amiga-bootcamp/01_hardware/common/zorro_bus.md

← Home · Hardware

Zorro Bus — Expansion Architecture

Overview

The Amiga uses the Zorro expansion bus for add-on cards. There are two generations:

• Zorro II — 16-bit, 24-bit addressing, 7 MHz, compatible with A2000/A3000/A4000
• Zorro III — 32-bit, 32-bit addressing, up to 33 MHz burst, A3000/A4000 only

Zorro uses AutoConfig — a standardised plug-and-play configuration protocol that predates PCI by several years.

Zorro II

Parameter	Value
Data bus	16-bit
Address bus	24-bit
Clock	7.14 MHz (bus cycle ≈ 280 ns)
Max transfer	~5 MB/s (DMA)
Address space	$A00000–$EFFFFF (I/O), $200000–$9FFFFF (RAM)
Slots	5 (A2000), 3 (A3000)

Zorro II cards appear in the 16 MB address space. RAM cards are configured into $200000–$9FFFFF. I/O cards use $A00000–$DEFFFF.

Zorro III

Parameter	Value
Data bus	32-bit
Address bus	32-bit
Clock	Up to 33 MHz burst
Max transfer	~40 MB/s (DMA)
Address space	$01000000 and above
Slots	4 (A3000), 5 (A4000)

Zorro III extends into the 32-bit address space, allowing large RAM cards (32–128 MB) and fast peripherals. Requires a 32-bit CPU (68030+) and OS support.

AutoConfig Protocol

AutoConfig allows the OS to discover and configure cards without jumpers:

sequenceDiagram
    participant OS as AmigaOS (expansion.library)
    participant Card as Zorro Card

    OS->>Card: Read $E80000 (config space)
    Card-->>OS: Manufacturer ID (16-bit)
    OS->>Card: Read $E80002
    Card-->>OS: Product ID, flags
    OS->>Card: Read board size, type
    OS->>OS: AllocAbs() / ConfigBoard()
    OS->>Card: Write base address
    Card-->>OS: Card configured, moves off $E80000

Key AutoConfig registers (read from $E80000–$E8007F before configuration):

Offset	Content
$00	er_Type (board type: RAM/IO, Zorro II/III)
$02	er_Product (product ID)
$04	er_Flags
$06	er_Reserved03
$08–$0A	er_Manufacturer (16-bit)
$0C–$0F	er_SerialNumber
$10–$11	er_InitDiagVec (diagnostic ROM vector)

Board types (er_Type bits):

#define ERT_TYPEMASK   0xC0
#define ERT_ZORROII    0xC0   /* Zorro II card */
#define ERT_ZORROIII   0x80   /* Zorro III card */
#define ERTB_MEMLIST   5      /* board is RAM, add to free list */
#define ERTB_DIAGVALID 4      /* DiagArea ROM is valid */
#define ERTB_CHAINEDCONFIG 3  /* more boards to configure */

expansion.library

AmigaOS provides expansion.library to manage Zorro configuration:

#include <libraries/expansion.h>
#include <clib/expansion_protos.h>

/* Find a configured board by manufacturer/product */
struct ConfigDev *cd = NULL;
while ((cd = FindConfigDev(cd, MANUF_ID, PROD_ID)) != NULL) {
    APTR base = cd->cd_BoardAddr;
    ULONG size = cd->cd_BoardSize;
    /* use board at base */
}

Key structures:

struct ConfigDev {
    struct Node    cd_Node;
    UBYTE          cd_Flags;
    UBYTE          cd_Pad;
    struct ExpansionRom cd_Rom;   /* copy of autoconfig ROM area */
    APTR           cd_BoardAddr;  /* configured base address */
    ULONG          cd_BoardSize;
    UWORD          cd_SlotAddr;
    UWORD          cd_SlotSize;
    APTR           cd_Driver;
    struct ConfigDev *cd_NextCD;
    ULONG          cd_Unused[4];
};

DiagArea — Card ROM

Cards with ERTB_DIAGVALID have a small ROM (DiagArea) that the OS calls during boot:

struct DiagArea {
    UBYTE da_Config;     /* flags */
    UBYTE da_Flags;
    UWORD da_Size;
    UWORD da_DiagPoint; /* offset to diagnostic code */
    UWORD da_BootPoint; /* offset to boot code */
    UWORD da_Name;      /* offset to name string */
    UWORD da_Reserved01;
    UWORD da_Reserved02;
};

The boot vector is called by ConfigChain() during the early boot sequence — this is how SCSI controllers install their filesystem handlers.

References

• NDK39: libraries/expansion.h, libraries/configregs.h, libraries/configvars.h
• ADCD 2.1 Autodocs: expansion — http://amigadev.elowar.com/read/ADCD_2.1/Includes_and_Autodocs_3._guide/node025B.html
• Amiga Hardware Reference Manual 3rd ed. — AutoConfig chapter
• Dave Haynie's Zorro III specification documents