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--##############################################################################
-- light8080 : Intel 8080 binary compatible core
--##############################################################################
-- v1.1    (20 sep 2008) Microcode bug in INR fixed.
-- v1.0    (05 nov 2007) First release. Jose A. Ruiz.
--
-- This file and all the light8080 project files are freeware (See COPYING.TXT)
--##############################################################################
-- (See timing diagrams at bottom of file. More comprehensive explainations can 
-- be found in the design notes)
--##############################################################################
 
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
 
--##############################################################################
-- vma :      enable a memory or io r/w access.
-- io :       access in progress is io (and not memory) 
-- rd :       read memory or io 
-- wr :       write memory or io
-- data_out : data output
-- addr_out : memory and io address
-- data_in :  data input
-- halt :     halt status (1 when in halt state)
-- inte :     interrupt status (1 when enabled)
-- intr :     interrupt request
-- inta :     interrupt acknowledge
-- reset :    synchronous reset
-- clk :      clock
--
-- (see timing diagrams at bottom of file)
--##############################################################################
entity light8080 is
    Port (  
            addr_out :  out std_logic_vector(15 downto 0);
 
            inta :      out std_logic;
            inte :      out std_logic;
            halt :      out std_logic;                
            intr :      in std_logic;
 
            vma :       out std_logic;
            io :        out std_logic;
            rd :        out std_logic;
            wr :        out std_logic;
            fetch :     out std_logic;
            data_in :   in std_logic_vector(7 downto 0);  
            data_out :  out std_logic_vector(7 downto 0);
 
            clk :       in std_logic;
            reset :     in std_logic );
end light8080;
 
--##############################################################################
-- All memory and io accesses are synchronous (rising clock edge). Signal vma 
-- works as the master memory and io synchronous enable. More specifically:
--
--    * All memory/io control signals (io,rd,wr) are valid only when vma is 
--      high. They never activate when vms is inactive. 
--    * Signals data_out and address are only valid when vma='1'. The high 
--      address byte is 0x00 for all io accesses.
--    * Signal data_in should be valid by the end of the cycle after vma='1', 
--      data is clocked in by the rising clock edge.
--
-- All signals are assumed to be synchronous to the master clock. Prevention of
-- metastability, if necessary, is up to you.
-- 
-- Signal reset needs to be active for just 1 clock cycle (it is sampled on a 
-- positive clock edge and is subject to setup and hold times).
-- Once reset is deasserted, the first fetch at address 0x0000 will happen 4
-- cycles later.
--
-- Signal intr is sampled on all positive clock edges. If asserted when inte is
-- high, interrupts will be disabled, inta will be asserted high and a fetch 
-- cycle will occur. The fetched instruction will be executed normally, except 
-- PC will not be valid in any subsequent fetch cycles of the same instruction, 
-- and will not be incremented (In practice, the same as the original 8080).
-- inta will remain high for the duration of the fetched instruction (in the 
-- original 8080 it was high only for the opcode fetch cycle). 
-- PC will not be autoincremented while inta is high, but it can be explicitly 
-- modified (e.g. RTS, CALL, etc.). Again, the same as the original.
-- Interrupts will be disabled upon assertion of inta, and remain disabled 
-- until explicitly enabled by the program (as in the original).
--
-- The above means that any instruction can be supplied in an inta cycle, 
-- either single byte or multibyte. See the design notes.
--##############################################################################
 
architecture microcoded of light8080 is
 
-- addr_low: low byte of address
signal addr_low :     std_logic_vector(7 downto 0);
-- IR: instruction register. some bits left unused.  
signal IR :           std_logic_vector(7 downto 0);
-- s_field: IR field, sss source reg code
signal s_field :      std_logic_vector(2 downto 0);
-- d_field: IR field, ddd destination reg code
signal d_field :      std_logic_vector(2 downto 0);
-- p_field: IR field, pp 16-bit reg pair code
signal p_field :      std_logic_vector(1 downto 0);
-- rbh: 1 when p_field=11, used in reg bank addressing for 'special' regs
signal rbh :          std_logic; -- 1 when P=11 (special case)  
-- alu_op: uinst field, ALU operation code 
signal alu_op :       std_logic_vector(3 downto 0);
-- DI: data input to ALU block from data_in, unregistered
signal DI :           std_logic_vector(7 downto 0);
-- uc_addr: microcode (ucode) address 
signal uc_addr :      std_logic_vector(7 downto 0);
-- next_uc_addr: computed next microcode address (uaddr++/jump/ret/fetch)
signal next_uc_addr : std_logic_vector(8 downto 0);
-- uc_jmp_addr: uinst field, absolute ucode jump address
signal uc_jmp_addr :  std_logic_vector(7 downto 0);
-- uc_ret_address: ucode return address saved in previous jump
signal uc_ret_addr :  std_logic_vector(7 downto 0);  
-- addr_plus_1: uaddr + 1
signal addr_plus_1 :  std_logic_vector(7 downto 0);  
-- do_reset: reset, delayed 1 cycle -- used to reset the microcode sequencer
signal do_reset :     std_logic;  
 
