DP8390D datasheet(11/56 Pages) NSC

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70 Packet Reception (Continued)

3 After a packet is DMAed from the Receive Buffer Ring

the Next Page Pointer (second byte in NIC buffer header)

is used to update BNDRY and next

pkt

pkt e Next Page Pointer

BNDRY e Next Page Pointer b 1

If BNDRY k PSTART then BNDRY e PSTOP b 1

Note the size of the Receive Buffer Ring is reduced by one

256-byte buffer this will not however impede the operation

of the NIC

In StarLAN applications using bus clock frequencies greater

than 4 MHz the NIC does not update the buffer header

information properly because of the disparity between the

network and bus clock speeds The lower byte count is cop-

ied twice into the third and fourth locations of the buffer

header and the upper byte count is not written The upper

byte count however can be calculated from the current

next page pointer (second byte in the buffer header) and the

previous next page pointer (stored in memory by the CPU)

The following routine calculates the upper byte count and

allows StarLAN applications to be insensitive to bus clock

speeds Next

pkt is defined similarly as above

1st Received Packet Removed By Remote DMA

TLF8582 – 57

upper byte count e next page pointer b next

pkt b 1

if (upper byte count) k 0 then

upper byte count e (PSTOP b next

pkt) a

(next page pointer b PSTART) b 1

if (lower byte count) l 0 fch then

upper byte count e upper byte count a 1

STORAGE FORMAT FOR RECEIVED PACKETS

The following diagrams describe the format for how re-

ceived packets are placed into memory by the local DMA

channel These modes are selected in the Data Configura-

tion Register

Storage Format

AD15

AD8

AD7

AD0

Next Packet

Receive

Pointer

Status

Receive

Byte Count 1

Byte Count 0

Byte 2

Byte 1

BOS e 0 WTS e 1 in Data Configuration Register

This format used with Series 32000 808X type processors

AD15

AD8

AD7

AD0

Next Packet

Receive

Pointer

Status

Receive

Byte Count 0

Byte Count 1

Byte 1

Byte 2

BOS e 1 WTS e 1 in Data Configuration Register

This format used with 68000 type processors

Note

The Receive Byte Count ordering remains the same for BOSe0or1

AD7

AD0

Receive Status

Next Packet

Pointer

Receive Byte

Count 0

Receive Byte

Count 1

Byte 0

Byte 1

BOS e 0 WTS e 0 in Data Configuration Register

This format used with general 8-bit CPUs

80 Packet Transmission

The Local DMA is also used during transmission of a pack-

et Three registers control the DMA transfer during trans-

mission a Transmit Page Start Address Register (TPSR)

and the Transmit Byte Count Registers (TBCR01) When

the NIC receives a command to transmit the packet pointed

to by these registers buffer memory data will be moved into

the FIFO as required during transmission The NIC will gen-

erate and append the preamble synch and CRC fields

TRANSMIT PACKET ASSEMBLY

The NIC requires a contiguous assembled packet with the

format shown The transmit byte count includes the Destina-

tion Address Source Address Length Field and Data It

does not include preamble and CRC When transmitting

data smaller than 46 bytes the packet must be padded to a

minimum size of 64 bytes The programmer is responsible

for adding and stripping pad bytes

General Transmit Packet Format

TLF8582 – 58

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