Technical information

CHAPTER 2

Hardware Design

I/O System 27

■ A 64-bit message mailbox with a 32-bit command port

■ Power-on reset logic

■ Autoconﬁguration logic

The Pretzel Logic IC functions as a slave device on the PC system bus. On the Macintosh

system bus, the Pretzel Logic IC functions both as a slave and as an alternate bus master.

To the Macintosh host computer, the DOS Compatibility Card appears as a PDS

expansion card capable of generating slot $E interrupts to the PPC 601 and either

responding as a system bus slave or becoming a system bus master. The Pretzel Logic IC

communicates with the host computer as a bus master when the PC is performing ﬂoppy

disk or hard disk accesses or when sharing Macintosh memory. The Pretzel Logic IC

responds as a system bus slave on the Macintosh host computer during interrupt

acknowledge cycles, keyboard and mouse accesses, and message mailbox accesses.

DMA Channels 2

When the DOS Compatibility Card is conﬁgured to operate in shared memory mode (no

SIMM installed), the Pretzel Logic IC uses one of its DMA channels for access to memory

in the Macintosh host computer. The DMA channel incorporates separate FIFOs for read

and write operations; each FIFO is four longwords deep. The write FIFO allows the

80486 to post up to four longword writes before forcing the processor to wait.

The address translation register provides 32-bit address translation between the PC and

the host computer. This feature supports block sizes of 2 to 64 MB and allows the PC

memory to be relocated anywhere within the unreserved memory area on the Macintosh

host computer.

The second DMA channel is used to perform I/O data transfers between Macintosh

peripherals and PC memory. This I/O DMA channel is used when a DRAM SIMM is

installed on the DOS Compatibility Card.

Serial Port Support 2

To support serial ports, the Pretzel Logic IC contains two identical sets of UART

emulation registers. These registers emulate the hardware of the standard 16C450

serial port ICs found in many PC/AT computers. When the PC accesses these registers,

interrupts are generated in the Macintosh host computer that cause the serial driver

in the Mac OS to route the data to the Macintosh serial ports.

The Macintosh serial ports are RS-422 ports and do not support all RS-232 signals.

In particular, the Carrier Detect (CD), Data Set Ready (DSR), Request To Send (RTS),

and Ring Indicator (RI) signals are not available. Table 1-2 (in Chapter 1) shows the

corresponding signals on the two types of serial ports.

Note

Not all RS-232 devices work properly using the RS-422 protocol. ◆