amiga-bootcamp/01_hardware/ocs_a500/cdtv_hardware.md

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CDTV — Platform Hardware

Overview

The Commodore Dynamic Total Vision (CDTV, 1991) is an A500-class OCS computer packaged as a consumer CD-ROM set-top box. Internally it uses the same Agnus/Denise/Paula chipset as the A500, but adds several unique subsystems:

1. DMAC + WD33C93 SCSI controller — drives the internal CD-ROM via DMA
2. Extended ROM — 512 KB of additional firmware for CD filesystem and player software
3. NVRAM — 64 KB battery-backed non-volatile storage
4. IR remote receiver — proprietary 40 kHz infrared protocol
5. Real-Time Clock — Oki MSM6242B at $DC0000

The CDTV has no keyboard (optional external), no internal floppy (optional external), and no expansion slots visible to the end user. It targets the living room, not the desktop.

System Specifications

Parameter	Value
CPU	68000 @ 7.09 MHz (PAL) / 7.16 MHz (NTSC)
Chipset	OCS (Agnus 8361/8367, Denise 8362, Paula 8364)
Chip RAM	1 MB (expandable to 2 MB with Super Agnus mod)
Fast RAM	None stock
ROM	256 KB Kickstart 1.3 + 512 KB Extended ROM
CD-ROM	Single-speed (1×), SCSI-based via DMAC/WD33C93
Audio	4-channel Paula DMA + CD-DA passthrough
Video	Composite, S-Video, RF (no RGB without mod)
Controller	IR remote, optional external keyboard
Storage	CD-ROM, 64 KB NVRAM

Address Map — CDTV-Specific Regions

The standard OCS 24-bit address map applies. The CDTV adds these regions:

Address Range	Size	Region
$DC0000–$DC003F	64 B	Real-Time Clock (Oki MSM6242B)
$E00000–$E3FFFF	256 KB	Extended ROM bank 1 (CD filesystem, player UI)
$E40000–$E7FFFF	256 KB	Extended ROM bank 2 (DMAC driver, boot logic)
$E90000–$E9FFFF	64 KB	DMAC registers (WD33C93 SCSI DMA controller)
$F00000–$F3FFFF	256 KB	NVRAM (battery-backed, 64 KB actual usable)

See also: Address Space — full 24-bit map with CDTV-specific entries.

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DMAC and WD33C93 SCSI Controller

Architecture

The CDTV's CD-ROM is connected via a SCSI bus — the same architecture used in the A2091 and A590 hard drive controllers. The system uses two chips:

Chip	Role
DMAC (Commodore custom)	DMA controller — transfers data between the WD33C93 and Amiga memory
WD33C93 (Western Digital)	SCSI Bus Interface Controller (SBIC) — handles SCSI protocol with the CD-ROM drive

This is fundamentally different from the CD32, where Akiko handles CD-ROM control directly via PIO.

DMAC Register Map

The DMAC is mapped at $E90000:

Offset	Register	R/W	Description
$E90000	DAWR	W	DMAC address write (DMA destination)
$E90002	WTCH	W	Word Transfer Count High
$E90004	CNTR	RW	Control register (DMA direction, interrupt enable)
$E90040	ACR_H	W	Address Counter Register high word
$E90042	ACR_L	W	Address Counter Register low word
$E90048	ST_DMA	W	Start DMA transfer (write any value)
$E9004A	FLUSH	W	Flush DMA FIFO
$E9004C	CINT	W	Clear DMAC interrupt
$E9004E	ISTR	R	Interrupt Status Register
$E90090	SASR	W	WD33C93 Address/Select Register
$E90091	SCMD	RW	WD33C93 Data Register (read/write SBIC registers)

WD33C93 SBIC Registers

The WD33C93 is accessed indirectly through the DMAC. Write the target register number to SASR ($E90090), then read/write data via SCMD ($E90091):

Register	Name	Description
$00	Own ID	SCSI initiator ID (typically 7)
$01	Control	DMA mode, interrupt enables
$02	Timeout	Selection timeout period
$03–$0B	CDB	Command Descriptor Block (SCSI command bytes)
$0F	Command	SCSI command register (initiate bus phases)
$10	Data	Data transfer register (PIO mode)
$17	Status	SCSI bus phase and completion status

SCSI Commands for CD-ROM

The CD-ROM drive responds to standard SCSI-2 commands:

Command	Opcode	Use
TEST UNIT READY	$00	Check drive presence
READ(6)	$08	Read data sectors (Mode 1)
READ(10)	$28	Read data sectors (extended addressing)
READ TOC	$43	Read Table of Contents
PLAY AUDIO(10)	$45	Play CD-DA audio tracks
PAUSE/RESUME	$4B	Pause/resume audio playback
READ SUB-CHANNEL	$42	Read current position, ISRC, UPC/EAN

AmigaOS Access

Under AmigaOS, the CD-ROM is accessed through scsi.device loaded from Extended ROM:

Application → dos.library → cdfs (CD filesystem handler) → scsi.device → DMAC → WD33C93 → CD-ROM

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Extended ROM

The CDTV includes 512 KB of additional ROM mapped at $E00000–$E7FFFF, split across two 256 KB banks. This ROM contains:

Bank	Address Range	Contents
Bank 1	$E00000–$E3FFFF	CD filesystem (CDFS), audio player UI, bookmark manager
Bank 2	$E40000–$E7FFFF	DMAC/WD33C93 device driver (scsi.device), boot sequence, system initialization

The Extended ROM is not present on standard A500/A2000 machines. Software that detects CDTV typically checks for the presence of cdtv.device or reads the Extended ROM base.

