AM486DX2 datasheet(18/67 Pages) AMD

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Am5X86 Microprocessor

AMD

PRELIMINARY

4.3.3

Protected Mode

Protected mode provides access to the sophisticated

memory management paging and privilege capabilities

of the processor.

4.3.4

System Management Mode

SMM is a special operating mode described in detail in

Section 7.

4.4

Cache Architecture

The Am5X86 microprocessor family supports a superset

architecture of the standard 486 cache implementation.

This architectural enhancement improves not only CPU

performance, but total system performance.

4.4.1

Write-Through Cache

The standard 486DX-type write-through cache architec-

ture is characterized by the following:

s External read accesses are placed in the cache if

they meet proper caching requirements.

s Subsequent reads to the data in the cache are made

if the address is stored in the cache tag array.

s Write operations to a valid address in the cache are

updated in the cache

and to external memory. This

data writing technique is called

write-through.

The write-through cache implementation forces all

writes to flow through to the external bus and back to

main memory. Consequently, the write-through cache

generates a large amount of bus traffic on the external

data bus.

4.4.2

Write-Back Cache

The microprocessor write-back cache architecture is

characterized by the following:

s External read accesses are placed in the cache if

they meet proper caching requirements.

s Subsequent reads to the data in the cache are made

if the address is stored in the cache tag array.

s Write operations to a valid address in the cache that

is in the write-through (shared) state is updated in

the cache and to external memory.

s Write operations to a valid address in the cache that

is in the write-back (exclusive or modified) state is

updated

only in the cache. External memory is not

updated at the time of the cache update.

s Modified data is written back to external memory

when the modified cache line is being replaced with

a new cache line (copy-back operation) or an exter-

nal bus master has snooped a modified cache line

(write-back).

The write-back cache feature significantly reduces the

amount of bus traffic on the external bus; however, it

also adds complexity to the system design to maintain

memory coherency. The write-back cache requires en-

FUNCTIONAL DESCRIPTION

4.1

Overview

Am5X86 microprocessors use a 32-bit architecture with

on-chip memory management and cache memory units.

The instruction set includes the complete 486 micropro-

cessor instruction set along with extensions to serve the

new extended applications. All software written for the

486 microprocessor and previous members of the x86

architectural family can run on the Am5X86 micropro-

cessor without modification.

The on-chip Memory Management Unit (MMU) is com-

pletely compatible with the 486 MMU. The MMU in-

cludes a segmentation unit and a paging unit.

Segmentation allows management of the logical ad-

dress space by providing easy data and code relocati-

bility and efficient sharing of global resources. The

paging mechanism operates beneath segmentation and

is transparent to the segmentation process. Paging is

optional and can be disabled by system software. Each

segment can be divided into one or more 4-Kbyte seg-

ments. To implement a virtual memory system, the

Am5X86 microprocessor supports full restartability for

all page and segment faults.

4.2

Memory

Memory is organized into one or more variable length

segments, each up to 4 Gbytes (232 bytes). A segment

can have attributes associated with it, including its lo-

cation, size, type (i.e., stack, code, or data), and protec-

tion characteristics. Each task on a microprocessor can

have a maximum of 16,381 segments, each up to 4

Gbytes. Thus, each task has a maximum of 64 Tbytes

of virtual memory.

The segmentation unit provides four levels of protection

for isolating and protecting applications and the operat-

ing system from each other. The hardware-enforced

protection allows high-integrity system designs.

4.3

Modes of Operation

The Am5X86 microprocessor has four modes of opera-

tion: Real Address mode (Real mode), Virtual 8086 Ad-

dress mode (Virtual mode), Protected Address mode

(Protected mode), and System Management mode

(SMM).

4.3.1

Real Mode

In Real mode, the Am5X86 microprocessor operates as

a fast 8086. Real mode is required primarily to set up

the processor for Protected mode operation.

4.3.2

Virtual Mode

In Virtual mode, the processor appears to be in Real

mode, but can use the extended memory accessing of

Protected mode.

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