How does the PowerPC address memory?
PowerPC microprocessors are 32 bit processors that have 4 Gbytes of address space. This means that the processor instructions are encoded in 32 bits, and that they can address or access 4 Gbytes of data. Although the PowerPC has 4G of address space, only a portion of the 4G is allocated for main memory (DRAM memory). For example, the total RAM expansion on entry-level Macs can be as much as 136 Mbytes, while RAM expansion on Power Mac 9500 can be as much as 1.5G. The remaining portion of the address space is allocated for system ROM, PCI cards, system control, etc. Data is transferred between the PowerPC and main memory via the 64 bit data bus by memory reads or memory writes. There are two types of memory transactions: single- and four-beat transfers. A single-beat memory transaction reads or writes between one to 8 bytes of data. (Equating 8 bytes to 64 bits and noting that the data bus is 64 bits wide, the PowerPC can read or write eight bytes in a single memory operation.) A four-b
