Servile : Convenience wrapper
=============================

.. figure:: servile.png

   Servile convenience wrapper

Servile is a helper component that takes care of common configuration and is used as a building block in the other designs and subsystems. It exposes a memory bus intended to be connected to a combined data and instruction memory, an extension bus for peripheral controllers and accelerators and an RF interface for connecting to an SRAM for GPR and CSR registers.

.. figure:: servile_int.png

   Servile block diagram

Internally, Servile contains logic for optionally instantiating an MDU core for the M extension, an arbiter to route instruction fetches and main memory accesses through the same interface and a mux to split up the memory map into memory (0x00000000-0x3FFFFFFF) and external accesses (0x40000000-0xFFFFFFFF). The mux also contains simulation-only logic to write output to a log file and stop the simulation.

Parameters
----------

.. list-table:: Parameters
   :header-rows: 1
   :widths: 10 20 80

   * - Parameter
     - Values
     - Description
   * - reset_pc
     - 0x00000000 (default) - 0xFFFFFFFC
     - Address of first instruction to fetch from memory after reset (Reset vector)
   * - reset_strategy
     - "MINI" (default), "NONE"
     - | Amount of reset applied to design
       | "NONE" : No reset at all. Relies on a POR to set correct initialization values and that core isn't reset during runtime
       | "MINI" : Standard setting. Resets the minimal amount of FFs needed to restart execution from the instruction at RESET_PC.
   * - rf_width
     - 2 (default), 4, 8, 16, 32
     - Width of the data bus to the RF memory. Typically smaller values use less resources, but can be implementation-dependant.
   * - sim
     - 0 (default), 1
     - | Enable simulation mode. In simulation mode, two memory addresses have special purposes.
       | 0x80000000: Writes to this address puts the byte in the lowest data byte into a log file decided by the "signature" plusarg.
       | 0x90000000: Writes to this address ends the simulation.
   * - with_c
     - 0 (default), 1
     - Enable the C extension. This also makes SERV support misaligned loads and stores.
   * - with_csr
     - 0 (default), 1
     - Enable the Zicsr extension. This also enables timer IRQ and exception handling. Note that SERV only implements a small subset of the CSR registers.
   * - with_mdu
     - 0 (default), 1
     - Enables the Multiplication and Division Unit (MDU) to support the M extension. Note that this only enables the interface and the decoder logic. The MDU itself is external from SERV.

Signals
-------

.. list-table:: Signals
   :header-rows: 1
   :widths: 30 10 5 75

   * - Signal
     - Width
     - Direction
     -  Description
   * - i_clk
     - 1
     - in
     - Clock
   * - i_rst
     - 1
     - in
     - Synchronous reset
   * - i_timer_irq
     - 1
     - in
     - Timer interrupt
   * - **Memory interface**
     -
     -
     - Connect to instruction/data memory
   * - o_wb_mem_adr
     - 32
     - out
     - Memory bus address
   * - o_wb_mem_dat
     - 32
     - out
     - Memory bus data
   * - o_wb_mem_sel
     - 4
     - out
     - Memory bus write data byte select mask
   * - o_wb_mem_we
     - 1
     - out
     - Memory bus write transaction
   * - o_wb_mem_stb
     - 1
     - out
     - Memory bus active strobe
   * - i_wb_mem_rdt
     - 32
     - in
     - Memory bus read data
   * - i_wb_mem_ack
     - 1
     - in
     - Memory bus cycle acknowledged
   * - **Extension interface**
     -
     -
     - Connect to peripheral controllers
   * - o_wb_ext_adr
     - 32
     - out
     - Data bus address
   * - o_wb_ext_dat
     - 32
     - out
     - Data bus write data
   * - o_wb_ext_sel
     - 4
     - out
     - Data bus write data byte select mask
   * - o_wb_ext_we
     - 1
     - out
     - Data bus write transaction
   * - o_wb_ext_stb
     - 1
     - out
     - Data bus active cycle
   * - i_wb_ext_rdt
     - 32
     - in
     - Data bus return data
   * - i_wb_ext_ack
     - 1
     - in
     - Data bus return data valid
   * - **RF (SRAM) interface**
     -
     -
     -
   * - o_rf_waddr
     - ceil(log2(regs*32/rf_width)
     - out
     - RF memory write address
   * - o_rf_wdata
     - rf_width
     - out
     - RF memory write data
   * - o_rf_wen
     - 1
     - out
     - RF memory write enable
   * - o_rf_raddr
     - ceil(log2(regs*32/rf_width)
     - out
     - RF memory read address
   * - i_rf_rdata
     - rf_width
     - out
     - RF memory read data
   * - o_rf_ren
     - 1
     - out
     - RF memory read enable