6.42

IDT7035S/L

High-Speed 8K x 18 Dual-Port Static RAM

Industrial and Commercial Temperature Ranges

18

The blocks do not have to be any particular size and can even be

variable, depending upon the complexity of the software using the

semaphoreflags.AlleightsemaphorescouldbeusedtodividetheDual-

Port RAM or other shared resources into eight parts. Semaphores can

evenbeassigneddifferentmeaningsondifferentsidesratherthanbeing

given a common meaning as was shown in the example above.

Semaphores are a useful form of arbitration in systems like disk

interfaceswheretheCPUmustbelockedoutofasectionofmemoryduring

atransferandtheI/Odevicecannottolerateanywaitstates.Withtheuse

ofsemaphores,oncethetwodeviceshasdeterminedwhichmemoryarea

was“off-limits”totheCPU,boththeCPUandtheI/Odevicescouldaccess

theirassignedportionsofmemorycontinuouslywithoutanywaitstates.

Semaphoresarealsousefulinapplicationswherenomemory“WAIT”

stateisavailableononeorbothsides.Onceasemaphorehandshakehas

Figure 4. IDT7035 Semaphore Logic

D

4088 drw 18

0

D

Q

WRITE

D

Q

WRITE

SEMAPHORE

REQUEST FLIP FLOP

SEMAPHORE

REQUEST FLIP FLOP

LPORT

RPORT

SEMAPHORE

READ

SEMAPHORE

READ

,

been performed, both processors can access their assigned RAM

segmentsatfullspeed.

Another application is in the area of complex data structures. In this

case,blockarbitrationisveryimportant.Forthisapplicationoneprocessor

mayberesponsibleforbuildingandupdatingadatastructure.Theother

processorthenreadsandinterpretsthatdatastructure.Iftheinterpreting

processorreadsanincompletedatastructure,amajorerrorconditionmay

exist.Therefore,somesortofarbitrationmustbeusedbetweenthetwo

differentprocessors.Thebuildingprocessorarbitratesfortheblock,locks

itandthenisabletogoinandupdatethedatastructure.Whentheupdate

is completed, the data structure block is released. This allows the

interpretingprocessortocomebackandreadthecompletedatastructure,

therebyguaranteeingaconsistentdatastructure.