Hardware Overview (Mega CD)
The Mega CD circuit is mostly made up of the following chips:
- 68000 CPU (Motorola MC68000)
- Program RAM (512kb) for 68000 (1x HM514270 256k x 16 bit or 4x MB81C4256A 256k x 4 bit)
- CD Processor (LC8951)
- PCM Sound Source (315-5476)
- Mega CD Engine (315-5477 / 315-5548)
- Video RAM for Mega CD Engine (2x TC511664BZ)
- BIOS (27C210A / 27C1024)
- Battery Backup IC (MB3790)
- Work RAM (64kb) for Mega Drive (MB8464A 64k)
Some great info on the Mega CD architecture can be found here.
The main CPU of the Mega CD is the 16-bit Motorola 68000. It runs at 12.5 MHz processor. The Mega Drive/Genesis has the same processor, but at a lower clock rate of 7.67 MHz (NTSC) / 7.61 MHz (PAL).
The SRAM attached to the 68k processor is either a single 256k x 16bit SRAM, or 4 lots of 256k x 4 bit SRAM, depending on revision.
This megabit (512 kilobyte) of RAM is used primarily by the Mega CD 68k CPU for program code, runtime memory and data processing, with occassional data transfer to the Mega Drive CPU.
The Sanyo LC8951 is a CD processor IC responsible for processing the CD drive on the Mega CD.
Sega had various versions of the custom ASIC, including 315-5477 and 315-5548.
This is the main workhorse of the Mega CD, sometimes called the Mega CD Engine, the Gate Array or the Graphics Processor.
- Graphics Processor: Custom ASIC
- Number of simultaneous colors on screen: 64 out of 512
- Display resolution: 320 x 224 pixels and 256 x 224, video size from ¼ to full screen
- Advanced compression scheme
- Software-based upgrade
- Scaling and rotation effects
The Video RAM is 2 megabits (256 kilboytes) split over two RAM chips TC511664BZ (64k x 16bit).
The Video RAM be used by both the Mega CD CPU and the Mega Drive CPU to read or write data.
There are two modes the system can run in:
- 2M mode: The whole 2 megabits available to a single CPU at a time.
- 1M mode: Memory is split into 1 megabit chunks, with each CPU having simultaneous access to each opposite bank. Access to the other bank can then be swapped at any time.
2M mode is useful for loading and transferring large blocks of data around, while 1M mode is more suited for more immediate transfers, like calculated graphics or video playback.
The BIOS is a 1Mbit (64k x 16bit) EEPROM. It stores the BIOS to the console.
The PCM Sound Source labelled 315-5476, is just a rebranded Ricoh PCM Sound Source RF5C164A.
The battery back IC is used to provide stable power to the volatile memory (Work RAM).
The Work RAM on the Mega CD is 64bit x 8bit that is used exclusively by the 68000 CPU on the Mega Drive main program code and runtime memory, and is not used directly by the Mega CD.
The Work RAM is volatile memory used to store date and time, game saves and has its power provided by the battery backup IC.