-- uc_flags1: uinst field, encoded flag of group 1 (see ucode file)
signal uc_flags1 :    std_logic_vector(2 downto 0);
-- uc_flags2: uinst field, encoded flag of group 2 (see ucode file)
signal uc_flags2 :    std_logic_vector(2 downto 0);  
-- uc_addr_sel: selection of next uc_addr, composition of 4 flags
signal uc_addr_sel :  std_logic_vector(3 downto 0);
-- NOTE: see microcode file for information on flags
signal uc_jsr :       std_logic;  -- uinst field, decoded 'jsr' flag
signal uc_tjsr :      std_logic;  -- uinst field, decoded 'tjsr' flag
signal uc_decode :    std_logic;  -- uinst field, decoded 'decode' flag
signal uc_end :       std_logic;  -- uinst field, decoded 'end' flag
signal condition_reg :std_logic;  -- registered tjst condition
-- condition: tjsr condition (computed ccc condition from '80 instructions)
signal condition :    std_logic;
-- condition_sel: IR field, ccc condition code
signal condition_sel :std_logic_vector(2 downto 0);
signal uc_do_jmp :    std_logic;  -- uinst jump (jsr/tjsr) flag, pipelined
signal uc_do_ret :    std_logic;  -- ret flag, pipelined
signal uc_halt_flag : std_logic;  -- uinst field, decoded 'halt' flag
signal uc_halt :      std_logic;  -- halt command
signal halt_reg :     std_logic;  -- halt status reg, output as 'halt' signal
signal uc_ei :        std_logic;  -- uinst field, decoded 'ei' flag
signal uc_di :        std_logic;  -- uinst field, decoded 'ei' flag
signal inte_reg :     std_logic;  -- inte status reg, output as 'inte' signal
signal int_pending :  std_logic;  -- intr requested, inta not active yet
signal inta_reg :     std_logic;  -- inta status reg, output as 'inta'
signal clr_t1 :       std_logic;  -- uinst field, explicitly erase T1
signal do_clr_t1 :    std_logic;  -- clr_t1 pipelined
signal clr_t2 :       std_logic;  -- uinst field, explicitly erase T2
signal do_clr_t2 :    std_logic;  -- clr_t2 pipelined
signal ucode :        std_logic_vector(31 downto 0); -- microcode word
signal ucode_field2 : std_logic_vector(24 downto 0); -- pipelined microcode
 
-- microcode ROM : see design notes and microcode source file 
type t_rom is array (0 to 511) of std_logic_vector(31 downto 0);
 
