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Voodoo2 Memory Question

Voodoo2 Memory Question. Why does the Voodoo2 have “so many memory chips”? (as noted in a recent review) 12 MB (MB => MegaBytes) onboard memory 4 MB Framebuffer Two 4MB Texture memories Uses EDO DRAM 3DFX claims a 192 datapath operating at 2.2 GB/s, interleaved memory

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Voodoo2 Memory Question

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  1. Voodoo2 Memory Question • Why does the Voodoo2 have “so many memory chips”? (as noted in a recent review) • 12 MB (MB => MegaBytes) onboard memory • 4 MB Framebuffer • Two 4MB Texture memories • Uses EDO DRAM • 3DFX claims a 192 datapath • operating at 2.2 GB/s, interleaved memory • Supports 800x600 with double buffering, 65K colors, 16-bit Z-buffer

  2. Framebuffer guesses • How many bits per pixel? • 4MB = 32Mb (Megabits) • 32Mb/ (800x600) => 69 bits per pixel, round down to 64 bits (5 wasted bits per pixel) • 64 bits/pixel • 16 bits front buffer • 16 bits back buffer • 16 bits Z-buffer • 16 bits left for misc functions • Assume 64 bit datapath to Framebuffer

  3. More Guesses….. • Two 4MB texture memories, assume 64 bit datapaths to each • 64 bit framepath + 64 bit texture path * 2 => 192 bits!!! • Current EDO DRAM chips are 16Mb. • 12MB = 96Mb • 96Mb/16Mb = 6 chips to meet storage spec • Widest 16Mb EDO DRAM is 16 bits (1M x 16) • 192/16 => 12 chips to meet bus width spec • So must be using at least 12 chips as one bank • Right or Wrong? Call this guess #1.

  4. What about Bandwidth? • 2.2GBytes/s claimed for 192 bit path • 17.6Gb/s / 192 = 96 Mhz access rate! • Voodoo2 cards actually use 90Mhz onboard bus speed. • Also claimed “interleaved memory” • Have to use multiple banks to interleave • If use multiple banks, then using 4Mb DRAMs • 256K x 16 • Still 12 chips in a bank • 12 chips * 4 Mb = 48 Mb per bank • Two banks of 48 Mb => 96 Mb => 12 MB • So Guess #2 = 24 chips.

  5. Does two banks = 90 Mhz?? • Diamond Monster Board says it uses 25 ns EDO DRAM (fast page mode) • 25 ns => 40 Mhz, 2 banks => 80Mhz, close enough? • Other option would be 4 rows (48 chips) - too many devices, odd memory size? • On the web, found picture of 8 MB board which had 12 DRAM chips on one side, other side not shown. • 8 MB => 64Mb/4Mb = 16 chips (did other side of board contain 4 chips??)

  6. Diamond Monster Voodoo2 Board (8MB) One Pixel engine (Pixelfx), two Texture engines (Texelfx) Texelfx Texelfx Pixelfx DRAM

  7. Actual Chip Count is…. • 24 chips, using 4Mb (256K x 16) EDO DRAM • 12 on front, 12 on back. • Silicon Magic is one supplier… • (www.simagic.com/simagic.htm)

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