= VCI protocol adaptations for SoCLib =

VCI has some ambiguous features, and other are unusable. Here we'll try to clarify the VCI extensions made in the SoCLib context.

= Endianness =

VCI is quite unclear about endianness interpretation. SoCLib defines some rules on top of VCI:
 * VCI addresses words. LSBs of addresses are non-significant. i.e. on a 32-data-bits VCI network, ADDR[1:0] are useless, and can be kept 0.
 * VCI subword addressing is done through BE field
  * BE![0] is associated to DATA![7:0]
  * BE![1] is associated to DATA![15:8]
  * BE![2] is associated to DATA![23:16]
  * BE![3] is associated to DATA![31:24]
 * DATA Words are little endian so considering an (aligned) address @ of a word in DATA,
  * byte at address @ is in DATA![7:0]
  * byte at address @+1 is in DATA![15:8]
  * byte at address @+2 is in DATA![23:16]
  * byte at address @+3 is in DATA![31:24]

== Peripherals considerations ==

It's up to the peripherals to define their endianness, i.e. the association between lower addresses and lower-significant-bits.

As our convention aligns lower addressable byte to LSBs of DATA, thus is Little-Endian, it may be easier to have little-endian-only peripherals and use them seamlessly from any CPU with encapsulated accesses.

== Memory considerations ==

Memory is endianness-agnostic. It serves reading and writing from/to memory words (in the VCI way defined above)

== Cache considerations ==

VciXCache does not care about endianness of attached processor BUT asserts the VCI definitions above.

When accessing memory with a big-endian processor, it's up to the processor to swap the word from LE to BE.

= Atomic Operations =

VCI defines a `LOCKED_READ` operation. Unfortunately, it can't be properly used with a NoC-based interconnect as putting the reservation on the "read" operation:
 * Can't guarantee the absence of livelocks
 * Necessitates the addition of a timeout mechanism to workaround software doing locked read without associated write.

Therefore, SoCLib uses the "NOOP" VCI command code (redundant with CMDVAL = 0) to introduce the "Store conditional" mechanism.

A "STORE_COND" response uses the RDATA field for STORE completion:
 `RDATA == 0`::
  STORE_COND was atomic
 `RDATA == 1`::
  STORE_COND was not atomic, a complete `LOCKED_READ` / `STORE_COND` cycle must be done again.