The logic board's 68020 communicates with each of the six synchronous
NuBus slots through the memory management unit. A full 32-bit address/data
transfer is available between each slot and the 68020. Each slot is
identified to the microprocessor by four ID lines, which the 68020 uses to
determine the source of the communication. Power, timing, acknowledge, and
parity lines are also implemented. Apple has changed Texas Instruments'
NuBus definition by adding an interrupt line from each slot, so that each
card can generate an interrupt to the 68020.
The 32-bit address space (equating to 4G bytes) available for all NuBus
slots is partitioned to provide space for each slot. First, the top 256M
of address space is divided into 16 "slots." This allows each physical
slot to "own" 16M, which is referred to as its Slot Space. Each NuBus slot
is also allocated 256M of "SuperSlot Space" from the portion of memory
remaining in the 4G of address space.
Each NuBus card should contain a configuration ROM mapped to the top of
its Slot Space to provide information for the Macintosh II operating
system at startup time. The Macintosh II looks at the ROM to determine the
type of card, how it is to be accessed, and slot resource data.
Pre-defined card categories include display, network, terminal emulator,
serial, parallel, intelligent bus, and human input devices. Each category
is further defined by a type indicator. For example, the network category
is subdivided into AppleTalk, EtherNet, Token Ring and so on. NuBus device
drivers can then locate any appropriate card by checking these bytes in
configuration ROM on each card.
NuBus cards can be designed to be "masters" of the bus, or as slaves only.
A master card must be able to initiate bus transfers of 8, 16, or 32 bits,
and must be able to arbitrate requests for bus mastership. It may include
the ability to lock the bus from access by other NuBus devices for a
specified period of time. A slave card responds to requests, but can only
send a "non-master request." It need not support the full 32-bit transfer.
Card manufacturers may choose to implement a NuBus card with only a 24-bit
logical address mode to provide compatibility with existing Macintosh
products. This mode is supported by using only address lines 0 through 23,
and is called "24-bit aliasing."