Power Macintosh: Amount of Virtual Memory Reported

When virtual memory is enabled in the Memory control panel, the amount that is reported in the "About This Macintosh" window (under System 7.0 to 7.5.5) or "About This Computer" (in Mac OS 7.6 and later) is more than expected. For example, with virtual memory set to 16 MB, this window reports 17,228 KB available.

Even taking into account the 1024K memory value instead of 1000, the memory allocation does not add up. What is taking up the additional memory?
The total memory value in "About this computer" (7.6 and later) and "About this Macintosh"(7.1.2 - 7.5.x) is obtained from the "lram" (logical RAM) Gestalt selector. This number is calculated slightly different on the Power Macintosh due to memory used by the NanoKernel (68K emulator). An adjustment to the number is performed to yield the best possible estimate of the remaining RAM as shown in the value for the largest unused block.

The discrepancy in values returned from different machines are usually caused by various color depth settings for on-board video. The expected discrepancy for the NanoKernel should be around 870K.
Published Date: Feb 19, 2012