Apple Desktop Computer manual Video RAM Expansion

C H A P T E R 4

RAM Expansion

Active	1.2 A (8 devices at 150 mA each)
Sleep	6 mA per bank

The maximum current specified for active operation generally rules out the use of 4-bit-wide SDRAM devices in a RAM expansion card. Such a card would have 16 such devices, and the 1.2 A maximum current would allow only about 75 mA per device. To stay within the current limits, RAM expansion cards should use only 8-bit or 16-bit SDRAM devices.

Video RAM Expansion

The iMac computer includes 2 MB of SGRAM for video on the main logic board. The video RAM can be expanded up to 6 MB with an additional 2 or 4 MB of SGRAM on a 144-pin SO-DIMM (small outline dual inline memory module). The mechanical design of the SGRAM SO-DIMM is defined by the JEDEC MO-190 specification. The SGRAM SO-DIMM specification can be downloaded at

http://www.jedec.org/download/freestd/pub95/

The devices on the DIMM must be 3.3 V SGRAM devices. The speed of the SGRAM devices must be 100 MHz or greater, corresponding to a cycle time of 10 ns or less. The 100 MHz/10ns SGRAM SO-DIMM specification provides suitable margins for compatibility with the iMac architecture.

Increasing the size of the installed video RAM from 2 MB to 4 or 6 MB increases the maximum color depth available at the highest monitor resolutions.

Any additional video RAM that isn’t used for the current display mode is available to QuickDraw 3D applications for more data storage, such as textures, which results in faster 3D graphics performance. A more detailed explanation of video RAM and QuickDraw 3D graphics performance can be found in Chapter 5 of the Power Macintosh 5500/6500 Developer Note, where the use of the video frame buffer for front frame buffer, back frame buffer, optional Z buffer, and available texture memory are described.

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