Blast Processing

Blast Processing was a marketing term coined by Sega to advertise the fact that the Sega Mega Drive/Genesis could calculate faster motion than the Super Nintendo Entertainment System and was generally taken by the public to refer to the main system processors. Strictly the term refers to a technical feature of the Mega Drive that wasn't replicated on the SNES - the ability for the CPU to be working on one visible section of map while the graphics processor displays another. Since only the visible part of the map is uploaded at any one time, this feature greatly increases the distance that the map can scroll from one frame to the next, but few if any people will have been able to discern that meaning from the advertising.

The term originates from the advertising material for Sonic the Hedgehog 2. The U.S. ad campaign featured commercials with races between 2 vehicles - an F1 car and a broken down ice cream truck, the former with a Genesis strapped to it and the latter a SNES.

While the claim is true even if taken to refer to the central processing units, the campaign misleadingly implied that the Mega Drive was more powerful overall. While the SNES CPU ran slower in clock cycles per second (3.6 MHz to Sega's 7.6), it had superior graphics display hardware - sprites and backgrounds could use more colours and backgrounds could be scaled or rotated largely without CPU intervention. This meant that on the whole SNES games looked better.

A further source of controversy was the difference in architectures between the two machines. The Mega Drive boasted an Motorola 68000 as its central processor and a secondary Zilog Z80 for sound generation. The 68000 is an internally 32bit processor (i.e. all internal calculations and values are stored in 32bit) which communicates with the outside world in 16bit chunks. It includes 8 general purpose registers and 8 index registers and can do a 32bit register to register add in 4 cycles. The cost of reading or writing a 32bit value to or from memory is a further 5 cycles, due at least partly to having to send it as two 16bit values.

The Z80 is an 8bit register with some internal capacity for 16bit arithmetic and was inherited from the Master System. The Mega Drive was compatible with the Master System at a hardware level - the Powerbase addon that allowed Master System games to be played on a Mega Drive did no more than provide the correct shape of cartridge slot and the Master System boot ROM.

Conversely, the SNES used an entirely 16bit 65c816 derivative with one general purpose register and two index registers. It has a clock rate of approximately half the 68000 of the Mega Drive. In the most commonly used mode, a 16bit add takes 5 cycles. It would superficially appear that a SNES should be heavily handicapped by requiring 10 cycles to do the 32bit add the Megadrive can do in 4 cycles, especially given that the Megadrive runs through twice as many cycles per second. However in 5 cycles the SNES adds a number from memory to its general purpose register whereas the Mega Drive only achieves a register to register add - so to do the same algorithm implemented in the same way would incur substantial extra costs in memory loading on the 68000 versus the 65c816.

In reality, algorithms written for the 68000 will do most of their processing in registers, making the 4 cycles the relevant number. However many 16bit games can get away with using 16bit values and don't require a full 32bit add so 5 is the relevant number for the 65c816. Furthermore, add is not the only operation and with others the differences vary.

Perhaps the best measure of processing speed is the games. Whereas the Mega Drive was able to do competent versions of early polygon shifters such as LHX Attack Chopper and Hard Drivin' in software, the first substantial polygon SNES games were those utilising the Super FX accelerator chip, intended to extend the existing SNES processing power for those purposes and at the same time leap frog anything the Mega Drive could do in software by a substantial margin.

External links

• =RGA= - Features - Blast Processing (http://www.rebelgamers.com/features/blastprocessing.htm)
• 1UP.com's Essential 50 - #28 Sonic The Hedgehog (http://www.1up.com/do/feature?cId=3134008)