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-- $Id: ibd_iist.vhd 335 2010-10-24 22:24:23Z mueller $

--

-- Copyright 2009-2010 by Walter F.J. Mueller <W.F.J.Mueller@gsi.de>

--

-- This program is free software; you may redistribute and/or modify it under

-- the terms of the GNU General Public License as published by the Free

-- Software Foundation, either version 2, or at your option any later version.

--

-- This program is distributed in the hope that it will be useful, but

-- WITHOUT ANY WARRANTY, without even the implied warranty of MERCHANTABILITY

-- or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

-- for complete details.

--

------------------------------------------------------------------------------

-- Module Name:    ibd_iist - syn

-- Description:    ibus dev(loc): IIST

--

-- Dependencies:   -

-- Test bench:     -

-- Target Devices: generic

-- Tool versions:  xst 8.1, 8.2, 9.1, 9.2, 12.1; ghdl 0.18-0.29

--

-- Synthesized (xst):

-- Date         Rev  ise         Target      flop lutl lutm slic t peri

-- 2010-10-17   333  12.1    M53 xc3s1000-4   112  510    0  291 s 15.8

-- 2010-10-17   314  12.1    M53 xc3s1000-4   111  504    0  290 s 15.6

-- 2009-06-01   223  10.1.03 K39 xc3s1000-4   111  439    0  256 s  9.8

-- 2009-06-01   221  10.1.03 K39 xc3s1000-4   111  449    0  258 s 13.3

--

-- Revision History: 

-- Date         Rev Version  Comment

-- 2010-10-17   333   0.8    use ibus V2 interface

-- 2009-06-07   224   0.7    send inverted stc_stp; remove pgc_err; honor msk_im

--                           also for dcf_dcf and exc_rte; add iist_mreq and

--                           iist_sreq, boot and lock interfaces

-- 2009-06-05   223   0.6    level interrupt, parity logic, exc.ui logic

--                           st logic modified (partially tested)

-- 2009-06-01   221   0.5    Initial version (untested, lock&boot missing)

------------------------------------------------------------------------------

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_arith.all;

use work.slvtypes.all;

use work.iblib.all;

use work.ibdlib.all;

-- ----------------------------------------------------------------------------

entity ibd_iist is                      -- ibus dev(loc): IIST

                                        -- fixed address: 177500

  generic (

    SID : slv2 := "00");                -- self id

  port (

    CLK : in slbit;                     -- clock

    CE_USEC : in slbit;                 -- usec pulse

    RESET : in slbit;                   -- system reset

    BRESET : in slbit;                  -- ibus reset

    IB_MREQ : in ib_mreq_type;          -- ibus request

    IB_SRES : out ib_sres_type;         -- ibus response

    EI_REQ : out slbit;                 -- interrupt request

    EI_ACK : in slbit;                  -- interrupt acknowledge

    IIST_BUS : in iist_bus_type;        -- iist bus (input from all iist's)

    IIST_OUT : out iist_line_type;      -- iist output

    IIST_MREQ : out iist_mreq_type;     -- iist->cpu requests

    IIST_SRES : in iist_sres_type       -- cpu->iist responses

  );

end ibd_iist;

architecture syn of ibd_iist is

  constant ibaddr_iist : slv16 := conv_std_logic_vector(8#177500#,16);

