchart LR
    CPU["68020/68030<br/>(fast bus)"] -->|"PIO transfer<br/>WORD at a time"| GAYLE["Gayle IDE<br/>(no DMA!)"]
    GAYLE -->|"16-bit ATA"| DRIVE["IDE Drive"]

    style GAYLE fill:#ffcdd2,stroke:#c62828,color:#333

Bottleneck	Explanation
No DMA	Gayle has no DMA engine — every word must be moved by the CPU (MOVE.W loop)
16-bit bus	Gayle connects via 16-bit data path even on 32-bit CPUs
PIO mode 0	Stock Gayle supports only PIO mode 0 (~3.3 MB/s theoretical, ~1.5 MB/s actual)
CIA timing	CIA chip access introduces wait states
CPU overhead	100% CPU utilization during transfers — no multitasking during disk I/O

Community Solutions — Fast IDE

Solution	Method	Improvement
FastATA (E-Matrix/Elbox)	Zorro SCSI/IDE card with DMA	Up to ~10 MB/s, frees CPU
Buddha/Catweasel	Clock port / Zorro IDE with optimized driver	~2–3 MB/s, multiple IDE channels
Blizzard SCSI	Accelerator-integrated SCSI	Up to ~5 MB/s with DMA
GVP Series II	Zorro II SCSI with custom DMA ASIC	~3 MB/s, DMA frees CPU
A4091	Zorro III SCSI (NCR 53C710)	~10 MB/s — fastest stock Amiga SCSI
CF adapters	CompactFlash in IDE socket	Same speed, but no mechanical seek latency
SD/microSD	Adapter on clock port	Varies, typically slow but no noise

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Native vs. Vendor Drivers — Software Differences

All drivers expose the same CMD_READ/CMD_WRITE/HD_SCSICMD API, but internal differences matter:

Aspect	Commodore scsi.device	Vendor Drivers (GVP, A4091, etc.)
Source	Commodore ROM or Devs:	Third-party (ships with hardware)
DMA support	None (Gayle PIO)	Often DMA-capable
Interrupt handling	Level 2 interrupt (CIA)	Card-specific interrupt level
TD64 / NSD	Added via patches	Often built-in from factory
Multi-LUN	Usually ignored	Proper LUN scanning on SCSI
Disconnect/reselect	Not supported (Gayle)	SCSI disconnect on good controllers
Tagged queuing	No	Some advanced SCSI controllers
CD-ROM support	Basic (atapi.device)	Full ATAPI/SCSI CD support
Removable media	Basic change detection	Proper unit attention handling

Important

When writing software that accesses disks, always use the device name from the mountlist — don't hardcode scsi.device. Different systems use different device names for the same physical drive.

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Opening

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

/* Unit 0 = first device on the bus (master/ID 0): */
if (OpenDevice("scsi.device", 0, (struct IORequest *)diskReq, 0))
{
    Printf("Cannot open scsi.device unit 0\n");
}

Units

Unit	SCSI Meaning	IDE Meaning
0	SCSI ID 0	Master
1	SCSI ID 1	Slave
2–6	SCSI ID 2–6	N/A
7	Host adapter (reserved)	N/A

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Standard Commands

/* Read 512 bytes from byte offset on disk: */
diskReq->io_Command = CMD_READ;
diskReq->io_Data    = buffer;
diskReq->io_Length  = 512;
diskReq->io_Offset  = 0;           /* byte offset */
DoIO((struct IORequest *)diskReq);

/* Write: */
diskReq->io_Command = CMD_WRITE;
diskReq->io_Data    = buffer;
diskReq->io_Length  = 512;
diskReq->io_Offset  = 512;
DoIO((struct IORequest *)diskReq);

/* Flush write cache: */
diskReq->io_Command = CMD_UPDATE;
DoIO((struct IORequest *)diskReq);

Important

The io_Offset field is a ULONG (32-bit), limiting addressable space to 4 GB. For larger drives, use TD64/NSD 64-bit commands.

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Direct SCSI Commands (HD_SCSICMD)

For operations beyond read/write — device identification, mode pages, CD-ROM commands:

#include <devices/scsidisk.h>

struct SCSICmd scsicmd;
UBYTE cdb[10], sense[20], data[512];

/* READ(10) — read 1 sector at LBA 0: */
memset(cdb, 0, sizeof(cdb));
cdb[0] = 0x28;                    /* READ(10) opcode */
cdb[7] = 0; cdb[8] = 1;          /* transfer length = 1 sector */

scsicmd.scsi_Data       = (UWORD *)data;
scsicmd.scsi_Length      = 512;
scsicmd.scsi_Command     = cdb;
scsicmd.scsi_CmdLength   = 10;
scsicmd.scsi_Flags       = SCSIF_READ | SCSIF_AUTOSENSE;
scsicmd.scsi_SenseData   = sense;
scsicmd.scsi_SenseLength = sizeof(sense);

diskReq->io_Command = HD_SCSICMD;
diskReq->io_Data    = &scsicmd;
diskReq->io_Length  = sizeof(scsicmd);
DoIO((struct IORequest *)diskReq);

if (scsicmd.scsi_Status != 0)
    Printf("SCSI error: status=%ld, sense key=%ld\n",
           scsicmd.scsi_Status, sense[2] & 0x0F);

