Blast processing

Blast Processing was a marketing term coined by Sega of America to advertise the faster processing performance of the Sega Mega Drive (Sega Genesis in that region).

Sonic the Hedgehog 2 was the posterboy for this campaign, being faster than any other platform game at the time. The ad campaign featured commercials with races between two vehicles, with the SNES strapped to one and the Mega Drive strapped to the other.

Sega originally coined the term to refer to the high-speed bandwidth and fillrate of the Mega Drive VDP graphics processor's DMA controller, but many later assumed it was referring to the 68000 CPU's higher clock rate.

Origins of term

<mediaplayer>File:Blast Processing Commercial.flv</mediaplayer>

According to Sega staff involved in its development and marketing, it was the high-speed DMA controller, rather than the CPU MHz, that the term was actually referring to. According to Sega of America's former technical director Scot Bayless:

“	the PR guys interviewed me about what made the platform interesting from a technical standpoint and somewhere in there I mentioned the fact that you could just "blast data into the DAC's". Well they loved the word 'blast' and the next thing I knew Blast Processing was born.	„
— Scot Bayless

One of the specific DMA programming techniques he was referring to was the mid-frame palette swap, where the color could be changed every scanline, increasing the colors displayed on screen, a technique that was used in Sonic 2:

“	Marty Franz [Sega technical director] discovered that you could do this nifty trick with the display system by hooking the scan line interrupt and firing off a DMA at just the right time. The result was that you could effectively jam data onto the graphics chip while the scan line was being drawn – which meant you could drive the DAC's with 8 bits per pixel. Assuming you could get the timing just right you could draw 256 color static images. There were all kinds of subtleties to the timing and the trick didn't work reliably on all iterations of the hardware but you could do it and it was cool as heck.	„
— Scot Bayless

Many of these DMA programmable techniques were originally intended by the Mega Drive's original product designer Masami Ishikawa:

“	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.	„
— Masami Ishikawa

Technical details

For more technical details on Mega Drive, see Mega Drive: Technical specifications and Mega Drive: Blast Processing

The term was used to refer to either the faster CPU processor or the VDP graphics processor's faster DMA controller.

CPU

The main CPU processor was clocked over two times faster than the one in its rival product, the Super NES. Sega's Motorola 68000 processor was clocked at 7.67 MHz, compared to the 3.58 MHz clock speed of Nintendo's Ricoh 5A22 processor. However, the idea of simply comparing CPU clock rates to determine performance, regardless of other characteristics, is commonly known as the megahertz myth. While Nintendo's 5A22 did run slower in clock cycles per second, it would put out more instructions per clock cycle, giving it a similar MIPS (million instructions per second) performance to Sega's 68000.

The 68000's faster performance came from other advantages, such as a wider 32-bit internal data bus and 16-bit external data bus (the SNES CPU had a 16-bit internal data bus and 8-bit external data bus), faster external data bus clock rate and bandwidth, more registers, 32-bit instructions, and shared codebase with arcade games (where the 68000 saw widespread use).

Console		Sega Mega Drive	Super Nintendo Entertainment System
Main CPU		Motorola 68000	Ricoh 5A22
Clock rate	Internal	7.670453 MHz (NTSC), 7.600489 MHz (PAL)	2.684658–3.579545 MHz (NTSC), 2.660171–3.546895 MHz (PAL)
	External	5 MHz (RAM/ROM)	2.660171–2.684658 MHz (RAM), 2.660171–3.579545 MHz (ROM)
Bits	Data bus width	32-bit internal, 16-bit external	16-bit internal, 8-bit external
	Arithmetic logic units	16-bit data ALU, 32-bit address ALU (2x 16-bit ALU)	16-bit ALU
	Word length	16-bit	16-bit
Internal instructions	Registers	16x 32-bit registers	4x 16-bit registers, 4x 8-bit registers
	Instruction set	16-bit, 32-bit	8-bit, 16-bit
	Instructions per second	1.342329 MIPS (NTSC), 1.330085 MIPS (PAL)	1.125–1.5 MIPS (NTSC), 1.114738–1.486318 MIPS (PAL)
Work RAM	Memory	64 KB PSRAM (16-bit, 5.263157 MHz)	128 KB DRAM (8-bit, 2.660171–2.684658 MHz)
	Bandwidth	5 MB/s (CPU), 10.526314 MB/s (VDP) CPU access per frame: 81 KB (NTSC), 98 KB (PAL)	2.684658 MB/s (NTSC), 2.660171 MB/s (PAL) CPU access per frame: 43 KB (NTSC), 51 KB (PAL)
	Transfer rate	3.835226 MB/s (NTSC), 3.800244 MB/s (PAL) CPU write per frame: 62 KB (NTSC), 73 KB (PAL)	2.684658 MB/s (NTSC), 2.660171 MB/s (PAL) CPU write per frame: 43 KB (NTSC), 51 KB (PAL)
Cartridge ROM	Memory	128 KB to 8 MB	128 KB to 6 MB
	Bandwidth	5 MB/s (CPU), 10–15.340906 MB/s (VDP)	2.5–3.579545 MB/s