  constant tdlysnd : natural := 150;    -- send delay timer

  constant ibaddr_acr : slv1 := "0";    -- acr address offset

  constant ibaddr_adr : slv1 := "1";    -- adr address offset

  constant acr_ibf_clr :    integer := 15;                  -- clear flag

  subtype  acr_ibf_sid      is integer range  9 downto  8;  -- self id

  subtype  acr_ibf_ac       is integer range  3 downto  0;  -- ac code

  constant ac_pge : slv4 := "0000";     --  0 program generated enables

  constant ac_pgc : slv4 := "0001";     --  1 program generated control/status

  constant ac_ste : slv4 := "0010";     --  2 sanity timer enables

  constant ac_stc : slv4 := "0011";     --  3 sanity timer control/status

  constant ac_msk : slv4 := "0100";     --  4 input masks

  constant ac_pgf : slv4 := "0101";     --  5 program generated flags

  constant ac_stf : slv4 := "0110";     --  6 sanity timer flags

  constant ac_dcf : slv4 := "0111";     --  7 disconnect flags

  constant ac_exc : slv4 := "1000";     -- 10 exceptions

  constant ac_mtc : slv4 := "1101";     -- 15 maintenance control

  subtype  pge_ibf_pbe      is integer range 11 downto  8;  -- pg boot ena

  subtype  pge_ibf_pie      is integer range  3 downto  0;  -- pg int ena

  constant pgc_ibf_err :    integer := 15;                  -- error

  constant pgc_ibf_grj :    integer := 14;                  -- go reject

  constant pgc_ibf_pgrmr :  integer := 13;                  -- pg req refused

  constant pgc_ibf_strmr :  integer := 12;                  -- st req refused

  constant pgc_ibf_rdy :    integer := 11;                  -- ready flag

  subtype  pgc_ibf_sid      is integer range  9 downto  8;  -- self id

  constant pgc_ibf_ip :     integer :=  3;                  -- int pending

  constant pgc_ibf_ie :     integer :=  2;                  -- int enable

  constant pgc_ibf_ptp :    integer :=  1;                  -- pg parity

  constant pgc_ibf_go :     integer :=  0;                  -- go flag

  subtype  ste_ibf_sbe      is integer range 11 downto  8;  -- st boot enable

  subtype  ste_ibf_sie      is integer range  3 downto  0;  -- st int  enable

  subtype  stc_ibf_count    is integer range 15 downto  8;  -- count

  constant stc_ibf_tmo :    integer :=  3;                  -- timeout

  constant stc_ibf_lke :    integer :=  2;                  -- lockup enable

  constant stc_ibf_stp :    integer :=  1;                  -- st parity

  constant stc_ibf_enb :    integer :=  0;                  -- enable

  subtype  msk_ibf_bm       is integer range 11 downto  8;  -- boot mask

  subtype  msk_ibf_im       is integer range  3 downto  0;  -- int  mask

  subtype  pgf_ibf_pbf      is integer range 11 downto  8;  -- boot flags

  subtype  pgf_ibf_pif      is integer range  3 downto  0;  -- int  flags

  subtype  stf_ibf_sbf      is integer range 11 downto  8;  -- boot flags

  subtype  stf_ibf_sif      is integer range  3 downto  0;  -- int  flags

  subtype  dcf_ibf_brk      is integer range 11 downto  8;  -- break flags

  subtype  dcf_ibf_dcf      is integer range  3 downto  0;  -- disconnect flags

  subtype  exc_ibf_ui       is integer range 11 downto  8;  -- unexpected int

  subtype  exc_ibf_rte      is integer range  3 downto  0;  -- transm. error

  constant mtc_ibf_mttp :   integer := 11;                  -- maint. type

  constant mtc_ibf_mfrm :   integer := 10;                  -- maint. frame err

  subtype  mtc_ibf_mid      is integer range  9 downto  8;  -- maint. id

  constant mtc_ibf_dsbt :   integer :=  3;                  -- disable boot

  constant mtc_ibf_enmxd :  integer :=  2;                  -- enable maint mux

  constant mtc_ibf_enmlp :  integer :=  1;                  -- enable maint loop

  constant mtc_ibf_dsdrv :  integer :=  0;                  -- disable driver

  type state_type is (

    s_idle,                             -- idle state

    s_clear,                            -- handle acr clr

    s_stsnd,                            -- handle st transmit 

    s_pgsnd                             -- handle pg transmit

  );

