Bit-Lane Definitions (Memory Device Usage) - 2024.1 English

UpdateMEM User Guide (UG1580)

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2024.1 English

A bit-lane definition determines which bits in a CPU bus access are assigned to particular block RAMs. Each definition takes the form of MemType with Placement data, followed by the bit numbers and AddressRange the bit lane occupies. The syntax is, as follows:

<BitLane MemType=”RAMB32” Placement=”X2Y19”>
          <DataWidth MSB=”31” LSB=”16”/>
          <AddressRange Begin=”0” End=”2047”/>
          <Parity ON=”false” NumBits=”0”/>
Important: Although bit-lane parity is defined in the MMI file, it is not supported by UpdateMEM.

Typically, the bit numbers are given in the following order:

<DataWidth MSB=bit_num LSB=bit_num>

If the order is reversed to have the least significant bit (LSB) first and the most significant bit (MSB) second, UpdateMEM bit-reverses the bit-lane value before placing it into the block RAM.

As with BusBlocks, the order in which bit-lanes are defined is important. But in the case of bit-lanes, the order infers which part of BusBlock CPU access a bit-lane occupies. The first bit-lane defined is inferred to be the most significant bit-lane value, and the last defined is the least significant bit-lane value. In the following figure, the most significant bit-lane is BRAM7, and the least significant bit-lane is BRAM0. As seen in example Figure 1 layout, this corresponds with the order in which the Bit Lanes are defined.

When UpdateMEM inputs data, it takes data from data input files in Bit Lane sized chunks, from the most right value first to the left most. For example, if the first 64 bits of input data are 0xB47DDE02826A8419, the value 0xB4 is the first value to be set into a block RAM.

The Bit Lane order BRAM7 is set to 0xB4 and BRAM6 to 0x7D until BRAM0 is set to 0x19. This process repeats for each successive Bus Block access block RAM set until the memory space is filled or until the input data is exhausted. The following figure expands the first Bus Block to illustrate this process.

Figure 1. Bit Lane Fill Order

The Bit Lane definitions must match the hardware configuration.
Warning: If the MMI is defined differently from the way the hardware actually works, incorrect data is retrieved from the memory components.

Bit Lane definitions also have some optional syntax, depending on the device type keyword used in the Address Block definition.

When specifying block RAM cells, indicate the physical row and column location within the FPGA. The following examples displays physical row and column location.


Use the Placement= keyword to assign the corresponding block RAM to a specific resource location in the FPGA. In this case, the block RAM is placed at column 3 and row 5 in the FPGA.


In addition to using correct syntax for bit-lane and BusBlock definitions, the following limitations are applicable.

  • The examples in this document use only byte-wide data widths for clarity, the same principles apply to any data width for which a block RAM is configured.
  • Ensure there are no gaps or overlaps in bit-lane numbering. All bit-lanes in an Address Block must be the same number of bits wide.
  • The bit-lane widths are valid for the memory device specified by the device type keyword.
  • The amount of byte storage occupied by the Bit Lane block RAMs in a BusBlock must equal the range of addresses inferred by the start and end addresses for a BusBlock.
    • All BusBlocks equals the same number of bytes in size.
    • Specify a block RAM instance name only once.
    • A BusBlock contains one or more valid bit-lane definitions.
    • An address block contains one or more valid BusBlock definitions.

    UpdateMEM checks for all these conditions and transmits an error message if it detects a violation.