DMA

The Sega Mega Drive's Yamaha YM7101 VDP graphics processor had a powerful DMA controller that could handle DMA (direct memory access) operations at much faster speeds than the Super Nintendo. The Mega Drive could write to VRAM during active display and VBlank, and had a faster memory bandwidth than the SNES. The quicker DMA transfer rates and bandwidth gave the Mega Drive a faster performance than the SNES, and helped give the Mega Drive a higher fillrate, higher gameplay resolution, faster parallax scrolling, fast data blitting, and high frame-rate with many moving objects on screen, and allowed it to display more unique tiles (background and sprite tiles) and large sprites (32×32 and higher) on screen, and quickly transfer more unique tiles and large sprites (16×16 and higher) on screen.

The Mega Drive's DMA capabilities also helped give it more flexibility, allowing the hardware to be programmed in various different ways. With DMA programming, it could replicate some of the Super Nintendo's hardware features, such as larger 64×64 sprites (combining 32×32 sprites), background scaling and rotation (like the Sega X Board and Mode 7), and direct color (increasing colors on screen). Other DMA programmable capabilities of the Mega Drive include mid-frame palette swaps (increasing colors per scanline), sprite scaling and rotation, ray casting, bitmap framebuffers, and 3D polygon graphics; the base Mega Drive hardware (without needing any enhancement chips) could render 3D polygons with a performance comparable to the Super Nintendo's optional Super FX enhancement chip, which itself is significantly outperformed by the Mega Drive's optional Sega Virtua Processor enhancement chip.

