LAN91C111 datasheet(84/125 Pages) MICROCHIP

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DS00002276A-page 85

LAN91C111

10.5

Memory Partitioning

Unlike other controllers, the LAN91C111 does not require a fixed memory partitioning between transmit and receive

resources. The MMU allocates and de-allocates memory upon different events. An additional mechanism allows the

CPU to prevent the receive process from starving the transmit memory allocation.

Memory is always requested by the side that needs to write into it, that is: the CPU for transmit or the MAC for receive.

The CPU can control the number of bytes it requests for transmit but it cannot determine the number of bytes the receive

process is going to demand. Furthermore, the receive process requests will be dependent on network traffic, in particular

on the arrival of broadcast and multicast packets that might not be for the node, and that are not subject to upper layer

software flow control.

FIGURE 10-5:

DRIVE SEND AND ALLOCATE ROUTINES

AL L O C AT E

Issu e "A llo cate M e m o ry"

Co m m a nd to M M U

R ead In te rrup t S tatus Re gister

E nqu eue Pa cket

S e t "R eady fo r Pa cke t" Flag

Re tu rn

Copy Re m a in in g T X D a ta

Pa cke t in to R A M

R e tu rn B u ffe rs to U p p e r L aye r

W rite A llo ca te d Pa cke t in to

Pa cke t # Re gister

W rite Add ress Po in te r R e g ister

C o py Pa rt o f T X D a ta Pa cke t

in to R A M

W rite S o urce A d d ress in to

P rop er Loca tio n

S tore Da ta B u ffer Po in te r

C lea r "R e ady fo r Pa cket" F lag

E n a ble A llo ca tio n In te rru p t

A llo ca tio n

Pa sse d ?

Ye s

N o

DRIV E R SE ND

C hoo se B ank S e le ct

Re gister 2

Call A L LO CAT E

E xit D river S end

R e a d A llo ca tio n R e su lt

Re gister

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