CRO Format – Containers ROms for Amstrad CPC

This document describes the CRO (Containers ROms) format, designed to group and describe ROMs for Amstrad CPC computers, all generations.

CRO is based on the Resource Interchange File Format (RIFF) using chunks to organize data.

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1. RIFF Overview

1.1 General Principle

RIFF is a binary format organized in independent chunks. It does not define the data semantics, only the structure.

A RIFF file consists of:

• a global header
• a sequence of chunks, possibly nested

Unknown chunks can be ignored safely.

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1.2 RIFF Header

Offset	Size	Description
0x00	4	ASCII "RIFF"
0x04	4	Total file size minus 8 bytes (uint32 LE)
0x08	4	Form Type for application format

CRO files: Form Type = "CRO ".

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1.3 Chunk Structure

Field	Size	Description
Chunk ID	4	ASCII identifier
Chunk Size	4	Data size (uint32 LE)
Chunk Data	N	Payload
Padding	0 or 1	Align to even size

Padding byte added if data size is odd.

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2. CRO File Identification

CRO files: RIFF with Form Type "CRO "
Describes hierarchical ROM organization.

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3. CRO Structure

RIFF
└─ CRO␠
├─ GRRO
│ ├─ GLBL
│ ├─ GNUM
│ ├─ GMSK
│ ├─ ROM␠
│ │ ├─ RID␠
│ │ ├─ RTYP
│ │ ├─ RLOG
│ │ ├─ RPHY
│ │ └─ RDT␠
│ └─ ...
└─ ...

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4. GRRO – ROM Group

4.1 Sub-chunks

Chunk ID	Purpose
GNUM	Unique group number
GLBL	Descriptive group label
GMSK	Address mask
ROM	ROM description

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4.2 Binary

Offset	Size	Description
0x00	4	"GRRO"
0x04	4	Chunk size (uint32 LE)
0x08	N	Sub-chunks: GNUM + GLBL + GMSK+ ROMs

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4.3 GNUM – Group Number

Offset	Size	Description
0x00	4	Unique number of group (uint32 LE, start at 0)

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4.4 GLBL – Group Label

Offset	Size	Description
0x00	N	Description ASCII String

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4.5 GMSK – Address Mask

Offset	Size	Description
0x00	4	Address mask (uint32 little-endian)

GMSK defines a binary mask applied to the ROM address: effective_address = address & GMSK

This allows emulation of EEPROMs smaller than the available address space (e.g. CPC PLUS 19-bit addressing), by forcing ROM mirroring beyond the physical size.

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5. ROM Chunk

5.1 Sub-chunks

Chunk ID	Role
RID	ROM identifier
RTYP	Logical type
RLOG	Logical number
RPHY	Physical number
RDT	Binary data

5.2 Binary

RID

Offset	Size	Description
0x00	N	ASCII string

RTYP

Offset	Size	Description
0x00	4	Logical type uint32 LE

Values:

• 0x00000000 : ROM_LOW
• 0x00000001 : ROM_HIGH
• 0x00000002 : ROM_BANKABLE
• 0x00000003 : ROM_MF2

RLOG

Offset	Size	Description
0x00	4	Logical selection number (uint32 LE, 0–255)

RPHY

Offset	Size	Description
0x00	4	Physical ROM number (uint32 LE)

RDT

Offset	Size	Description
0x00	N	ROM raw contents

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6. Hardware Neutrality

CRO describes only:

• ROM organization
• intrinsic properties
• logical grouping

Not included:

• hardware ports
• selection registers
• activation/mapping

Interpretation is up to emulation or real hardware.

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7. Extensibility

RIFF allows:

• new chunks to be added without breaking compatibility
• new properties to be introduced
• unrecognized chunks to be ignored

CRO is durable, extensible, and compatible with all CPC architectures.