Console		Sega Mega Drive	Super Nintendo Entertainment System
DMA controller		Sega 315‑5313 VDP (Yamaha YM7101)	Ricoh 5A22
Clock rate	Internal	13.423294 MHz (NTSC), 13.300856 MHz (PAL)	2.684658–3.579545 MHz (NTSC), 2.660171–3.546895 MHz (PAL)
	External	8–11.764705 MHz (RAM), 5–7.67 MHz (ROM)	2.660171–2.684658 MHz (RAM), 2.660171–3.579545 MHz (ROM)
Video RAM	Memory	64 KB VRAM (64 KB FPM DRAM, 256 bytes SAM), 232 bytes VDP cache (CRAM, VSRAM, sprite buffer)	64 KB SRAM, 1056 bytes PPU cache (CGRAM, OAM)
	Bandwidth	8–11.764705 MB/s (VRAM), 26.846588 MB/s (VDP cache)	7.15909 MB/s (NTSC), 7.09379 MB/s (PAL)
DMA blitting transfer rate	Inactive display	VRAM: 3.21845 MB/s, 205 bytes per scanline VDP cache: 6.4369 MB/s, 410 bytes per scanline	2.660171–2.684658 MB/s, 170.5 bytes per scanline
	Active display	VRAM: 708.406 KB/s to 1.702026 MB/s VDP cache: 1.416813 MB/s to 3.404052 MB/s	443.228 KB/s (389.38 KB/s DMA, 53.848 KB/s HDMA)
Fillrate	Video clock rate	13.300856–13.423294 MHz (VDP)	5.320342–5.369317 MHz (PPU)
	Read fillrate	6.934358 MPixels/s (108,340 tiles/sec)	5.369317 MPixels/s (83,895 tiles/sec)
	Write fillrate (inactive display)	6.4369 MPixels/s (100,576 tiles/sec), 410 pixels (51 tiles) per scanline	5.36 MPixels/s (83,750 tiles/sec), 341 pixels (42 tiles) per scanline
	Write fillrate (active display)	1.416813–3.404052 MPixels/s Pixels per frame: 23,644–68,490 pixels	886,457 pixels/s Pixels per frame: 14,750 pixels
	Tiles on screen (active display)	Display: 1808 tiles Blit transfer per frame: 369–1070 tiles	Display: 1395 tiles Blit transfer per frame: 230 tiles
Sprites	Sprite fillrate	4.90887 MTexels/s, 320 texels per scanline	4.282881 MTexels/s, 272 texels per scanline
	Sprite tiles	1280 sprite tiles on screen	512 sprite tiles on screen
	Sprites on screen	80 sprites (8×8 to 32×32), 20 sprites (64×64), 5 sprites (128×128)	128 sprites (8×8, 16×16), 69 sprites (32×32), 17 sprites (64×64), 4 sprites (128×128)
	Unique sprites on screen	80 sprites (8×8 to 32×32), 20 sprites (64×64), 5 sprites (128×128)	128 sprites (8×8, 16×16), 32 sprites (32×32), 8 sprites (64×64), 2 sprites (128×128)
	Blit transfer per frame	80 sprites (8×8 to 16×16), 41–80 sprites (24×24), 23–66 sprites (32×32), 5–16 sprites (64×64)	128 sprites (8×8), 57 sprites (16×16), 14 sprites (32×32), 3 sprites (64×64)
	Sprites per scanline	20 sprites (8×8 to 16×16), 13 sprites (24×24), 10 sprites (32×32), 5 sprites (64×64)	32 sprites (8×8), 17 sprites (16×16), 8 sprites (32×32), 4 sprites (64×64)
Background planes	Background tiles on screen	1344–1808 background tiles	256–1024 background tiles
	Tilemap planes	2 scrolling planes (1344–1808 tiles), 1 static window plane, 40–64 overlapping scrolling layers (20–32 layers per plane)	1–4 planes (256–1024 tiles)
	Tilemap resolution	256×256 to 512×512 (2 planes, 1344–1808 tiles), 1024×256 (2 planes, 1344–1424 tiles)	256×256 to 512×512 (1–4 planes, 256–1024 tiles), 1024×1024 (1 plane, 256 tiles)
	Scrolling capabilities	Parallax scrolling, line scrolling, tile scrolling, row/column scrolling, overlapping scrolling layers	Parallax scrolling, line scrolling, tile scrolling
Resolution	Overscan	427×262 (NTSC), 423×312 (PAL)	341×262
	Display resolution	Gameplay: 256×224 to 320×480 (default 320×224) Custom: 128×160 to 256×160, 128×224 to 160×224	Gameplay: 256×224 to 256×239 (default 256×224) Pseudo-hires text: 512×448, 512×478 (half-pixels)
Colors	Color palettes	512 colors (default), 1536 colors (Shadow/Highlight), 256–512 colors (direct color)	32,768 colors (default), 256–2048 colors (direct color)
	Colors on screen	61–64 colors (default), 75–256 colors (palette swap), 183–1536 colors (Shadow/Highlight), 256–512 colors (direct color)	128–256 colors (1–2 planes), 128–160 colors (3 planes), 128 colors (4 planes), 256–2048 colors (direct color)
	Colors per tile	16 colors (2 planes), 16–256 colors (mid-frame palette swap), 256–512 colors (direct color)	16 colors (1–2 planes), 8 colors (3 planes), 4 colors (4 planes), 256 colors (direct color)
3D polygon graphics	Base hardware	Geometry: 10,000 vertices/s, 3000 polys/s Rendering: 1600 polys/s (flat), 150 polys/s (textured)	~100 polys/s (flat)
	Enhancement chips	SVP: 9000 polys/s (flat), ~800 polys/s (textured)	SFX: 1000 polys/s (flat), ~100 polys/s (textured) SFX2: 2000 polys/s (flat), ~200 polys/s (textured)

External links

• 1UP's Essential 50 - #28 Sonic The Hedgehog
• gamesTM - Gaming News '92 "Hogging the World Stage"