HARDWARE IMPLEMENTATION

Pipelining instruction fetch, decode, and execu- tion units on a single chip is not unusual in modern microprocessors. On the other hand, placing the memory management unit (MMU) in the on-chip pipeline is quite unusual. Incor- porating the MMU on the processor chip im- proves the speed of addresss translation by reducing signal propagation delays (most off- chip MMUs introduce at least one wait state), and exploiting the half-clock boundaries that are accessible within the chip (the 80386 clock input is twice the frequency of the chip). The 80386

MMU consists of the segment and page units shown in Figure 5-1.

The segment unit translates logical addresses to linear addresses, and checks each access for consistency with segment protection attributes. For the majority of instructions, the segment unit obtains the translation and protection data from the 80386's on-chip segment and descriptor registers. The page unit is enabled or disabled by operating system software. When disabled, the linear addresses produced by the segment unit

82384OPTIONAL CLOCK

GENERATOR

CACHE

CONTROL

80287

OR

 

 

DRAM

80387

80386

DRAM

 

NUMERIC

CPU

 

CONTROL

 

COPROCESSOR

 

 

 

 

 

 

 

MULTIBUSINTERFACE1/11

K:====:> MULTIBUS® 1/11

 

INTR

 

 

 

 

82258

LOCAL

 

 

ADVANCED

BUS

 

 

DMA

CONTROL

EPROM

Figure 5-2. Representative System Block Diagram

5-2

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Intel 80386 manual Hardware Implementation