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Voodoo2 Memory Question

Explore why Voodoo2 has multiple memory chips for optimal performance. Dive into the technical details of bandwidth, bit allocation, and chip configurations. Unravel the mysteries of interleaved memory and bus width specifications. Gain insights into the inner workings of Diamond Monster Voodoo2 board.

richardgmartinez
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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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