signal rom : t_rom := (
"00000000000000000000000000000000", -- 000
"00000000000001001000000001000100", -- 001
"00000000000001000000000001000100", -- 002
"10111101101001001000000001001101", -- 003
"10110110101001000000000001001101", -- 004
"00100000000000000000000000000000", -- 005
"00000000000000000000000000000000", -- 006
"11100100000000000000000000000000", -- 007
"00000000101010000000000000000000", -- 008
"00000100000100000000000001010111", -- 009
"00001000000000000000110000011001", -- 00a
"00000100000100000000000001010111", -- 00b
"00000000101010000000000010010111", -- 00c
"00001000000000000000110000011100", -- 00d
"00001000000000000000110000011111", -- 00e
"00000100000100000000000001010111", -- 00f
"00001000000000000000110000011111", -- 010
"00001000000000000000110000011100", -- 011
"00001000000000000000110000011111", -- 012
"00000000000110001000000001010111", -- 013
"00001000000000000000110000011111", -- 014
"00000100000110000000000001010111", -- 015
"00001000000000000000110000101110", -- 016
"00001000000000000000110000100010", -- 017
"00000100000000111000000001010111", -- 018
"00001000000000000000110000101110", -- 019
"00000000101000111000000010010111", -- 01a
"00001000000000000000110000100101", -- 01b
"00001000000000000000110000101110", -- 01c
"10111101101001100000000001001101", -- 01d
"10110110101001101000000001001101", -- 01e
"00000000100000101000000001010111", -- 01f
"00001000000000000000110000100010", -- 020
"00000100000000100000000001010111", -- 021
"00001000000000000000110000101110", -- 022
"00000000101000101000000010010111", -- 023
"10111101101001100000000001001101", -- 024
"10111010101001101000000001001101", -- 025
"00000000101000100000000010010111", -- 026
"00001000000000000000110000100101", -- 027
"00001000000000000000110000101000", -- 028
"00000100000000111000000001010111", -- 029
"00000000101000111000000010010111", -- 02a
"00001000000000000000110000101011", -- 02b
"00000000101000010000000000000000", -- 02c
"00000000000001010000000001010111", -- 02d
"00000000101000011000000000000000", -- 02e
"00000000000001011000000001010111", -- 02f
"00000000101000100000000000000000", -- 030
"00000000000000010000000001010111", -- 031
"00000000101000101000000000000000", -- 032
"00000000000000011000000001010111", -- 033
"00000000101001010000000000000000", -- 034
"00000000000000100000000001010111", -- 035
"00000000101001011000000000000000", -- 036
"00000100000000101000000001010111", -- 037
"00001000000000000000110000011111", -- 038
"00000100011000111000001101001100", -- 039
"00001000000000000000110000011111", -- 03a
"00000100011000111000001101001101", -- 03b
"00001000000000000000110000011111", -- 03c
"00000100011000111000001101001110", -- 03d
"00001000000000000000110000011111", -- 03e
"00000100011000111000001101001111", -- 03f
"00001000000000000000110000011111", -- 040
"00000100011000111000001101000100", -- 041
"00001000000000000000110000011111", -- 042
"00000100011000111000001101000101", -- 043
"00001000000000000000110000011111", -- 044
"00000100011000111000001101000110", -- 045
"00001000000000000000110000011111", -- 046
"00000100011000111000001110001110", -- 047
"00000000101010000000000000000000", -- 048
"00000100011000111000001101001100", -- 049
"00000000101010000000000000000000", -- 04a
"00000100011000111000001101001101", -- 04b
"00000000101010000000000000000000", -- 04c
"00000100011000111000001101001110", -- 04d
"00000000101010000000000000000000", -- 04e
"00000100011000111000001101001111", -- 04f
"00000000101010000000000000000000", -- 050
"00000100011000111000001101000100", -- 051
"00000000101010000000000000000000", -- 052
"00000100011000111000001101000101", -- 053
"00000000101010000000000000000000", -- 054
"00000100011000111000001101000110", -- 055
"00000000101010000000000000000000", -- 056
"00000100011000111000001110001110", -- 057
"00001000000000000000110000011001", -- 058
"00000100011000111000001101001100", -- 059
"00001000000000000000110000011001", -- 05a
"00000100011000111000001101001101", -- 05b
"00001000000000000000110000011001", -- 05c
"00000100011000111000001101001110", -- 05d
"00001000000000000000110000011001", -- 05e
"00000100011000111000001101001111", -- 05f
"00001000000000000000110000011001", -- 060
"00000100011000111000001101000100", -- 061
"00001000000000000000110000011001", -- 062
"00000100011000111000001101000101", -- 063
"00001000000000000000110000011001", -- 064
"00000100011000111000001101000110", -- 065
"00001000000000000000110000011001", -- 066
"00000100011000111000001110001110", -- 067
"10111100101100000000001001001101", -- 068
"00000100000000000000000000000000", -- 069
"00001000000000000000110000011001", -- 06a
"10111100000000000000001010001101", -- 06b
"00001000000000000000110000011100", -- 06c
"10111100011100000000001001001111", -- 06d
"00000100000000000000000000000000", -- 06e
"00001000000000000000110000011001", -- 06f
"11000000000000000000000000000000", -- 070
"10111100011001010000001010001111", -- 071
"00001000000000000000110000011100", -- 072
"10111100101110001000000001001101", -- 073
"10100100101110000000000001001101", -- 074
"10111100011110001000000001001111", -- 075
"10100100011110000000000001001111", -- 076
"00000000011110001000000000000000", -- 077
"00000000101000101000000101001100", -- 078
"00000000011110000000000000000000", -- 079
"00000100101000100000000101001101", -- 07a
"00000000101000111000000010101000", -- 07b
"00000100101000111000001101101000", -- 07c
"00000100101000111000000101000000", -- 07d
"00000100101000111000000101000001", -- 07e
"00000100101000111000000101000010", -- 07f
