Technical information

CHAPTER 2
Hardware Design
14
Processor and Memory System
Processor and Memory System 2
The processor and memory system includes the 80486DX2 microprocessor and the
control devices for the onboard memory: the 84031 memory controller and the
84035 data path controller.
80486DX2 Microprocessor 2
The DOS Compatibility Card has a 80486DX2 microprocessor running at 66 MHz
(33 MHz processor bus clock). The 80486DX2 supports 32-bit data paths and 32-bit
addresses; that allows up to 4 GB of physically addressable memory. The DX2 version
of the 80486 is pin compatible with the DX version but operates at twice the clock
frequency.
Some of the key features of the 80486DX2 are listed below. Please refer to the Intel
80486SX/DX Hardware Reference Manual for further information.
■
full 32-bit addressing architecture with 32 bit data interface
■
internal 8 KB unified instruction and data cache
■
instruction prefetch mechanism during idle bus activity
■
internal memory management unit supporting both memory segmentation
and paging
■
internal write buffer (1 longword deep) to support posted writes
■
dynamic bus sizing to support 8-bit and 16-bit peripherals
■
support for synchronous 16-byte block reads
■
backward compatible with existing 80x86 code
The 80486DX2 in the DOS Compatibility Card is in a 168-pin ceramic PGA package. With
a clock speed of 66 MHz, this package requires a heat sink.
PC System Bus and Devices 2
The PC system bus is defined as the unbuffered 80486 pins that are required to support
slave and alternate bus masters. This bus operates synchronously at the same clock
speed as the processor bus clock (33 MHz). The bus supports burst reads and compelled
writes (due to the write-through cache). The key devices attached to this bus are the
memory and bus controller IC and the Pretzel Logic bus interface IC.