🚀 go-pugleaf

	There has been a discussion in this newsgroup as to whether any
	NuBus cards larger than one MByte (in particular National
	Semiconductor's 16 MByte Memory Card) will work on the Mac II.

	The current version of the Macintosh O/S memory management scheme
	assumes a 24-bit (16 MByte) address space. Each of the six NuBus
	slots in the Mac II is allocated one MByte of the system's address
	space when in 24-bit compatibility mode (currently the standard mode
	of operation for the Mac O/S).

	The NuBus specification defines a 32-bit (4 GByte) address space,
	where the top 256 megabytes are divided into 16 "slot spaces" (16
	MByte each). To make the NuBus slots visible to the CPU when in 24-bit
	mode, Apple has developed a memory management gate array, the HMMU,
	which translates 6 megabytes of CPU addresses into the lowest megabyte
	of each of the six slots. 
	
	The address space occupied by the slots in this mode is in the I/O
	space of the classical Macintosh address map, not in the RAM space.
	This means that memory of any kind that is resident on a NuBus board
	is not currently managed by the Mac O/S but must be accessed via
	dedicated "I/O drivers"; An example of this is the RAMdisk device
	driver that National is bundling with the NS8/16 NuBus memory card.

	The HMMU can however be reconfigured through a Mac O/S system call to
	run in 32-bit mode. In this mode, all addresses generated by the CPU
	are passed directly to the NuBus. This is the mechanism used by NSC's
	RAMdisk software to make all 16 Megabytes accessible to the system.
	In a similar manner, any applications developed for use in 32-bit
	mode are able to access memory directly.

	In summary, NuBus memory can certainly be used in the Mac II running
	under the current version of the Mac O/S, but not as main system
	memory that applications can access by requesting blocks of memory
	from the memory manager. When the Mac II is running A/UX (which is
	implemented in 32-bit mode), the entire NuBus address space (and
	thus any NuBus memory) is directly accessible to the CPU.

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  | The opinions stated herein are my own and do not necessarily represent |
  |  the policies or opinions of my employer 				   |
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-- 
			UUCP: {amdahl,decwrl,hplabs,pyramid,sun}!nsc!ernesto
  * Ernesto Rey
  * Compatible Products Group
  * National Semiconductor, Santa Clara, CA