"00000100101000111000000101000011", -- 080
"00000100101000111000000001000111", -- 081
"00000100000000000000000100101100", -- 082
"00000100000000000000000100101101", -- 083
"00001000000000000000110000101110", -- 084
"00000000101001100000000000000000", -- 085
"00000000000001001000000001010111", -- 086
"00000000101001101000000000000000", -- 087
"00000100000001000000000001010111", -- 088
"00000100000000000000000000000000", -- 089
"00001000000000000000110000101110", -- 08a
"00010000000000000000100000000101", -- 08b
"00001000000000000000110000101110", -- 08c
"11000000101001000000000010010111", -- 08d
"00001000000000000000110000110100", -- 08e
"11000000101001001000000010010111", -- 08f
"00001000000000000000110000110100", -- 090
"00000000101001100000000000000000", -- 091
"00000000000001001000000001010111", -- 092
"00000000101001101000000000000000", -- 093
"00000100000001000000000001010111", -- 094
"00001000000000000000110000101110", -- 095
"00010000000000000000100000001101", -- 096
"00001000000000000000110000111001", -- 097
"00000000000001001000000001010111", -- 098
"00001000000000000000110000111001", -- 099
"00000100000001000000000001010111", -- 09a
"00010000000000000000100000010111", -- 09b
"11000000101001000000000010010111", -- 09c
"00001000000000000000110000110100", -- 09d
"11000000101001001000000010010111", -- 09e
"00001000000000000000110000110100", -- 09f
"11000000000001001000000001011111", -- 0a0
"00000100000001000000000001000100", -- 0a1
"00000000101000101000000000000000", -- 0a2
"00000000000001001000000001010111", -- 0a3
"00000000101000100000000000000000", -- 0a4
"00000100000001000000000001010111", -- 0a5
"11000000101110000000000010010111", -- 0a6
"00001000000000000000110000110100", -- 0a7
"11000000101110001000000010010111", -- 0a8
"00001000000000000000110000110100", -- 0a9
"00000100000000000000000000000000", -- 0aa
"11000000101000111000000010010111", -- 0ab
"00001000000000000000110000110100", -- 0ac
"11000000000000000000000010110000", -- 0ad
"00001000000000000000110000110100", -- 0ae
"00000100000000000000000000000000", -- 0af
"00001000000000000000110000111001", -- 0b0
"00000000000110001000000001010111", -- 0b1
"00001000000000000000110000111001", -- 0b2
"00000100000110000000000001010111", -- 0b3
"00001000000000000000110000111001", -- 0b4
"00000000000000110000001101010111", -- 0b5
"00001000000000000000110000111001", -- 0b6
"00000100000000111000000001010111", -- 0b7
"00001000000000000000110000111001", -- 0b8
"00000000000001100000000001010111", -- 0b9
"00001000000000000000110000111001", -- 0ba
"00000000000001101000000001010111", -- 0bb
"11000000101000100000000010010111", -- 0bc
"00001000000000000000110000110100", -- 0bd
"11000000101000101000000010010111", -- 0be
"00001000000000000000110000110100", -- 0bf
"00000000101001100000000000000000", -- 0c0
"00000000000000101000000001010111", -- 0c1
"00000000101001101000000000000000", -- 0c2
"00000100000000100000000001010111", -- 0c3
"00000000101000101000000000000000", -- 0c4
"00000000000001111000000001010111", -- 0c5
"00000000101000100000000000000000", -- 0c6
"00000100000001110000000001010111", -- 0c7
"01100100000000000000000000000000", -- 0c8
"01000100000000000000000000000000", -- 0c9
"00000000000001101000000001010111", -- 0ca
"00001000000000000000110000011111", -- 0cb
"00000000000001100000000001010111", -- 0cc
"00000000000000000000000000000000", -- 0cd
"00000001101001100000000000000000", -- 0ce
"10010110101001101000000000000000", -- 0cf
"00000100100000111000000001010111", -- 0d0
"00000000000001101000000001010111", -- 0d1
"00001000000000000000110000011111", -- 0d2
"00000000000001100000000001010111", -- 0d3
"00000000101000111000000010010111", -- 0d4
"00000001101001100000000000000000", -- 0d5
"10011010101001101000000000000000", -- 0d6
"00000100000000000000000000000000", -- 0d7
"11100100000000000000000000000000", -- 0d8
"00000001101000101000000000000000", -- 0d9
"00010110101000100000000000000000", -- 0da
"00001100100001010000000001010111", -- 0db
"00000001101000101000000000000000", -- 0dc
"00011010101000100000000000000000", -- 0dd
"00000100000000000000000000000000", -- 0de
"10111101101001001000000001001101", -- 0df
"10110110101001000000000001001101", -- 0e0
"00001100100000000000000010010111", -- 0e1
"00000001101001100000000000000000", -- 0e2
"00010110101001101000000000000000", -- 0e3
"00001100100000000000000000000000", -- 0e4
"00000001101001100000000000000000", -- 0e5
"00011010101001101000000000000000", -- 0e6
"00000100000000000000000000000000", -- 0e7
"00000001101110001000000000000000", -- 0e8
"00010110101110000000000000000000", -- 0e9
"00001100100000000000000000000000", -- 0ea
"00000001101110001000000000000000", -- 0eb
"00011010101110000000000000000000", -- 0ec
"00000100000000000000000000000000", -- 0ed
"10111101101001001000000001001101", -- 0ee
"10110110101001000000000001001101", -- 0ef
"00000000100001100000000001010111", -- 0f0
"10111101101001001000000001001101", -- 0f1
"10110110101001000000000001001101", -- 0f2
"00001100100001101000000001010111", -- 0f3
"10111100011001111000000001001111", -- 0f4
"10100000011001110000000001001111", -- 0f5
"00000001101001111000000000000000", -- 0f6
"00011010101001110000000000000000", -- 0f7
"00001100000000000000000000000000", -- 0f8
"10111101101001111000000001001101", -- 0f9
"10110110101001110000000001001101", -- 0fa
"00001100100000000000000000000000", -- 0fb
"00000100000000000000000000000000", -- 0fc
"00000100000000000000000000000000", -- 0fd
"00000100000000000000000000000000", -- 0fe
"00000100000000000000000000000000", -- 0ff
