Technical information

CHAPTER 2

Hardware Design

Processor and Memory System

When performing DMA cycles to the PC, the Pretzel Logic IC becomes a PowerPC 601

bus master.

When conﬁgured for shared memory operation, the Pretzel Logic IC provides a way to

limit bus monopolizing by the PDS slot. Internal to the Pretzel Logic IC is a count

pending number of posted writes is equal to or less than the speciﬁed count.

The DOS Compatibility Card relies on the host’s PowerPC 601 microprocessor to

maintain a watchdog timer for the PDS. This timer is necessary to prevent the Macintosh

from hanging while waiting for a response from the Pretzel Logic IC.

Expansion 2

The DOS Compatibility Card does not provide any way to add ISA or EISA expansion

boards. The local ISA bus (XD) is closed and supports only the 8242 keyboard controller,

80450 VGA controller, the 558 game timer, and the sound expansion card. The COM1,

COM2, and LPT1 peripherals usually found on the AT-ISA bus are directly accessible

from the Pretzel Logic IC through the processor system bus.

A 50-pin connector is provided on the card for access to a subset of the ISA signals. That

connector is normally occupied by the sound expansion card.

84031 Memory Controller 2

The 84031 memory controller IC performs the following system-level functions:

■

DRAM control

■

ROM control

■

system clock generation

■

ISA bus control

■

VL (local) bus arbitration

DRAM Control 2

The DRAM on the card is directly interfaced to the local data bus. The /RAS, /CAS, /

DWE, and MA lines are driven directly from the 84031 memory controller IC without

external buffers.

The DRAM controller in the 84031 supports page mode operation. 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 1-wait-state accesses. Both read and write operations are designed for

DRAM devices with 80 ns access time and have RAS-CAS delays of two T states.

The DOS Compatibility Card has a slot for one 32-bit-wide SIMM that supports up to

two banks of DRAM (for double-sided modules). No system DRAM is soldered on the

card. A single-sided SIMM can hold 1 MB, 4 MB, or 16 MB using 1, 4, or 16 Mbit DRAM

devices, respectively. Double-sided SIMM modules can hold double those amounts

of memory.