Apple Desktop Computer Performance, RAM Footprint, User Experience, Data Structures and Files

C H A P T E R 5

Software

Performance

Performance of an iMac computer using ROM in RAM should exceed performance measurements for other Macintosh computers with comparable CPUs and speeds due to improved interrupt handling with the New World approach. In addition, performance is improved due to executing code that normally exists in ROM in RAM, because the RAM devices operate faster than the ROM devices normally used.

RAM Footprint

The iMac is the first Macintosh computer to have its Toolbox ROM image stored in RAM. This removes approximately 3 megabytes of RAM from Mac OS usage. In effect, a system with 32 megabytes of RAM appears to have only 29 megabytes available. Some portion of the missing 3 megabytes is offset by having fewer patches in RAM. Other mechanisms are being explored in an attempt to minimize the impact of ROM-in-RAM.

User Experience

Setting the boot volume from the Startup Disk control panel makes all the changes to the boot process that are necessary to operate with a ToolBox image in RAM. The control panel user interface remains unchanged for this release.

Data Structures and Files

The ToolBox ROM image is contained in a new file, named “Mac OS ROM”, that is kept in the System Folder. The ToolBox ROM image is exactly the same as it would be if it were an actual ToolBox ROM, containing the ToolBox software, the kernel software, and the 68K emulator.

The Startup Disk control panel sets the Open Firmware’s boot-device configuration variable by modifying the Open Firmware NV-RAM partition that contains the Open Firmware’s configuration variables. The format of the NV-RAM partition is defined in the Open Firmware CHRP Binding. The partition is accessed using RTAS.

Compatibility

A Mac OS ToolBox image that is in write-protected RAM will appear to be a ROM to all MacOS software and applications. Because the image of the ToolBox

Why Make This Change? 59