MULTIMASTERING
Q) How many coprocessor cards can NuBus support?
A) Any single NuBus card can be the Master (a card that initiates the
addressing of another card) at any time. This action can be passed from
card to card. Because of Apple's implementation of six slots, only six
NuBus master cards can be supported in the Macintosh II.
NuBus itself can utilize an arbitrary number of slots, based on the
designated size of each slot. The upper one-sixteenth of the total address
space is available for slot space. This provides 256MB of addressing.
Apple provides six slots of 1MB each in 24-bit mode. In the 32-bit mode,
there are 16 possible slots on the Macintosh II, with an address space of
16MB each.
PERFORMANCE
Q) Can NuBus support processor speeds up to 25 MHz? 33 MHz? 45 MHz?
A) Cards can maintain their internal clock up to any possible speed.
There are no set limitations for a card's internal clocks. Limitation are
imposed by NuBus transactions. NuBus is clocked at 10 MHz.
ADDRESS SPACE
Q) Is NuBus hardwired at 24 address lines?
A) NuBus is not hardwired at 24 address lines. It is a full 32-bit bus.
The 24-bit addressing mode is a software-controlled mode imposed for
compatibility with the 24-bit addressing System 6.
MEMORY
Q) What's the maximum amount of memory addressable through today's NuBus?
A) The maximum amount of memory addressable on the NuBus is 4 gigabytes.
This is the total addressable range at 32 bits. On the Macintosh II, ROM
space is reserved for 1MB of space. RAM is reserved for 1 gigabyte.
Additionally, the NuBus slot address space includes a super slot space of
1.536 gigabytes allocated to the six slots for 256MB per slot and a slot
space of 256MB allocated for 16 slots at 16MB each.
The breakdown is:
Memory: ROM, RAM, and slot space
ROM: 1MB
RAM: 1000MB
Super Slot: 1536MB
Slot: 256MB
----------------------
Total: 2793MB
Q) How much memory can be added on a card or the motherboard?
A) The NuBus will access up to 16MB of RAM on a card in slot space and
256MB of RAM in Super Slot space. The maximum direct addressable range
through the Macintosh OS is currently 1MB per card.
The full address range (up to 256MB) can be addressed by NuBus or by page
flipping the memory on the card. The Macintosh II motherboard physically
can connect 8MB. The maximum possible when looking at the NuBus and
reserved memory addresses is 1 gigabyte.
DMA
Q) What are the DMA capabilities of NuBus?
A) DMA is not supported across NuBus.
NuBus-to-Macintosh II Address Mapping
-------------------------------------
24-bit Addresses 32-bit Addresses Used To Access
from MC68020 from MC68020 NuBus Addresses Macintosh II Sys
$xx00 0000 to $0000 0000 to $0000 0000 to Present RAM
$xx7F FFFF $007F FFFF $007F FFFF
$0080 0000 to $0080 0000 to Future RAM
$3FFF FFFF $3FFF FFFF
$xx80 0000 to $4000 0000 to $F080 0000 to ROM (aliased)
$xx8F FFFF $4FFF FFFF $F0FF FFFF
$xxF0 0000 to $5000 0000 to $F000 0000 to I/O (aliased); do
$xxFF FFFF to $5FFF FFFF $F070 FFFF not access from a
slot card
$6000 0000 to $60000 0000 to Presently unused
$8FFF FFFF $8FFF FFFF
$9000 0000 to $9000 0000 to Super slot space,
$EFFF FFFF $EFFF FFFF slots $9 to $E
$xxF0 0000 to $F000 0000 to $F000 0000 to Slot $0 (Macintosh
$xxFF FFFF $F0FF FFFF $F0FF FFFF system)
$F100 0000 to $F100 0000 to Presently unused
$F8FF FFFF $F8FF FFFF
$xxs0 0000 to $Fs00 0000 to $Fs00 0000 to Slot space, slot s
$xxsF FFFF $FsFF FFFF $FsFF FFFF (s in range $9-$E)
or or
$Fs10 0000 to $Fs10 0000 to
$FsFF FFFF $FsFF FFFF
$FF00 0000 to $FF00 0000 to Presently unused
$FFFF FFFF to $FFFF FFFF
FOR MORE INFORMATION
Check "Designing Cards and Drivers for Macintosh II and Macintosh SE" (ISBN
#0-201-19256-X) from Addison-Wesley; mechanical drawing of cards and
connectors available from APDA; and NuBus patent licenses from Texas
Instruments, Inc.
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