Common SCSI Commands

Opcode	Name	CDB	Description
0x00	TEST UNIT READY	6	Check device readiness
0x03	REQUEST SENSE	6	Get error details
0x12	INQUIRY	6	Device identification
0x1A	MODE SENSE(6)	6	Read device parameters
0x25	READ CAPACITY	10	Get total sectors and sector size
0x28	READ(10)	10	Read sectors (32-bit LBA)
0x2A	WRITE(10)	10	Write sectors
0x35	SYNCHRONIZE CACHE	10	Flush write cache
0x43	READ TOC	10	Read CD table of contents
0xBE	READ CD	12	Read CD sector (any format)

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CD-ROM Specifics

CD-ROM drives require special handling — they use ATAPI (IDE) or SCSI commands but with CD-specific extensions:

Opening a CD-ROM

/* CD-ROM is typically on a different device or unit: */
/* A1200 + ATAPI CD: */
OpenDevice("scsi.device", 2, req, 0);  /* unit 2 = slave on second channel */

/* External SCSI CD: */
OpenDevice("2nd.scsi.device", 3, req, 0);  /* SCSI ID 3 */

/* AmiCDFS or CacheCDFS handles mounting automatically via:
   DEVS:DOSDrivers/CD0 mountlist entry */

Reading the Table of Contents

UBYTE tocData[804];  /* max TOC size */
memset(cdb, 0, 10);
cdb[0] = 0x43;       /* READ TOC */
cdb[6] = 1;          /* starting track */
cdb[7] = (sizeof(tocData) >> 8) & 0xFF;
cdb[8] = sizeof(tocData) & 0xFF;

scsicmd.scsi_Data      = (UWORD *)tocData;
scsicmd.scsi_Length     = sizeof(tocData);
scsicmd.scsi_Command    = cdb;
scsicmd.scsi_CmdLength  = 10;
scsicmd.scsi_Flags      = SCSIF_READ | SCSIF_AUTOSENSE;
DoIO((struct IORequest *)diskReq);

Audio CD Playback

/* PLAY AUDIO MSF — play from start to end: */
memset(cdb, 0, 10);
cdb[0] = 0x47;       /* PLAY AUDIO MSF */
cdb[3] = 0;          /* start minute */
cdb[4] = 2;          /* start second */
cdb[5] = 0;          /* start frame */
cdb[6] = 60;         /* end minute */
cdb[7] = 0;          /* end second */
cdb[8] = 0;          /* end frame */

scsicmd.scsi_Flags = SCSIF_READ;
DoIO((struct IORequest *)diskReq);

CD Filesystem Drivers

Driver	Type	Features
AmiCDFS	Commodore (stock)	Basic ISO 9660, slow
CacheCDFS	Third-party (popular)	ISO 9660 + Joliet + RockRidge, caching, fast
AsimCDFS	Third-party	Similar to CacheCDFS, commercial

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64-Bit Addressing (TD64 / NSD)

For drives larger than 4 GB:

/* TD64 — uses io_Actual for high 32 bits: */
diskReq->io_Command = NSCMD_TD_READ64;
diskReq->io_Data    = buffer;
diskReq->io_Length  = 512;
diskReq->io_Offset  = lowOffset;      /* low 32 bits */
diskReq->io_Actual  = highOffset;     /* high 32 bits */
DoIO((struct IORequest *)diskReq);

Tip

New Style Device (NSD) provides a standard way to query 64-bit support: send NSCMD_DEVICEQUERY and check the returned command list.

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MiSTer / FPGA Notes

Aspect	Implementation
IDE emulation	MiSTer emulates Gayle IDE — presents virtual ATA backed by SD card .hdf
SCSI emulation	A2091/A3000 SCSI cores map targets to .hdf files
Sector size	Must be 512 bytes — all Amiga drivers assume this
RDB	Rigid Disk Block at sectors 0–15 — must be present for HDToolBox
Performance	Virtual IDE is faster than real Gayle (no PIO bottleneck)

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References

• NDK39: devices/scsidisk.h, devices/trackdisk.h
• ADCD 2.1: scsi.device autodocs
• SCSI-2 standard: ANSI X3.131-1994
• See also: trackdisk.md — floppy I/O (shares the same API model)
• See also: CDTV Hardware — DMAC/WD33C93 SCSI CD-ROM controller
• See also: Akiko — CD32 — CD32 CD-ROM controller (Akiko PIO, not SCSI)
• See also: Gayle IDE & PCMCIA — A600/A1200 IDE controller
• See also: Gary — A3000 — A3000 SDMAC/WD33C93 SCSI integration