Specifications

CHAPTER 2
Hardware Design
Hardware Features 17
a mechanism that allows each device (keyboard, serial port, and so forth) to interrupt the
CPU asynchronously. Table 2-1 defines the interrupts used in both cards.
All transfers between the PC system and the Macintosh host computer are interrupt
driven. The master interrupt register, which is part of the Mustard ASIC, contains the
state of all interrupt sources on the card. Each of these sources can be individually
masked by an accompanying master interrupt enable register. You can determine the
precise reason for the interrupt by examining the registers for the ports associated with
the interrupts listed in Table 2-1.
You can access the interrupt and status registers in the Mustard ASIC from the
Macintosh environment only. In the PC environment, you will see the standard
definitions for the COM1, COM2, and parallel ports.
The system interrupts for the 12” card are controlled by the 82C558 IC. On the 7” card,
the 85C497 IC controls the interrupts.
Bus Arbitration 2
On the PC side of the PCI bus there are two functional circuit blocks that can act as bus
master. They are the Mustard ASIC and the PC chip set.
Each bus master must arbitrate for control of the bus using a straightforward
request-grant handshake. Each bus master has a unique request signal (REQ L) that it
asserts to request access to the bus. An arbitration algorithm examines the request and
assigns access to the bus based on a rotating priority system. When access is granted, a
grant signal (GNT L), unique to each master, is returned.
A bus error on the PC system bus causes the PC system (12” or 7”) to hang. When this
happens, Macintosh operation is not affected, and you can use the Macintosh computer
to restart the PC system. You can restart from the Macintosh using Restart from the
Finder menu, or by using the Cmd-Ctl-Alt-Del key sequence on the Macintosh keyboard.
Note that on some keyboards, the Alt key is labeled Option or Alt Option. If the PC
Table 2-1 Definition of interrupts
Interrupt number Description
1 Keyboard data ready
3 COM2 port
4 COM1 port
5 Sound Blaster
6 Message interface
7 Parallel port 1 (printer)
12 Mouse