"00001000000000000000100000001001", -- 100
"00001000000000000000000000010010", -- 101
"00001000000000000000000000101010", -- 102
"00001000000000000000010000110011", -- 103
"00001000000000000000010000101000", -- 104
"00001000000000000000010000101101", -- 105
"00001000000000000000000000001110", -- 106
"00001000000000000000010000111101", -- 107
"00001000000000000000000000000000", -- 108
"00001000000000000000010000110111", -- 109
"00001000000000000000000000101000", -- 10a
"00001000000000000000010000110101", -- 10b
"00001000000000000000010000101000", -- 10c
"00001000000000000000010000101101", -- 10d
"00001000000000000000000000001110", -- 10e
"00001000000000000000010000111110", -- 10f
"00001000000000000000000000000000", -- 110
"00001000000000000000000000010010", -- 111
"00001000000000000000000000101010", -- 112
"00001000000000000000010000110011", -- 113
"00001000000000000000010000101000", -- 114
"00001000000000000000010000101101", -- 115
"00001000000000000000000000001110", -- 116
"00001000000000000000010000111111", -- 117
"00001000000000000000000000000000", -- 118
"00001000000000000000010000110111", -- 119
"00001000000000000000000000101000", -- 11a
"00001000000000000000010000110101", -- 11b
"00001000000000000000010000101000", -- 11c
"00001000000000000000010000101101", -- 11d
"00001000000000000000000000001110", -- 11e
"00001000000000000000100000000000", -- 11f
"00001000000000000000000000000000", -- 120
"00001000000000000000000000010010", -- 121
"00001000000000000000000000100010", -- 122
"00001000000000000000010000110011", -- 123
"00001000000000000000010000101000", -- 124
"00001000000000000000010000101101", -- 125
"00001000000000000000000000001110", -- 126
"00001000000000000000010000111011", -- 127
"00001000000000000000000000000000", -- 128
"00001000000000000000010000110111", -- 129
"00001000000000000000000000011100", -- 12a
"00001000000000000000010000110101", -- 12b
"00001000000000000000010000101000", -- 12c
"00001000000000000000010000101101", -- 12d
"00001000000000000000000000001110", -- 12e
"00001000000000000000100000000001", -- 12f
"00001000000000000000000000000000", -- 130
"00001000000000000000000000010010", -- 131
"00001000000000000000000000011001", -- 132
"00001000000000000000010000110011", -- 133
"00001000000000000000010000101010", -- 134
"00001000000000000000010000101111", -- 135
"00001000000000000000000000010000", -- 136
"00001000000000000000100000000011", -- 137
"00001000000000000000000000000000", -- 138
"00001000000000000000010000110111", -- 139
"00001000000000000000000000010110", -- 13a
"00001000000000000000010000110101", -- 13b
"00001000000000000000010000101000", -- 13c
"00001000000000000000010000101101", -- 13d
"00001000000000000000000000001110", -- 13e
"00001000000000000000100000000010", -- 13f
"00001000000000000000000000001000", -- 140
"00001000000000000000000000001000", -- 141
"00001000000000000000000000001000", -- 142
"00001000000000000000000000001000", -- 143
"00001000000000000000000000001000", -- 144
"00001000000000000000000000001000", -- 145
"00001000000000000000000000001010", -- 146
"00001000000000000000000000001000", -- 147
"00001000000000000000000000001000", -- 148
"00001000000000000000000000001000", -- 149
"00001000000000000000000000001000", -- 14a
"00001000000000000000000000001000", -- 14b
"00001000000000000000000000001000", -- 14c
"00001000000000000000000000001000", -- 14d
"00001000000000000000000000001010", -- 14e
"00001000000000000000000000001000", -- 14f
"00001000000000000000000000001000", -- 150
"00001000000000000000000000001000", -- 151
"00001000000000000000000000001000", -- 152
"00001000000000000000000000001000", -- 153
"00001000000000000000000000001000", -- 154
"00001000000000000000000000001000", -- 155
"00001000000000000000000000001010", -- 156
"00001000000000000000000000001000", -- 157
"00001000000000000000000000001000", -- 158
"00001000000000000000000000001000", -- 159
"00001000000000000000000000001000", -- 15a
"00001000000000000000000000001000", -- 15b
"00001000000000000000000000001000", -- 15c
"00001000000000000000000000001000", -- 15d
"00001000000000000000000000001010", -- 15e
"00001000000000000000000000001000", -- 15f
"00001000000000000000000000001000", -- 160
"00001000000000000000000000001000", -- 161
"00001000000000000000000000001000", -- 162
"00001000000000000000000000001000", -- 163
"00001000000000000000000000001000", -- 164
"00001000000000000000000000001000", -- 165
"00001000000000000000000000001010", -- 166
"00001000000000000000000000001000", -- 167
"00001000000000000000000000001000", -- 168
"00001000000000000000000000001000", -- 169
"00001000000000000000000000001000", -- 16a
"00001000000000000000000000001000", -- 16b
"00001000000000000000000000001000", -- 16c
"00001000000000000000000000001000", -- 16d
"00001000000000000000000000001010", -- 16e
"00001000000000000000000000001000", -- 16f
"00001000000000000000000000001100", -- 170
"00001000000000000000000000001100", -- 171
"00001000000000000000000000001100", -- 172
"00001000000000000000000000001100", -- 173
"00001000000000000000000000001100", -- 174
"00001000000000000000000000001100", -- 175
"00001000000000000000110000011000", -- 176
"00001000000000000000000000001100", -- 177
"00001000000000000000000000001000", -- 178
"00001000000000000000000000001000", -- 179
"00001000000000000000000000001000", -- 17a
"00001000000000000000000000001000", -- 17b
"00001000000000000000000000001000", -- 17c
"00001000000000000000000000001000", -- 17d
"00001000000000000000000000001010", -- 17e
"00001000000000000000000000001000", -- 17f
