In late 2014, publisher Read-Only Memory released Mega Drive/Genesis: Collected Works , a history book covering the hardware, games and legacy of Sega's 16-Bit system. Featuring console diagrams and translated design documents, it serves as part art book, part history lesson and part interview collection. And you can read one of those interviews below. Read-Only Memory has provided Polygon with an excerpt — an interview with product designer Masami Ishikawa on how the hardware came about.

16-BIT

Only one Sega product designer saw the Mega Drive through from initial plans to final execution. Joining Sega in 1979, initially to work in the amusement machine division, Masami Ishikawa then became leader of the company's home console R&D division. His early projects included SG-1000 console variants the Othello Multivision — a hardware clone that represented an early attempt to create a global standard for computer games — and the Game Pack — an add-on module that slotted into a Pioneer television system. The SG-1000 II followed, before Ishikawa took on the role of design team leader for the Mark III and the Master System, the technical building blocks of which underpinned the subsequent creation of the Mega Drive. Remaining at Sega, Ishikawa headed up the development of the TeraDrive — the combined PC/Mega Drive unit manufactured by IBM — and then returned to his roots, working at the forefront of arcade hardware innovation at the company.

All images in this story come from Read-Only Memory's Mega Drive/Genesis: Collected Works.



How did you get involved with the design of the Mega Drive?

Masami Ishikawa: I was the team leader working on the design of the Mark III, the predecessor to the Mega Drive. It had a similar underlying concept to the Mark III so the project was a natural progression for me.

What were your priorities with the Mega Drive architecture?

MI: We wanted the Mega Drive to have the basic performance of the preceding system boards — the System I, System II and System 16 — and we wanted to preserve compatibility with the Mark III. In fact, even SG-1000 II titles were playable on the Mega Drive. The top priority was the Mark III compatibility — in order to retain gamers who owned older systems — while at the same time maximising the graphic performance. The crux was how to optimise the efficiency of the memory access cycle with the graphic memory. We also separated the CPU into a graphic component and sound component to lessen the stress on the game program. The Mark III compatibility meant that it also had a Zilog Z80 CPU. When the Mega Drive was in Mark III mode, it was mainly running on the Z80, but when it was in Mega Drive mode, the Z80 was used only for sound.

How did you decide on the Motorola 68000 as the primary CPU?

MI: Arcade consoles were already using 16-bit systems, but cost considerations meant that the decision to use an 8-bit or 16-bit CPU was made quite late in the design process. I think the final decision was made by the manager at the time, Sato-san, who later became president of the company. By using the 68000 we could take advantage of the programming resources already available for arcade use, plus the hardware — bus components — and software — for coding — were relatively simple to get to grips with.





What were the major strengths of the hardware in terms of graphics?

MI: I think its strength was in having multiple displays. We were able to have two scrolling windows — with both vertical and horizontal line scrolling — and the sprite size could be changed to fill the whole display. It could also display the background screen behind the scrolling window and could change the color of each line. The number of available colors was limited compared to comparable arcade systems, but it could create shadows that matched each character's shape and was also capable of semi-transparency. The biggest hurdle was the size of the chip. We wanted to include enlarging and minimizing capabilities as well as sprite-spinning functionality, but the circuit design was becoming too large to fit on one chip, which would have lowered the production yield rate and hiked up costs, so we had to remove it from the spec. The number of available colors was also limited by the size of the circuit structure.

Did you ever experiment with different CPU or graphics processing chips?

MI: No, it was more about choosing between 8-bit and 16-bit. We were planning to use a customised, in-house-designed graphic processor from the beginning. The 68000 had all the qualities we were looking for, and there were no other comparable chips on the market.

What was the idea behind using two sound chips (the Yamaha YM2612 and the TI SN76489) in tandem?

MI: We already used the Yamaha FM sound chip for arcade games, while the TI sound generator was employed to retain SG-1000 and Mark III compatibility.

How long did the design process for the console take?

MI: The project started in mid-1986 and lasted approximately one and a half years. I was the only person in charge of the project, but an additional four people became involved during the debugging process, after which I think we had one full-time assistant.