  type regs_type is record              -- state registers

    ibsel : slbit;                      -- ibus select

    acr_ac : slv4;                      -- acr: ac 

    pge_pbe : slv4;                     -- pge: pg boot ena

    pge_pie : slv4;                     -- pge: pg int ena

    pgc_grj : slbit;                    -- pgc: go reject

    pgc_pgrmr : slbit;                  -- pgc: pg req refused

    pgc_strmr : slbit;                  -- pgc: st req refused

    pgc_ie  : slbit;                    -- pgc: int enable

    pgc_ptp : slbit;                    -- pgc: pg parity

    ste_sbe : slv4;                     -- ste: st boot enable

    ste_sie : slv4;                     -- ste: st int  enable

    stc_count : slv8;                   -- stc: count

    stc_tmo : slbit;                    -- stc: timeout

    stc_lke : slbit;                    -- stc: lockup enable

    stc_stp : slbit;                    -- stc: st parity

    stc_enb : slbit;                    -- stc: enable

    msk_bm  : slv4;                     -- msk: boot mask

    msk_im  : slv4;                     -- msk: int  mask

    pgf_pbf : slv4;                     -- pgf: boot flags

    pgf_pif : slv4;                     -- pgf: int  flags

    stf_sbf : slv4;                     -- stf: boot flags

    stf_sif : slv4;                     -- stf: int  flags

    dcf_brk : slv4;                     -- dcf: break flags

    dcf_dcf : slv4;                     -- dcf: disconnect flags

    exc_ui  : slv4;                     -- exc: unexpected int

    exc_rte : slv4;                     -- exc: transm. error

    mtc_mttp  : slbit;                  -- mtc: maint. type

    mtc_mfrm  : slbit;                  -- mtc: maint. frame err

    mtc_mid   : slv2;                   -- mtc: maint. id

    mtc_dsbt  : slbit;                  -- mtc: disable boot

    mtc_enmxd : slbit;                  -- mtc: enable maint mux

    mtc_enmlp : slbit;                  -- mtc: enable maint loop

    mtc_dsdrv : slbit;                  -- mtc: disable driver

    state : state_type;                 -- state

    req_clear : slbit;                  -- request clear

    req_stsnd : slbit;                  -- request sanity timer transmit

    req_pgsnd : slbit;                  -- request prog. gen.   transmit

    tcnt256 : slv8;                     -- usec clock divider for st clock

    tcntsnd : slv8;                     -- timer for transmit delay

    req_lock : slbit;                   -- cpu lock request

    req_boot : slbit;                   -- cpu boot request

   end record regs_type;

  constant regs_init : regs_type := (

    '0',                                -- ibsel

    "0000",                             -- acr_ac

    "0000","0000",                      -- pge_pbe, pge_pie

    '0',                                -- pgc_grj

    '0','0',                            -- pgc_pgrmr, pgc_strmr

    '0','0',                            -- pgc_ie, pgc_ptp

    "0000","0000",                      -- ste_sbe, ste_sie

    (others=>'0'),                      -- stc_count

    '0','0',                            -- stc_tmo, stc_lke

    '0','0',                            -- stc_stp, stc_enb

    "0000","0000",                      -- msk_bm, msk_im

    "0000","0000",                      -- pgf_pbf, pgf_pif

    "0000","0000",                      -- stf_sbf, stf_sif

    "0000","0000",                      -- dcf_brk, dcf_dcf

    "0000","0000",                      -- exc_ui, exc_rte

    '0','0',                            -- mtc_mttp, mtc_mfrm

    "00",                               -- mtc_mid

    '0','0',                            -- mtc_dsbt, mtc_enmxd

    '0','0',                            -- mtc_enmlp, mtc_dsdrv

    s_idle,                             -- state

    '0',                                -- req_clear

    '0','0',                            -- req_stsnd, req_pgsnd

    (others=>'0'),                      -- tcnt256

    (others=>'0'),                      -- tcntsnd

    '0','0'                             -- req_lock, req_boot

  );

  signal R_REGS : regs_type := regs_init;

  signal N_REGS : regs_type := regs_init;

begin

  proc_regs: process (CLK)

  begin

    if CLK'event and CLK='1' then

      if BRESET = '1' or                -- BRESET is 1 for system and ibus reset

         R_REGS.req_clear='1' then

        R_REGS <= regs_init;            --

        if RESET = '0' then               -- if RESET=0 we do just an ibus reset

          R_REGS.pgf_pbf <= N_REGS.pgf_pbf; -- don't reset pg boot flags

          R_REGS.stf_sbf <= N_REGS.stf_sbf; -- don't reset st boot flags

          R_REGS.tcnt256 <= N_REGS.tcnt256; -- don't reset st clock divider

        end if;