"00001000000000000000010000001000", -- 180
"00001000000000000000010000001000", -- 181
"00001000000000000000010000001000", -- 182
"00001000000000000000010000001000", -- 183
"00001000000000000000010000001000", -- 184
"00001000000000000000010000001000", -- 185
"00001000000000000000010000011000", -- 186
"00001000000000000000010000001000", -- 187
"00001000000000000000010000001010", -- 188
"00001000000000000000010000001010", -- 189
"00001000000000000000010000001010", -- 18a
"00001000000000000000010000001010", -- 18b
"00001000000000000000010000001010", -- 18c
"00001000000000000000010000001010", -- 18d
"00001000000000000000010000011010", -- 18e
"00001000000000000000010000001010", -- 18f
"00001000000000000000010000001100", -- 190
"00001000000000000000010000001100", -- 191
"00001000000000000000010000001100", -- 192
"00001000000000000000010000001100", -- 193
"00001000000000000000010000001100", -- 194
"00001000000000000000010000001100", -- 195
"00001000000000000000010000011100", -- 196
"00001000000000000000010000001100", -- 197
"00001000000000000000010000001110", -- 198
"00001000000000000000010000001110", -- 199
"00001000000000000000010000001110", -- 19a
"00001000000000000000010000001110", -- 19b
"00001000000000000000010000001110", -- 19c
"00001000000000000000010000001110", -- 19d
"00001000000000000000010000011110", -- 19e
"00001000000000000000010000001110", -- 19f
"00001000000000000000010000010000", -- 1a0
"00001000000000000000010000010000", -- 1a1
"00001000000000000000010000010000", -- 1a2
"00001000000000000000010000010000", -- 1a3
"00001000000000000000010000010000", -- 1a4
"00001000000000000000010000010000", -- 1a5
"00001000000000000000010000100000", -- 1a6
"00001000000000000000010000010000", -- 1a7
"00001000000000000000010000010010", -- 1a8
"00001000000000000000010000010010", -- 1a9
"00001000000000000000010000010010", -- 1aa
"00001000000000000000010000010010", -- 1ab
"00001000000000000000010000010010", -- 1ac
"00001000000000000000010000010010", -- 1ad
"00001000000000000000010000100010", -- 1ae
"00001000000000000000010000010010", -- 1af
"00001000000000000000010000010100", -- 1b0
"00001000000000000000010000010100", -- 1b1
"00001000000000000000010000010100", -- 1b2
"00001000000000000000010000010100", -- 1b3
"00001000000000000000010000010100", -- 1b4
"00001000000000000000010000010100", -- 1b5
"00001000000000000000010000100100", -- 1b6
"00001000000000000000010000010100", -- 1b7
"00001000000000000000010000010110", -- 1b8
"00001000000000000000010000010110", -- 1b9
"00001000000000000000010000010110", -- 1ba
"00001000000000000000010000010110", -- 1bb
"00001000000000000000010000010110", -- 1bc
"00001000000000000000010000010110", -- 1bd
"00001000000000000000010000100110", -- 1be
"00001000000000000000010000010110", -- 1bf
"00001000000000000000100000011011", -- 1c0
"00001000000000000000100000110000", -- 1c1
"00001000000000000000100000001010", -- 1c2
"00001000000000000000100000000100", -- 1c3
"00001000000000000000100000010101", -- 1c4
"00001000000000000000100000100110", -- 1c5
"00001000000000000000000000111000", -- 1c6
"00001000000000000000100000011100", -- 1c7
"00001000000000000000100000011011", -- 1c8
"00001000000000000000100000010111", -- 1c9
"00001000000000000000100000001010", -- 1ca
"00001000000000000000000000000000", -- 1cb
"00001000000000000000100000010101", -- 1cc
"00001000000000000000100000001100", -- 1cd
"00001000000000000000000000111010", -- 1ce
"00001000000000000000100000011100", -- 1cf
"00001000000000000000100000011011", -- 1d0
"00001000000000000000100000110000", -- 1d1
"00001000000000000000100000001010", -- 1d2
"00001000000000000000110000010001", -- 1d3
"00001000000000000000100000010101", -- 1d4
"00001000000000000000100000100110", -- 1d5
"00001000000000000000000000111100", -- 1d6
"00001000000000000000100000011100", -- 1d7
"00001000000000000000100000011011", -- 1d8
"00001000000000000000000000000000", -- 1d9
"00001000000000000000100000001010", -- 1da
"00001000000000000000110000001010", -- 1db
"00001000000000000000100000010101", -- 1dc
"00001000000000000000000000000000", -- 1dd
"00001000000000000000000000111110", -- 1de
"00001000000000000000100000011100", -- 1df
"00001000000000000000100000011011", -- 1e0
"00001000000000000000100000110000", -- 1e1
"00001000000000000000100000001010", -- 1e2
"00001000000000000000100000111000", -- 1e3
"00001000000000000000100000010101", -- 1e4
"00001000000000000000100000100110", -- 1e5
"00001000000000000000010000000000", -- 1e6
"00001000000000000000100000011100", -- 1e7
"00001000000000000000100000011011", -- 1e8
"00001000000000000000100000100010", -- 1e9
"00001000000000000000100000001010", -- 1ea
"00001000000000000000000000101100", -- 1eb
"00001000000000000000100000010101", -- 1ec
"00001000000000000000000000000000", -- 1ed
"00001000000000000000010000000010", -- 1ee
"00001000000000000000100000011100", -- 1ef
"00001000000000000000100000011011", -- 1f0
"00001000000000000000100000110100", -- 1f1
"00001000000000000000100000001010", -- 1f2
"00001000000000000000110000001001", -- 1f3
"00001000000000000000100000010101", -- 1f4
"00001000000000000000100000101011", -- 1f5
"00001000000000000000010000000100", -- 1f6
"00001000000000000000100000011100", -- 1f7
"00001000000000000000100000011011", -- 1f8
"00001000000000000000110000000100", -- 1f9
"00001000000000000000100000001010", -- 1fa
"00001000000000000000110000001000", -- 1fb
"00001000000000000000100000010101", -- 1fc
"00001000000000000000000000000000", -- 1fd
"00001000000000000000010000000110", -- 1fe
"00001000000000000000100000011100"  -- 1ff
 
