70V06 datasheet(21/24 Pages) RENESAS

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6.42

70V06L

High-Speed 3.3V 16K x 8 Dual-Port Static RAM

Industrial and Commercial Temperature Ranges

Feb.07.20

Using Semaphores-Some Examples

Perhapsthesimplestapplicationofsemaphoresistheirapplicationas

resourcemarkersfortheIDT70V06’sDual-PortSRAM.Saythe16Kx8

SRAM was to be divided into two 8K x 8 blocks which were to be

dedicated at any one time to servicing either the left or right port.

Semaphore0couldbeusedtoindicatethesidewhichwouldcontrolthe

lower section of memory, and Semaphore 1 could be defined as the

indicator for the upper section of memory.

Totakearesource,inthisexamplethelower8KofDual-PortSRAM,

the processor on the left port could write and then read a zero in to

Semaphore0.Ifthistaskweresuccessfullycompleted(azerowasread

back rather than a one), the left processor would assume control of the

lower8K.Meanwhiletherightprocessorwasattemptingtogaincontrolof

the resourceaftertheleftprocessor,itwouldreadbackaoneinresponse

to the zero it had attempted to write into Semaphore 0. At this point, the

softwarecouldchoosetotryandgaincontrolofthesecond8Ksectionby

writing,thenreadingazerointoSemaphore1.Ifitsucceededingaining

control,itwouldlockouttheleftside.

Once the left side was finished with its task, it would write a one to

Semaphore 0 and may then try to gain access to Semaphore 1. If

Semaphore 1 was still occupied by the right side, the left side could

undo its semaphore request and perform other tasks until it was able

to write, then read a zero into Semaphore 1. If the right processor

performs a similar task with Semaphore 0, this protocol would allow the

two processors to swap 8K blocks of Dual-Port SRAM with each other.

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

variable, depending upon the complexity of the software using the

2942 drw 19

WRITE

SEMAPHORE

REQUEST FLIP FLOP

SEMAPHORE

REQUEST FLIP FLOP

LPORT

RPORT

SEMAPHORE

READ

SEMAPHORE

READ

Figure 4. IDT70V06 Semaphore Logic

semaphoreflags.AlleightsemaphorescouldbeusedtodividetheDual-

PortSRAMorothersharedresourcesintoeightparts.Semaphorescan

evenbeassigneddifferentmeaningsondifferentsidesratherthanbeing

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

Semaphores are a useful form of arbitration in systems like disk

interfaces where the CPU must be locked out of a section of memory

during a transfer and the I/O device cannot tolerate any wait states.

With the use of semaphores, once the two devices has determined

which memory area was “off-limits” to the CPU, both the CPU and the

I/O devices could access their assigned portions of memory continu-

ously without any wait states.

Semaphores are also useful in applications where no memory

“WAIT” state is available on one or both sides. Once a semaphore

handshake has been performed, both processors can access their

assignedSRAMsegmentsatfullspeed.

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

case, block arbitration is very important. For this application one

processor may be responsible for building and updating a data

structure. The other processor then reads and interprets that data

structure. If the interpreting processor reads an incomplete data

structure, a major error condition may exist. Therefore, some sort of

arbitration must be used between the two different processors. The

building processor arbitrates for the block, locks it and then is able to

go in and update the data structure. When the update is completed, the

data structure block is released. This allows the interpreting processor

to come back and read the complete data structure, thereby guaran-

teeing a consistent data structure.

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