Because the SIMMs for the Power Macintosh 7100 and 8100 VRAM Expansion Cards have the same pin assignments and external dimensions, they may inadvertently be switched between computers. If a switch happens, the additional VRAM may not be recognized, or strange video activity may occur. Be sure the proper VRAM SIMMs are installed.
The SIMMs can be differentiated visually. SIMMs for the Power Macintosh 7100 have two chips per SIMM, while SIMMs for the Power Macintosh 8100 have four chips per SIMM.
Note that this information is applicable to Apple SIMMs only, third party SIMMs may be different in chip count. Be sure to check with your SIMM vendor or installing dealer as to the compatibility of the VRAM SIMMs prior to installation.