);
 
-- end of microcode ROM
 
signal load_al :      std_logic; -- uinst field, load AL reg from rbank
signal load_addr :    std_logic; -- uinst field, enable external addr reg load
signal load_t1 :      std_logic; -- uinst field, load reg T1 
signal load_t2 :      std_logic; -- uinst field, load reg T2
signal mux_in :       std_logic; -- uinst field, T1/T2 input data selection
signal load_do :      std_logic; -- uinst field, pipelined, load DO reg
-- rb_addr_sel: uinst field, rbank address selection: (sss,ddd,pp,ra_field)
signal rb_addr_sel :  std_logic_vector(1 downto 0);
-- ra_field: uinst field, explicit reg bank address
signal ra_field :     std_logic_vector(3 downto 0);
signal rbank_data :   std_logic_vector(7 downto 0); -- rbank output
signal alu_output :   std_logic_vector(7 downto 0); -- ALU output
-- data_output: datapath output: ALU output vs. F reg 
signal data_output :  std_logic_vector(7 downto 0); 
signal T1 :           std_logic_vector(7 downto 0); -- T1 reg (ALU operand)
signal T2 :           std_logic_vector(7 downto 0); -- T2 reg (ALU operand)
-- alu_input: data loaded into T1, T2: rbank data vs. DI
signal alu_input :    std_logic_vector(7 downto 0);
signal we_rb :        std_logic; -- uinst field, commands a write to the rbank
signal inhibit_pc_increment : std_logic; -- avoid PC changes (during INTA)
signal rbank_rd_addr: std_logic_vector(3 downto 0); -- rbank rd addr
signal rbank_wr_addr: std_logic_vector(3 downto 0); -- rbank wr addr
signal DO :           std_logic_vector(7 downto 0); -- data output reg
 
