The steaming interface (for example the Avalon ST)

input logic data[255:0] in_data; // a 256 bits input stream data

The local signals of data like this:

logic[255:0] bit_data;           // local signal for the input data

I have business messages with max length = 1500 bytes (12000 bits)

So I have to model an integer for representing the message length in BIT:

logic[13:0] msg_bit_len; // 14 bits unsigned int with max value = 16383

If I model the data with BYTE based:

typedef logic[7:0] logic8x32_t [31:0]; // a 32 bytes structure

logic[7:0] byte_data [31:0]; // local signal for the input data
byte_data = logic8x32_t'(in_data);

with BYTE based I can model msg_byte_len (max = 1500) with lesser number of bits.

logic[10:0] msg_byte_len; // 11 bits unsigned integer with max value = 2047

In this case 3 bits are saved comparing to BIT based.

It sounds good but I am afraid if there exist any issue with byte base design (for example, the incoming data are not in completed byte).

Please advise.

  • 1
    \$\begingroup\$ Designing messages that way makes the life of people much harder that work with them on ordinary processors \$\endgroup\$ – PlasmaHH Aug 31 '18 at 6:01
  • 1
    \$\begingroup\$ Either way, you are comparing the same number of bits. "Saving 3 bits" on your counter, i would argue, is a big ol' "don't care" when compared to what makes logical sense and is easiest to design. BTW, aren't ethernet fields on byte boundaries? When would you have a valid message that wasn't a "complete byte"? \$\endgroup\$ – CapnJJ Sep 4 '18 at 19:29

It depends, for example:

  • if you models a work over MII, you need to represent a packet as a stream of consecutive nibbles, nibble = a half of 8-bit byte = 4 bit;
  • if you models it over GMII, you can represent a packet as a stream of consecutive octets directly, octet = 8-bit byte;
  • etc...

If you models a work over a wide (N x 8 in the case you shown) parallel bus, such a bus must contain masking signals (N in the case of 8-bit byte) to speak out which byte(s) on the bus contain(s) information during a transfer cycle and which do(es) not.


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