      else

        R_REGS <= N_REGS;

      end if;

    end if;

  end process proc_regs;

  proc_next : process (R_REGS, CE_USEC, IB_MREQ, EI_ACK, EI_ACK,

                       IIST_BUS(0), IIST_BUS(1), IIST_BUS(2), IIST_BUS(3),

                       IIST_SRES)

    variable r : regs_type := regs_init;

    variable n : regs_type := regs_init;

    variable ibhold : slbit := '0';

    variable idout : slv16 := (others=>'0');

    variable ibreq : slbit := '0';

    variable ibrd : slbit := '0';

    variable ibw0 : slbit := '0';

    variable ibw1 : slbit := '0';

    variable int_or : slbit := '0';

    variable tcnt256_end : slbit := '0';

    variable tcntsnd_end : slbit := '0';

    variable eff_id  : slv2 := "00";

    variable eff_bus : iist_bus_type := iist_bus_init;

    variable par_err : slbit := '0';

    variable act_ibit : slbit := '0';

    variable act_bbit : slbit := '0';

    variable iout : iist_line_type := iist_line_init;

  begin

    r := R_REGS;

    n := R_REGS;

    ibhold := '0';

    idout  := (others=>'0');

    ibreq  := IB_MREQ.re or IB_MREQ.we;

    ibrd   := IB_MREQ.re;

    ibw0   := IB_MREQ.we and IB_MREQ.be0;

    ibw1   := IB_MREQ.we and IB_MREQ.be1;

    int_or := r.pgc_grj or r.pgc_pgrmr or r.pgc_strmr;

    for i in r.dcf_dcf'range loop

      int_or := int_or or r.dcf_dcf(i) or

                          r.exc_rte(i) or

                          r.pgf_pif(i) or

                          r.stf_sif(i);

    end loop;  -- i

    tcnt256_end := '0';

    if CE_USEC='1' and r.stc_enb='1'then   -- if st enabled on every usec

      n.tcnt256 := unsigned(r.tcnt256) + 1;  -- advance 8 bit counter

      if unsigned(r.tcnt256) = 255 then      -- if wrap

        tcnt256_end := '1';                  -- signal 256 usec passed

      end if;

    end if;

    tcntsnd_end := '0';

    n.tcntsnd := unsigned(r.tcntsnd) + 1;  -- advance send timer counter

    if unsigned(r.tcntsnd) = tdlysnd-1 then -- if delay time reached 

      tcntsnd_end := '1';                    -- signal end

    end if;

    eff_id := SID;                      -- effective self-id, normally SID

    if r.mtc_enmxd = '1' then           -- if maint. mux enabled

      eff_id := r.mtc_mid;                -- use maint. id

    end if;

    eff_bus  := IIST_BUS;

    par_err  := '0';

    act_ibit := '0';

    act_bbit := '0';

    iout     := iist_line_init;            -- default state of out line

    -- ibus address decoder

    n.ibsel := '0';

    if IB_MREQ.aval='1' and

       IB_MREQ.addr(12 downto 2)=ibaddr_iist(12 downto 2) then

      n.ibsel := '1';

    end if;

    -- internal state machine

    case r.state is

      when s_idle =>                    -- idle state

        n.tcntsnd := (others=>'0');       -- keep send delay timer zero

        if r.req_stsnd = '1' then         -- sanity timer request pending

          n.state := s_stsnd;

        elsif r.req_pgsnd = '1' then      -- prog. gen. request pending

          n.state := s_pgsnd;

        end if;

      when s_clear =>                   -- handle acr clr

        ibhold := r.ibsel;              -- keep req pending if selected

        -- r.req_clear is set when in this state and cause a reset in prog_regs

        --   --> n.req_clear := '0';

        --   --> n.state := s_idle;

      when s_stsnd =>                   -- handle st transmit

        if tcntsnd_end = '1' then         -- send delay expired

          n.req_stsnd := '0';               -- clear st transmit request

          iout.req   := '1';                -- do transmit

          iout.stf   := '1';                -- signal type = st

          iout.imask := r.ste_sie;          -- int  enables

          iout.bmask := r.ste_sbe;          -- boot enables

          iout.par   := not r.stc_stp;      -- send parity (odd incl. stf!)