Boot Sequence

1. Kickstart 1.3 loads from ROM ($F80000)
2. Extended ROM at $E00000 is detected and initialized
3. scsi.device from Extended ROM initializes DMAC + WD33C93
4. CD-ROM drive is probed for a bootable disc
5. If a valid Amiga boot block is found on the CD → boot from CD
6. If no CD → fall through to standard floppy boot (if external floppy present)

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NVRAM (Non-Volatile Storage)

Hardware

The CDTV includes 64 KB of battery-backed SRAM mapped at $F00000–$F3FFFF. This is fundamentally different from the CD32's tiny serial EEPROM — the CDTV has substantially more storage.

Parameter	Value
Type	Battery-backed SRAM
Capacity	64 KB (512 Kbit)
Address	$F00000–$F0FFFF (64 KB usable within 256 KB window)
Persistence	Battery-backed (internal lithium cell)
Access	Direct memory-mapped (byte-addressable)
Typical use	Bookmarks, game saves, user preferences

Access

Unlike the CD32's I²C EEPROM, CDTV NVRAM is directly memory-mapped — the CPU reads and writes it like normal RAM:

; Read a byte from NVRAM
    move.b  $F00000, d0         ; read first byte of NVRAM

; Write a byte to NVRAM
    move.b  d0, $F00000         ; write to NVRAM — persists across power cycles

AmigaOS Interface

The CDTV provides bookmark.device for structured NVRAM access:

/* CDTV bookmark access — stores named data blocks in NVRAM */
struct IOStdReq *io;
/* ... open bookmark.device ... */
io->io_Command = CMD_WRITE;
io->io_Data    = save_data;
io->io_Length  = data_length;
DoIO((struct IORequest *)io);

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Infrared Remote Controller

Hardware

The CDTV includes a dedicated IR receiver module and ships with a full remote control unit. The remote provides media playback buttons, a numeric keypad, and navigation controls.

Protocol

The CDTV uses a proprietary IR protocol (not NEC, not RC-5):

Parameter	Value
Carrier frequency	40 kHz
Frame size	12-bit command + 12-bit inverted (24-bit total)
Header	9 ms pulse + 4.5 ms space
Bit encoding	400 µs pulse + variable space (400 µs = 0, 1200 µs = 1)
Repeat	9 ms pulse + 2.1 ms space + 400 µs end (every ~60 ms)
Receiver	Dedicated module tuned for 40 kHz (TSOP38240 compatible)

Button Mapping

Button	Code	Function
Play/Pause	$01	Media playback toggle
Stop	$02	Stop playback
Forward	$03	Next track / fast forward
Rewind	$04	Previous track / rewind
Vol+	$05	Volume up
Vol−	$06	Volume down
0–9	$10–$19	Numeric keypad
Enter	$1A	Confirm / select
Escape	$1B	Cancel / back
Up/Down/Left/Right	$1C–$1F	Navigation
A / B	$20 / $21	Assignable (game buttons)

Note

The IR receiver connects to the system as a keyboard-like input device. AmigaOS treats remote button presses as IECLASS_RAWKEY input events with dedicated qualifier codes. Standard input.device handlers receive these events transparently.

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Real-Time Clock

The CDTV includes an Oki MSM6242B RTC chip at $DC0000:

Address	Register	Description
$DC0001	Seconds (units)	BCD 0–9
$DC0003	Seconds (tens)	BCD 0–5
$DC0005	Minutes (units)	BCD 0–9
$DC0007	Minutes (tens)	BCD 0–5
$DC0009	Hours (units)	BCD 0–9
$DC000B	Hours (tens)	BCD 0–2
$DC000D	Day (units)	BCD 0–9
$DC000F	Day (tens)	BCD 0–3
$DC0011	Month (units)	BCD 0–9
$DC0013	Month (tens)	BCD 0–1
$DC0015	Year (units)	BCD 0–9
$DC0017	Year (tens)	BCD 0–9
$DC0019	Day of week	0–6
$DC001B	Control D	AM/PM, 12/24 mode
$DC001D	Control E	IRQ enable, test
$DC001F	Control F	Reset, busy flag

Note

The same MSM6242B RTC is used in the A2000 and A3000. The register layout is identical across all models.

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CDTV vs A500 — Hardware Comparison

Feature	A500	CDTV
CPU	68000 @ 7.09 MHz	68000 @ 7.09 MHz
Chipset	OCS	OCS (identical)
Chip RAM	512 KB–1 MB	1 MB
ROM	256 KB Kickstart	256 KB Kickstart + 512 KB Extended ROM
Storage	Internal floppy (880 KB)	CD-ROM (1×, 680 MB) + 64 KB NVRAM
Input	Keyboard + mouse + joystick	IR remote + optional ext. keyboard
Audio	Paula 4-channel	Paula + CD-DA passthrough
Video output	RGB (DB23)	Composite, S-Video, RF
Expansion	Trapdoor + side	None visible (internal A2000-compatible)
RTC	None stock (add-on)	Oki MSM6242B

Software Compatibility

The CDTV runs standard AmigaOS 1.3 software from CD or external floppy. The Extended ROM adds CD-specific functionality but does not break compatibility. Most A500 games work unmodified when loaded from compatible media.

References

• Commodore CDTV Technical Reference Manual (internal)
• WD33C93 SCSI Bus Interface Controller datasheet (Western Digital)
• Oki MSM6242B Real-Time Clock datasheet
• WinUAE source: CDTV emulation code (Toni Wilen)
• Big Book of Amiga Hardware — CDTV
• ADCD 2.1: scsi.device Autodocs

See Also

• OCS Chipset — Agnus/Denise/Paula (shared with CDTV)
• Memory Types — CDTV memory configuration
• Address Space — CDTV address map with Extended ROM and NVRAM regions
• CIA Chips — CIA A/B (shared across all models)