Locking and Unlocking Memory
You can use the LockMemory function to make a portion of the address space immovable in physical memory and ineligible for paging. The Operating
System may move the contents of the specified range of logical addresses to a
more convenient location in physical memory during the locking operation, but
on completion, the contents of the specified range of logical addresses are
resident and do not move in physical memory.
LockMemory is used by drivers and other code when hardware other than the Macintosh CPU is transferring data to or from user buffers, such as any
NuBus master peripheral card or DMA hardware. This function prevents both
paging and physical relocation of a specified memory area and enables the
physical addresses of a memory area to be exported to the non-CPU hardware.
Typically, you would use this service for the duration of a single I/O request.
However, you could use this service to lock data structures that are
permanently shared between driver or other code and a NuBus master.
Note: Do not confuse locking addresses in RAM ( using
handle can still be paged out.
The main reason to disable movement of pages in physical memory is to
translate virtual addresses to physical addresses. This translation is needed by
bus masters, which must write to memory in the physical address space. To
avoid stale data, the memory locked in RAM is marked non cacheable in the CPU
cache.
You can lock a range of memory into contiguous physical memory by using the
NuBus master or driver and DMA hardware combinations when a non-CPU
device accessing memory is unable to handle physically discontiguous data
transfers. You can also use this service when it is possible to handle
physically discontiguous data transfers, but doing so causes performance
because entire pages might need to be copied in order to make a range
contiguous.
Note: It might not be possible to make a range physically
contiguous if any of the pages contained in the range are already
to return the desired results, all code that uses
locking the necessary ranges of memory.
To unlock a range of previously locked pages, use the UnlockMemory function. This function reverses the effects of LockMemory or Locking, contiguous locking, and unlocking operations are applied to ranges of
the logical address space. If necessary to force the ranges onto page boundaries,
the Operating System performs rounding of addresses and sizes, as described