          iout.frm   := '0';                -- frame always ok

          n.state   := s_idle;

        end if;

      when s_pgsnd =>                   -- handle pg transmit

        if tcntsnd_end = '1' then         -- send delay expired

          n.req_pgsnd := '0';               -- clear pg transmit request

          iout.req   := '1';                -- do transmit

          iout.stf   := '0';                -- signal type = pg

          iout.imask := r.pge_pie;          -- int  enables

          iout.bmask := r.pge_pbe;          -- boot enables

          iout.par   := r.pgc_ptp;          -- send parity

          iout.frm   := '0';                -- frame always ok

          n.state := s_idle;

        end if;

      when others => null;

    end case;

    if r.mtc_enmxd = '1' then              -- if maintenance mux enabled

      iout.stf := r.mtc_mttp;                -- force type  from mtc_mttp

      iout.frm := r.mtc_mfrm;                -- force frame from mtc_mfrm

    end if;

    -- ibus transactions

    if r.ibsel = '1' and ibhold='0' then

      if IB_MREQ.addr(1 downto 1) = "0" then -- ACR -- access control reg -----

        idout(acr_ibf_sid) := SID;

        idout(acr_ibf_ac)  := r.acr_ac;

        if ibw1 = '1' then

          if IB_MREQ.din(acr_ibf_clr) = '1' then

            n.req_clear := '1';

            n.state     := s_clear;

          end if;

        end if;

        if ibw0 = '1' then

          n.acr_ac  := IB_MREQ.din(acr_ibf_ac);

        end if;

      else                                  -- ADR -- access data reg --------

        case r.acr_ac is

          when ac_pge =>                -- PGE -- program gen enables --------

            idout(pge_ibf_pbe) := r.pge_pbe;

            idout(pge_ibf_pie) := r.pge_pie;

            if IB_MREQ.we = '1' then

              if r.req_pgsnd = '0' then   -- no pg transmit pending

                if ibw1 = '1' then

                  n.pge_pbe := IB_MREQ.din(pge_ibf_pbe);

                end if;

                if ibw0 = '1' then

                  n.pge_pie := IB_MREQ.din(pge_ibf_pie);

                end if;

              else                        -- if collision with pg transmit

                n.pgc_pgrmr := '1';         -- set pge refused flag

              end if;

            end if;

          when ac_pgc =>                -- PGC -- program gen control/status -

            idout(pgc_ibf_err)   := r.pgc_grj or r.pgc_pgrmr or r.pgc_strmr;

            idout(pgc_ibf_grj)   := r.pgc_grj;

            idout(pgc_ibf_pgrmr) := r.pgc_pgrmr;

            idout(pgc_ibf_strmr) := r.pgc_strmr;

            idout(pgc_ibf_rdy)   := not r.req_pgsnd;

            idout(pgc_ibf_sid)   := eff_id;

            idout(pgc_ibf_ip)    := int_or;

            idout(pgc_ibf_ie)    := r.pgc_ie;

            idout(pgc_ibf_ptp)   := r.pgc_ptp;

            if ibw1 = '1' then

              if IB_MREQ.din(pgc_ibf_err) = '1' then -- '1' written into ERR

                n.pgc_grj   := '0';                  -- clears GRJ

                n.pgc_pgrmr := '0';                  -- clears PGRMR

                n.pgc_strmr := '0';                  -- clears STRMR

              end if;

            end if;

            if ibw0 = '1' then

              n.pgc_ie  := IB_MREQ.din(pgc_ibf_ie);

              n.pgc_ptp := IB_MREQ.din(pgc_ibf_ptp);

              if IB_MREQ.din(pgc_ibf_go) = '1' then -- GO bit set

                if r.req_pgsnd = '0' then           -- if ready (no pgsnd pend)

                  n.req_pgsnd := '1';                 -- request pgsnd

                else                                -- if not ready

                  n.pgc_grj := '1';                   -- set go reject flag

                end if;

              end if;

            end if;

