Register File

Source Files: rtl/ibex_register_file_ff.sv rtl/ibex_register_file_fpga.sv rtl/ibex_register_file_latch.sv

Ibex has either 31 or 15 32-bit registers if the RV32E extension is disabled or enabled, respectively. Register x0 is statically bound to 0 and can only be read, it does not contain any sequential logic.

The register file has two read ports and one write port, register file data is available the same cycle a read is requested. There is no write to read forwarding path so if one register is being both read and written the read will return the current value rather than the value being written.

There are three flavors of register file available, each having their own benefits and trade-offs. The register file flavor is selected via the enumerated parameter RegFile defined in rtl/ibex_pkg.sv.

Flip-Flop-Based Register File

The flip-flop-based register file uses regular, positive-edge-triggered flip-flops to implement the registers.

This makes it the first choice when simulating the design using Verilator.

This implementation can be selected by setting the RegFile parameter to “ibex_pkg::RegFileFF”. It is the default selection.

FPGA Register File

The FPGA register file leverages synchronous-write / asynchronous-read RAM design elements, where available on FPGA targets.

For Xilinx FPGAs, synthesis results in an implementation using RAM32M primitives. Using this design with a Xilinx Artya7-100 FPGA conserves around 600 Logic LUTs and 1000 flip-flops at the expense of 48 LUTRAMs for the 31-entry register file as compared to the flip-flop-based register file.

This makes it the first choice for FPGA synthesis.

To select the FPGA register file, set the RegFile parameter to “ibex_pkg::RegFileFPGA”.

Latch-Based Register File

The latch-based register file uses level-sensitive latches to implement the registers.

This allows for significant area savings compared to an implementation using regular flip-flops and thus makes the latch-based register file the first choice for ASIC implementations. Simulation of the latch-based register file is possible using commercial tools.

Note

The latch-based register file cannot be simulated using Verilator.

The latch-based register file can also be used for FPGA synthesis, but this is not recommended as FPGAs usually do not well support latches.

To select the latch-based register file, set the RegFile parameter to “ibex_pkg::RegFileLatch”. In addition, a technology-specific clock gating cell must be provided to keep the clock inactive when the latches are not written. This cell must be wrapped in a module called prim_clock_gating. For more information regarding the clock gating cell, checkout Getting Started with Ibex.

Note

The latch-based register file requires the gated clock to be enabled in the cycle after the write enable we_a_i signal was set high. This can be achieved by latching we_a_i in the clock gating cell during the low phase of clk_i.

The resulting behavior of the latch-based register file is visualized in Figure 9. The input data wdata_a_i is sampled into a flip-flop-based register wdata_a_q using clk_int. The actual latch-based registers mem[1] and mem[2] are transparent during high phases of mem_clk[1] and mem_clk[2], respectively. Their content is sampled from wdata_a_q on falling edges of these clocks.

Figure 9 Latch-based register file operation