6.42

IDT71342SA/LA

High-Speed 4K x 8 Dual-Port Static RAM with Semaphore

Industrial and Commercial Temperature Ranges

12

request latches feed into a semaphore flag. Whichever latch is first to

present a zero to the semaphore flag will force its side of the

semaphore flag LOW and the other side HIGH. This condition will

continue until a one is written to the same semaphore request latch.

Should the other side’s semaphore request latch have been written to

a zero in the meantime, the semaphore flag will now stay LOW until its

semaphore request latch is written to a one. From this it is easy to

understand that, if a semaphore is requested and the processor which

requesteditnolongerneedstheresource,theentiresystemcanhangup

untilaoneiswrittenintothatsemaphorerequestlatch.

The critical case of semaphore timing is when both sides request

a single token by attempting to write a zero into it at the same time. The

semaphore logic is specially designed to resolve this problem. If

simultaneous requests are made, the logic guarantees that only one

sidereceivesthetoken.Ifonesideisearlierthantheotherinmakingthe

request, the first side to make the request will receive the token. If both

requestsarriveatthesametime,theassignmentwillbearbitrarilymade

to one port or the other.

One caution that should be noted when using semaphores is that

semaphores alone do not guarantee that access to

a resource is secure. As with any powerful programming technique, if

semaphores are misused or misinterpreted, a software error can

easily happen. Code integrity is of the utmost importance when

semaphores are used instead of slower, more restrictive hardware

intensive schemes.

Initialization of the semaphores is not automatic and must be

handledviatheinitializationprogramatpowerup.Sinceanysemaphore

request flag which contains a zero must be reset to a one, all

NOTE:

NOTE:

1. This table denotes a sequence of events for only one of the eight semaphores on the IDT71342.

3. CE = VIH, SEM = VIL to access the semaphores. Refer to the semaphore Read/Write Control Truth Table.

R/W

CE

SEM

OE

Function

X

H

H

X

Z

Port Disabled and in Power Down Mode

HHL

L

Data in Semaphore Flag Output on Port

X

X

X

H

Z

Output Disabled

↑

HL

X

HL

HL

Data in Memory Output on Port

LL

H

X

Data on Port Written Into Memory

XL

L

X

____

Not Allowed

2721 tbl 11

Functions

Status

No Action

1

1

Semaphore free

Left Port Writes "0" to Semaphore

0

1

Left port has semaphore token

Right Port Writes "0" to Semaphore

0

1

No change. Right side has no write access to semaphore

Left Port Writes "1" to Semaphore

1

0

Right port obtains semaphore token

Left Port Writes "0" to Semaphore

1

0

No change. Left port has no write access to semaphore

Right Port Writes "1" to Semaphore

0

1

Left port obtains semaphore token

Left Port Writes "1" to Semaphore

1

1

Semaphore free

Right Port Writes "0" to Semaphore

1

0

Right port has semaphore token

Right Port Writes "1" to Semaphore

1

1

Semaphore free

Left Port Writes "0" to Semaphore

0

1

Left port has semaphore token

Left Port Writes "1" to Semaphore

1

1

Semaphore free

2721 tbl 12