          when ac_ste =>                -- STE -- sanity timer enables -------

            idout(ste_ibf_sbe)   := r.ste_sbe;

            idout(ste_ibf_sie)   := r.ste_sie;

            if IB_MREQ.we = '1' then

              if r.req_stsnd = '0' then   -- no st transmit pending

                if ibw1 = '1' then

                  n.ste_sbe := IB_MREQ.din(ste_ibf_sbe);

                end if;

                if ibw0 = '1' then

                  n.ste_sie := IB_MREQ.din(ste_ibf_sie);

                end if;

              else                        -- if collision with st transmit

                n.pgc_strmr := '1';         -- set ste refused flag

              end if;

            end if;

          when ac_stc =>                -- STC -- sanity timer control/status

            idout(stc_ibf_count) := r.stc_count;

            idout(stc_ibf_tmo)   := r.stc_tmo;

            idout(stc_ibf_lke)   := r.stc_lke;

            idout(stc_ibf_stp)   := r.stc_stp;

            idout(stc_ibf_enb)   := r.stc_enb;

            if ibw1 = '1' then

              n.stc_count := IB_MREQ.din(stc_ibf_count);    -- reset st count

              n.tcnt256   := (others=>'0');                 -- reset usec count

            end if;

            if ibw0 = '1' then

              if IB_MREQ.din(stc_ibf_tmo) = '1' then -- 1 written into TMO

                n.stc_tmo := '0';

              end if;

              n.stc_lke   := IB_MREQ.din(stc_ibf_lke);

              n.stc_stp   := IB_MREQ.din(stc_ibf_stp);

              n.stc_enb   := IB_MREQ.din(stc_ibf_enb);

            end if;

          when ac_msk =>                -- MSK -- input masks ----------------

            idout(msk_ibf_bm)    := r.msk_bm;

            idout(msk_ibf_im)    := r.msk_im;

            if ibw1 = '1' then

              n.msk_bm  := IB_MREQ.din(msk_ibf_bm);

            end if;

            if ibw0 = '1' then

              n.msk_im  := IB_MREQ.din(msk_ibf_im);

            end if;

          when ac_pgf =>                -- PGF -- program generated flags ----

            idout(pgf_ibf_pbf)   := r.pgf_pbf;

            idout(pgf_ibf_pif)   := r.pgf_pif;

            if ibw1 = '1' then

              n.pgf_pbf := r.pgf_pbf and not IB_MREQ.din(pgf_ibf_pbf);

            end if;

            if ibw0 = '1' then

              n.pgf_pif := r.pgf_pif and not IB_MREQ.din(pgf_ibf_pif);

            end if;

          when ac_stf =>                -- STF -- sanity timer flags ---------

            idout(stf_ibf_sbf)   := r.stf_sbf;

            idout(stf_ibf_sif)   := r.stf_sif;

            if ibw1 = '1' then

              n.stf_sbf := r.stf_sbf and not IB_MREQ.din(stf_ibf_sbf);

            end if;

            if ibw0 = '1' then

              n.stf_sif := r.stf_sif and not IB_MREQ.din(stf_ibf_sif);

            end if;

          when ac_dcf =>                -- DCE -- disconnect flags -----------

            idout(dcf_ibf_brk)   := r.dcf_brk;

            idout(dcf_ibf_dcf)   := r.dcf_dcf;

            if ibw0 = '1' then

              n.dcf_dcf := r.dcf_dcf and not IB_MREQ.din(dcf_ibf_dcf);

            end if;

          when ac_exc =>                -- EXC -- exceptions -----------------

            idout(exc_ibf_ui)    := r.exc_ui;

            idout(exc_ibf_rte)   := r.exc_rte;

            if ibw1 = '1' then

              n.exc_ui  := r.exc_ui  and not IB_MREQ.din(exc_ibf_ui);

            end if;

            if ibw0 = '1' then

              n.exc_rte := r.exc_rte and not IB_MREQ.din(exc_ibf_rte);

            end if;

          when ac_mtc =>                -- MTC -- maintenance control --------

            idout(mtc_ibf_mttp)  := r.mtc_mttp;

            idout(mtc_ibf_mfrm)  := r.mtc_mfrm;

