Accelerator Interfaces

Minimal Memory Interface Protocol

The Minimal Memory Interface Protocol is a hardware memory interface designed to act as an on-chip bus, and it is used by Bambu as the default accelerator interface. This protocol is specifically designed to offer a compact and straightforward interface while maintaining efficient data transfer for both read and write operations. The protocol utilizes a set of seven signals, each serving a specific purpose in the data transfer process.

Interface Signals

The Minimal Memory Interface Protocol is composed of the following seven signals:

Signal	Direction	Bitsize	Description
oe	Output	1 bit	Output Enable: Asserted by the master to indicate a read request.
we	Output	1 bit	Write Enable: Asserted by the master to indicate a write request.
addr	Output	32/64 bits	Address Bus: Carries the memory address of the data to be read from or written to.
data_size	Output	ceil(log2(Wdata))	Data Size Bus: Specifies the requested write bitsize as a binary number from 0 to the size of the Wdata bus.
Wdata	Output	Variable	Write Data Bus: Carries the data to be written into the memory module during write requests.
Rdata	Input	Variable	Read Data Bus: Carries the requested data from the memory module during read requests.
DataRdy	Input	1 bit	Data Ready: Asserted by the memory module to indicate the completion of a read or write request.

oe (Read Enable): This signal is asserted by the master to indicate a read request. When asserted, the slave memory module is expected to provide data back to the master.

we (Write Enable): This signal is asserted by the master to indicate a write request. When asserted, the slave memory module is ready to accept data from the master for storage.

addr (Address Bus): The address bus is a bus of 32 or 64 bits, depending on the implementation. It carries the memory address of the data to be read from or written to.

data_size (Data Size Bus): The data_size bus is used to specify the requested write bitsize as a binary number ranging from 0 to Wdata bus bitsize. This feature allows the master to indicate how many bits of the Wdata bus carry valid data during a write request.

Wdata (Write Data Bus): Wdata is an output bus of variable size. During write requests, it carries the data to be written into the memory module by the master. The number of valid bits carried on this bus is determined by the data_size signal.

Rdata (Read Data Bus): Rdata is an input bus of variable size. During read requests, the memory module places the requested data on this bus for the master to read.

DataRdy (Data Ready): This signal is asserted by the memory module to indicate the completion of a read or write request. When this signal is asserted, it signifies that valid data is available on the Rdata bus for read requests, or the data provided on the Wdata bus has been successfully stored for write requests.

Operation

The Minimal Memory Interface Protocol operates as follows:

1. Read Operation:

   • The memory address is placed on the addr bus by the master.
   • The master asserts the oe signal to indicate a read request.
   • The memory module responds by placing the requested data on the Rdata bus.
   • Once the data is available on the Rdata bus, the memory module asserts the DataRdy signal.
   • The master reads the data from the Rdata bus upon observing the DataRdy signal.

2. Write Operation:

   • The memory address is placed on the addr bus by the master.
   • The master asserts the we signal to indicate a write request.
   • The master specifies the number of valid bits in the data being written by setting the data_size signal.
   • The data to be written is placed on the Wdata bus by the master.
   • The memory module stores the data from the Wdata bus based on the value indicated by the data_size signal.
   • Once the write operation is complete, the memory module asserts the DataRdy signal.

The Minimal Memory Interface Protocol offers several advantages, including its simplicity, flexibility, efficiency, and low overhead. To enable multiple outstanding requests, it is possible to implement a pipelined version of the minimal interface protocol.

Inferred interfaces

When the --generate-interface=INFER option is specified, Bambu evaluates the following factors, in order of precedence, to set the correct interface protocol for each argument of the top function:

1. Presence of a user-specified XML file (--interface-xml-filename=<filename>)
2. Presence of HLS_interface pragmas in the code
3. Default assignment according to the argument type

In cases 1. and 2. the user selects the desired interfaces for one or more function arguments through the options described here. For all arguments without a specification, and in case 3., the following table applies:

C argument type	Direction	Interface protocol
Scalar (pass by value)	in	none
Array	in/out	array
Pointer or reference	in	none
Pointer or reference	out	valid
Pointer or reference	in/out	o_valid