FMPC CPU

CPU

Each computer has a CPU (Central Processing Unit) that understands a fixed set of instructions. All programs that operate on that computer are composed of those instructions and any constant values that the programmer defines, listed in the appropriate order. Instructions exist in all CPUs that allow information to be moved into the CPU from memory, or stored back into memory. Other instructions cause arithmetic or logical operations to be performed on values in the CPU or in memory locations. Another set of instructions compare values to each other and set 'flag' bits in the CPU based on the results. These flag bits may then be used by another class of instructions to cause execution of the program to jump to another part of the program instead of executing the next sequential instruction. This allows a program to take a different path depending on a condition that may be tested while the program is running, and is the basis for all 'decision making' that a program performs.

Each CPU has a unique instruction set, so programs that are compiled for one CPU will not run unaided on another CPU. The Mac OS is supported on two CPUs currently, the 68000 series and the PPC. The PPC based machines do not directly understand the instructions that were compiled to run on the 68000 series machines, but Apple has provided an emulator that allows the PPC machines to translate 68000 machine level instructions to PPC instructions on the fly and this emulator allows them to run most of the 68000 programs. There is no emulator that operates in the other direction, so programs that are compiled to run on the PPC cannot be executed on a 68000 machine.

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