            idout(mtc_ibf_mid)   := r.mtc_mid;

            idout(mtc_ibf_dsbt)  := r.mtc_dsbt;

            idout(mtc_ibf_enmxd) := r.mtc_enmxd;

            idout(mtc_ibf_enmlp) := r.mtc_enmlp;

            idout(mtc_ibf_dsdrv) := r.mtc_dsdrv;

            if ibw1 = '1' then

              n.mtc_mttp  := IB_MREQ.din(mtc_ibf_mttp);

              n.mtc_mfrm  := IB_MREQ.din(mtc_ibf_mfrm);

              n.mtc_mid   := IB_MREQ.din(mtc_ibf_mid);

            end if;

            if ibw0 = '1' then

              n.mtc_dsbt  := IB_MREQ.din(mtc_ibf_dsbt);

              n.mtc_enmxd := IB_MREQ.din(mtc_ibf_enmxd);

              n.mtc_enmlp := IB_MREQ.din(mtc_ibf_enmlp);

              n.mtc_dsdrv := IB_MREQ.din(mtc_ibf_dsdrv);

            end if;

          when others =>                -- access to undefined AC code -------

            null;

        end case;

        if unsigned(r.acr_ac) <= unsigned(ac_exc) then -- if ac 0,..,10

          if IB_MREQ.rmw = '0' then                    -- if not 1st part of rmw

            n.acr_ac := unsigned(r.acr_ac) + 1;          -- autoincrement

          end if;

        end if;

      end if;

    end if;

    -- sanity timer

    if tcnt256_end = '1' then           -- if 256 usec expired (and enabled)

      n.stc_count := unsigned(r.stc_count) - 1;

      if unsigned(r.stc_count) = 0 then   -- if sanity timer expired

        n.stc_tmo := '1';                   -- set timeout flag

        n.req_stsnd := '1';                 -- request st transmit

        if r.stc_lke = '1' then             -- if lockup enabled

          n.req_lock := '1';                  -- request lockup

        end if;

      end if;

    end if;

    -- process iist bus inputs

    if r.mtc_enmlp = '1' then           -- if mainentance loop

      for i in eff_bus'range loop

        eff_bus(i) := iout;               -- local signal on all input ports

        eff_bus(i).dcf := '0';            -- all ports considered connected

      end loop;  -- i

    end if;

    for i in eff_bus'range loop

      par_err := eff_bus(i).stf xor

                 eff_bus(i).imask(0) xor eff_bus(i).imask(1) xor

                 eff_bus(i).imask(2) xor eff_bus(i).imask(3) xor

                 eff_bus(i).bmask(0) xor eff_bus(i).bmask(1) xor

                 eff_bus(i).bmask(2) xor eff_bus(i).bmask(3) xor

                 not eff_bus(i).par;

      act_ibit := eff_bus(i).imask(conv_integer(unsigned(eff_id)));

      act_bbit := eff_bus(i).bmask(conv_integer(unsigned(eff_id)));

      n.dcf_brk(i) := eff_bus(i).dcf;     -- trace dcf state in brk

      if eff_bus(i).dcf = '1' then        -- if disconnected

        if r.msk_im(i) = '0' then           -- if not disabled

          n.dcf_dcf(i) := '1';                -- set dcf flag

        end if;

      else                                -- if connected

        if eff_bus(i).req = '1' then        -- request received ?

          if eff_bus(i).frm='1' or            -- frame error seen ?

             par_err='1' then                 -- parity error seen ?

            if r.msk_im(i) = '0' then           -- if not disabled

              n.exc_rte(i) := '1';                -- set rte flag

            end if;

          else                                -- here if valid request seen

            if act_ibit = '1' then              -- interrupt request

              if r.msk_im(i) = '1' then           -- if disabled

                n.exc_ui(i) := '1';                 -- set ui flag

              else                                -- if enabled

                n.req_lock := '0';                  -- release lock

                if eff_bus(i).stf = '0' then        -- and pg request

                  n.pgf_pif(i) := '1';                -- set pif flag

                else                                -- and st request

                  n.stf_sif(i) := '1';                -- set sif flag

                end if;

              end if;