Specifications

CHAPTER 2
Hardware Design
18 Hardware Features
keyboard is still responding, you can restart the PC system using the Ctl-Alt-Del key
sequence on the PC keyboard.
The Macintosh system bus supports three bus masters: the Macintosh memory
controller, the Mustard ASIC on the 12” or 7” card, and any other PCI card in the
Macintosh.
IMPORTANT
Any other PCI card in the Macintosh must be of a different type from the
12” or 7” card. ▲
Table 2-2 summarizes the fixed arbitration device assignments and priorities.
Memory Controller 2
Memory control for the 12” card is provided by the 82C557 IC, and by the 85C496 IC for
the 7” card. These ICs perform the following system-level functions for the related card:
■ control DRAM
■ control cache memory
■ act as a bridge between the PC system’s local bus and PCI bus
DRAM Control 2
The memory controller controls all transfers between the microprocessor and the
onboard DRAM and DIMM. It generates the row and column address strobe signals
(RAS, CAS) and the read and write enable signals (MEMR L and MEMW L) for the
DRAMs and the DIMM. It drives these signals and the memory address lines directly to
the memory devices without external buffers.
The DRAM controller in the memory controller chip supports page-mode operations.
For memory read operations the page hit cycles are either 3-2-2-2 or 4-2-2-2 bursts. For
write operations, the page hits are single wait-state accesses. Both read and write
operations are designed for DRAM devices with a 70ns access time.
Table 2-2 PC arbitration priorities
Priority Macintosh side PC system side
High I/O IC Memory controller (Mustard ASIC)
Low Any PCI device Any PCI device