Because the logical address space is bigger than the physical memory of the computer, it stands to reason that not all logical pages have a corresponding physical page. VM also utilitizes the processor's ability to stop executing instructions when a page translation fails. Always use Gestalt to determinewhether VM is operating, as outlined in Inside Macintosh: Memory. You should never assume the state of VM based onthe processor type. Some 68020 computers have aslot where an optional MMU can be installed. All 68030, 68040 and PowerPCprocessors have an MMU. Virtual Memory is only available when theprocessor has a Memory Management Unit (MMU). When the processor accesses a logical address, the MMU uses the page table to translate the access into a physical address, which is the address that's actually passed to the computer's memory subsystem. This is a table that maps all logical pages into their corresponding physical pages. There are two key features of the processor and its memory management unit (MMU) that you must grasp in order to understand how VM works. Other code should call Gestalt with the gestaltLogicalPageSize selector to determine the page size. Is there a queue emulator alternative for a powerpc mac drivers#PCI device drivers should call the Driver Services Library routine GetLogicalPageSize to find the page size. You should never assume the page size is 4K. The system then uses the computer's physical memory to give the illusion that the entire logical address space is made up of real memory. Each page in the logical address space has a corresponding page on the disk, in a special file known as the backing store. Under System 7-style virtual memory, each page is 4KB in size. The basic idea behind VM is that the system creates a logical address space that is larger than the installed physical memory and divides it up into uniformly-sized chunks of memory called pages. This section describes how VM works, both the theory and the specific implementation under System 7. Hardware Interrupt Handlers & Physical AddressingĪllocating Memory Above BufPtr at INIT time
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