IBM 600 manual Bus Adapter, Invalidated When Microprocessor Performs, Address Space, Shadowed

bus,

the

cache memory enters “snoop” mode and monitors all write

and

read

operations. If memory data is written

to

a

location

in

the

cache and the cache line is in

the “modified” state, the corresponding

cache line

is

written

back

to system

memory

and

invalidated.

When

the

microprocessor

performs

a

memory

read,

the

data

address

is

used

to find the data in the

cache. If

the

data is found (a hit), it is

read

from

the

cache

memory

and

no

external

bus

cycle

occurs. If

the

data

is

not

found

(a

miss),

an

external

bus

cycle is used to read

the

data

from

system

memory. If

the

address

of

the

missed

data

is

in

cacheable

address

space,

the data is

stored

in

the

cache

memory

and

the

remainder

of

the

cache

line

is

read.

When the microprocessor performs a memory write, the data

address

is

used

to

search

the

cache. If

the

address

is

found

(a hit),

the

data

is written to the cache

and

no

external

bus cycle is used to

write the data to system memory. (If the address of the write

operation

was not in the cache

memory but was in cacheable

address

space,

the

data

is

read

back

into

the

cache

memory and

the

remainder

of

the

cache line

is

read.)

Cacheable

Address

Space

Cacheable

address

space

is

defined

as

system

memory

that

resides

on the system board (0–640

KB and 1 MB–256 MB). Cacheability of

system

memory

is

up

to 64 MB for

Pentium

or 512 MB for Pentium II

in the L2 cache. Nothing in

address

range

hex A0000–BFFFF,

I/O

address

space,

or

memory

in

any

AT

slot

is

cached.

ROM address space (hex C0000–C9FFF and F0000–FFFFF) is L1

cacheable codefor read operations. Ifonlydata in this address

range

is

already

in

cache

memory

and the

address range is

written

to,

the

cached

line

is

invalidated

and

is

read

again

from

RAM, where

the

BIOS

is

shadowed.

Bus Adapter

When the computer is attached to the ThinkPad SelectaDock III docking system, the PCI adapters or AT-bus adapters can be used through the docking system.

System Board 2-3