A number of second-generation Power Macintosh models are based on PowerPC 603 and 604 processors. Unlike the 601, these (and all subsequent) processors do not provide hardware support for POWER instructions. These instructions were included in the 601 for backward compatibility of binaries created for the IBM POWER architecture.
Who should use the “POWER Emulator” Control Panel?
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It is unlikely that we will ship this control panel to users. It is provided to developers as a tool for finding and removing POWER instructions. Because newer processors do not support these instructions, low-level exception handlers have been written to emulate the effects of these instructions. This means that software which uses POWER ops will not crash, but will run more slowly on non-601 machines. It is in your best interest to remove all POWER instructions from your code. Failure to do so will directly (and perhaps substantially) affect the performance of your software. If your software is at risk (see the section below), try running your program with POWER emulation disabled. In this mode, an illegal instruction exception will occur when a POWER instruction is encountered.
Who should be concerned about POWER instructions?
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Since most PowerPC compilers and development tools do not produce POWER instructions, most developers need not be concerned about this issue. The only developers at risk are those who used early versions of the IBM “xlc” compiler, early versions of the gcc compiler or other compilers intended for or based on the IBM POWER architecture. Assembly language programmers are also at risk if care was not taken to avoid POWER instructions.
How does the Control Panel work?
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The control panel is very simple. It displays the current global state of the POWER Emulation feature (enabled or disabled). Note that all machines without a 601 will start up with POWER Emulation enabled. Selecting “disable” will immediately disable POWER emulation for all PowerPC code. The control panel also provides a counter which indicates the number of POWER instructions encountered since the last reboot.
