M80C286 datasheet(27/60 Pages) INTEL

27 / 60 page

M80C286

271103 – 26

Figure 28 Back to Back Write-Write Cycles

HOLD and HLDA

HOLD AND HLDA allow another bus master to gain

control of the local bus by placing the M80C286 bus

into the Th state The sequence of events required

to pass control between the M80C286 and another

local bus master are shown in Figure 29

In this example the M80C286 is initially in the Th

state as signaled by HLDA being active Upon leav-

ing Th as signaled by HLDA going inactive a write

operation is started During the write operation an-

other local bus master requests the local bus from

the M80C286 as shown by the HOLD signal After

completing the write operation the M80C286 per-

forms one Ti bus cycle to guarantee write data hold

time then enters Th as signaled by HLDA going ac-

tive

The CMDLY signal and ARDY ready are used to

start and stop the write bus command respectively

Note that SRDY must be inactive or disabled by

SRDYEN to guarantee ARDY will terminate the cy-

cle

HOLD must not be active during the time from the

leading edge of RESET until 34 CLKs following the

trailing edge of RESET

Lock

The CPU asserts an active lock signal during Inter-

rupt-Acknowledge cycles the XCHG instruction and

during some descriptor accesses Lock is also as-

serted when the LOCK prefix is used The LOCK

prefix may be used with the following ASM-286 as-

sembly instructions MOVS INS and OUTS For bus

cycles other than Interrupt-Acknowledge cycles

Lock will be active for the first and subsequent cy-

cles of a series of cycles to be locked Lock will not

be shown active during the last cycle to be locked

For the next-to-last cycle Lock will become inactive

at the end of the first Tc regardless of the number of

wait-states inserted For Interrupt-Acknowledge cy-

cles Lock will be active for each cycle and will be-

come inactive at the end of the first Tc for each cy-

cle regardless of the number of wait-states inserted

Instruction Fetching

The M80C286 Bus Unit (BU) will fetch instructions

ahead of the current instruction being executed This

activity is called prefetching It occurs when the local

bus would otherwise be idle and obeys the following

rules

A prefetch bus operation starts when at least two

bytes of the 6-byte prefetch queue are empty

The prefetcher normally performs word prefetches

independent of the byte alignment of the code seg-

ment base in physical memory

The prefetcher will perform only a byte code fetch

operation for control transfers to an instruction be-

ginning on a numerically odd physical address

Prefetching stops whenever a control transfer or

HLT instruction is decoded by the IU and placed into

the instruction queue

M80C286 Datasheet(PDF) 27 Page - Intel Corporation

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