Technical information

CHAPTER 2
Hardware Design
20
Processor and Memory System
The DOS Compatibility Card does not require a DRAM SIMM with parity.
The presence of a DRAM SIMM on the card is sensed by the Pretzel Logic IC at startup
and stored in a register in the IC. Upon reading this register bit, the startup software
determines the size of the memory and programs the 84031’s configuration registers
with starting and ending addresses for each bank. If a DRAM SIMM is not present,
shared memory is assumed and the software disables all local DRAM banks in the 84031.
BIOS Control 2
The DOS Compatibility Card has no ROM except for the declaration ROM common to
all Macintosh expansion cards. The BIOS is stored in the host computer’s RAM and
accessed by way of the shared memory channel in the Pretzel Logic IC.
Note
The BIOS and the BIOS extensions in the host computer’s memory are
always accessed by way of the shared memory interface, regardless of
whether a DRAM SIMM is installed on the card.
◆
At reset the 80486 microprocessor issues the starting reset-vector address from within the
address range of the BIOS image in the upper 64 KB of shared system memory. The
Pretzel Logic IC remaps this address range down to the lower 1 MB region where the
BIOS actually resides. The Pretzel Logic IC also performs the address translation
between the BIOS addresses on the PC side and the corresponding addresses in shared
memory on the Macintosh host computer.
Clock Generation 2
The 84031 memory controller IC receives a 2X clock and generates a low-skew 1X and 2X
clock for the system and the 80486 processor. In addition, it divides down the 2X clock to
generate the BUSCLK signal for the ISA bus.
ISA Bus Control 2
The 84031 handles all accesses to the ISA bus by the 80486. In addition, the 84031
performs data buffering to form the XD bus for local peripherals such as the keyboard,
joystick, and VGA controllers. The 84031 also provides support for local bus slaves such
as the Pretzel Logic IC.
84035 Data Path Controller 2
The 84035 data path controller IC performs the following system-level functions:
■
system reset
■
interrupt control
■
speaker drive