Technical information

CHAPTER 2

Hardware Design

28 I/O System

Printer Port Support 2

The Pretzel Logic IC implements all the registers of the standard Centronics parallel port

found on a PC. When the PC accesses these registers, interrupts are generated in the

Macintosh host computer that cause the driver software in the Mac OS to send data to a

print spooler ﬁle. The spooler ﬁle is then sent to whatever printer is selected by the user

in the Macintosh environment.

Note

The parallel port interface does not control printer hardware

signals and does not support bidirectional data transfer. ◆

Keyboard and Mouse Emulation 2

The Pretzel Logic IC emulates in hardware the PC’s keyboard and mouse. The 8242

keyboard and mouse controller is conﬁgured to support a PS2 mouse making the

protocol identical for the keyboard and mouse. The Pretzel Logic IC generates the

appropriate serial clock protocol and serial bit stream to communicate with the 8242.

Message Mailbox 2

The message-passing interface in the Pretzel Logic IC supports simple interrupt-driven

communication between the PC and the Macintosh host computer. The message-passing

interface contains two data registers and one command register. One of the data registers

incorporates byte swapping to allow address-invariant data to be moved between the

two systems. The interface uses a semaphore mechanism of arbitration and grants to

control the direction of the message passing. See “Passing Messages” beginning on

page 45 for a description of the software API for message passing.

Power-on Reset 2

The Pretzel Logic IC contains the reset logic that allows the Macintosh host computer to

start up the PC. Reset of the PC is controlled through the /PWRGOOD signal to the

84035 data path controller IC. Power for the PRAM on the PC is provided by the

Macintosh computer, so the PRAM is not invalidated when the PC is reset. When the

host computer is turned off, the PRAM becomes invalid; the next time the computer is

turned on, software on the Macintosh side reloads the PRAM on the PC side before the

PC system BIOS is executed.

Soft reset of the PC by way of the keyboard (Ctl-Alt-Del keys) is handled by the 8242

keyboard controller once the proper key code is sent by the Pretzel Logic IC through the

keyboard port.