Amstrad Mega PC
From Sega Retro
The Amstrad Mega PC is a special Amstrad computer with the added features of a Sega Mega Drive. It was initially released in 1993 in PAL regions for the expensive price of £999.99 (later £599.99). It is unrelated to the Sega Teradrive, though the two machines share similar specifications as the computer portion is also powered by an Intel 80386sx processor.
Though a rare collector's item now, at the time of release the machine was unsuccessful due to its high retail price (made worse by the fact in 1993 a 386 processor was considered outdated). A later revision, named the Amstrad Mega PC Plus used an 80486 processor and had a RAM upgrade.
The Mega PC can also play any Japanese and/or US-localized game as well as unlicensed games.
Althrough the video always originates from the 15-pin VGA connector, the video timings differ between Mega Drive mode and PC mode. The original monitor as a Multisync VGA monitor, which accepted 15Khz video from the Mega Drive portion and the 31Khz signal from the onboard VGA.
The hardware of the PC part is a design entirely based on a chipset from Western Design Center, the same company that produced the CPU for the Super Famicom.) The ISA bus/memory controller, combined floppy/harddisk controller, multi-IO controller and SVGA chipset are entirely made by WDC.
2 ISA connectors from the ISA riser-card are 16 bit, although one is reserved for the Mega Drive itself. The Mega Drive connection to the ISA bus is 8Bit though. The only connection between the Mega Drive and the PC itself is the Yamaha FM sound chip that can be used as an Adlib-compatible soundcard, including the 15-pin connector for a traditional analogue joystick or MIDI interface. Whether or not the Mega Drive can make use of this connector is unknown. One can change the IRQ of the card between IRQ2/3/5, though an Adlib card normally doesn't make use of it. Original Adlib cards and clones always use a fixed IRQ and port address.
The SVGA graphics memory can be upgraded to 512KB by using 2 additional 1Mbit chips (most likely 2 chips of the 414256/411000 variety with 20 pins.) Despite the printing on the mainboard, 41464 chips could not be very useful, since the total amount of video memory would be 320KB.
The board also has holes for a feature connector of the graphics card to allow the SVGA graphics to pass-through to cards like MPEG accelerators or TV cards. The pins would be needed to be soldered in to enable this "feature".
The onboard memory consisted of 4 30-pin SIMM sockets with up to 4MB/16MB of RAM. There are prepared connectors on the mainboard to allow up to 8MB of total RAM with 1MB modules.
Beside these obvious features, there are other leftovers that didn't made it into the final unit: -PC-type Joystick port on the mainboard, right to the serial ports. The entire hardware is missing, the case is not designed for this. -Onboard PC speaker. The speaker was replaced with a larger one, with volume control on the back. -Front panel connector for things like reset switches, etc.
It seems as though Amstrad simply bought the design from WDC and added a Mega Drive in the form of an 8 bit ISA card.