-- Register bank as an array of 16 bytes (asynch. LUT ram)
type t_reg_bank is array(0 to 15) of std_logic_vector(7 downto 0);
-- Register bank : BC, DE, HL, AF, [PC, XY, ZW, SP]
signal rbank :        t_reg_bank;
 
signal flag_reg :     std_logic_vector(7 downto 0); -- F register
-- flag_pattern: uinst field, F update pattern: which flags are updated
signal flag_pattern : std_logic_vector(1 downto 0);
signal flag_s :       std_logic; -- new computed S flag  
signal flag_z :       std_logic; -- new computed Z flag
signal flag_p :       std_logic; -- new computed P flag
signal flag_cy :      std_logic; -- new computed C flag
signal flag_cy_1 :    std_logic; -- C flag computed from arith/logic operation
signal flag_cy_2 :    std_logic; -- C flag computed from CPC circuit
signal do_cy_op :     std_logic; -- ALU explicit CY operation (CPC, etc.)
signal do_cy_op_d :   std_logic; -- do_cy_op, pipelined
signal do_cpc :       std_logic; -- ALU operation is CPC
signal do_cpc_d :     std_logic; -- do_cpc, pipelined
signal do_daa :       std_logic; -- ALU operation is DAA
signal flag_ac :      std_logic; -- new computed half carry flag
-- flag_aux_cy: new computed half carry flag (used in 16-bit ops)
signal flag_aux_cy :  std_logic;
signal load_psw :     std_logic; -- load F register
 
-- aux carry computation and control signals
signal use_aux :      std_logic; -- decoded from flags in 1st phase
signal use_aux_cy :   std_logic; -- 2nd phase signal
signal reg_aux_cy :   std_logic;
signal aux_cy_in :    std_logic;
signal set_aux_cy :   std_logic;
signal set_aux  :     std_logic;
 
-- ALU control signals -- together they select ALU operation
signal alu_fn :       std_logic_vector(1 downto 0);
signal use_logic :    std_logic; -- logic/arith mux control 
signal mux_fn :       std_logic_vector(1 downto 0); 
signal use_psw :      std_logic; -- ALU/F mux control
 
-- ALU arithmetic operands and result
signal arith_op1 :    std_logic_vector(8 downto 0);
signal arith_op2 :    std_logic_vector(8 downto 0);  
signal arith_op2_sgn: std_logic_vector(8 downto 0);
signal arith_res :    std_logic_vector(8 downto 0);
signal arith_res8 :   std_logic_vector(7 downto 0);  
 
-- ALU DAA intermediate signals (DAA has fully dedicated logic)
signal daa_res :      std_logic_vector(8 downto 0);
signal daa_res8 :     std_logic_vector(7 downto 0);
signal daa_res9 :     std_logic_vector(8 downto 0);    
signal daa_test1 :    std_logic;  
signal daa_test1a :   std_logic;  
signal daa_test2 :    std_logic;  
signal daa_test2a :   std_logic;  
signal arith_daa_res :std_logic_vector(7 downto 0);     
signal cy_daa :       std_logic;
 
-- ALU CY flag intermediate signals
signal cy_in_sgn :    std_logic;
signal cy_in :        std_logic;
signal cy_in_gated :  std_logic;
signal cy_adder :     std_logic;
signal cy_arith :     std_logic;
signal cy_shifter :   std_logic;
 
-- ALU intermediate results
signal logic_res :    std_logic_vector(7 downto 0);  
signal shift_res :    std_logic_vector(7 downto 0);    
signal alu_mux1 :     std_logic_vector(7 downto 0);
 
begin
 
DI <= data_in;
 
process(clk)    -- IR register, load when uc_decode flag activates
begin
  if clk'event and clk='1' then
    if uc_decode = '1' then
      IR <= DI;
    end if;
  end if;
end process;
 
s_field <= IR(2 downto 0); -- IR field extraction : sss reg code
d_field <= IR(5 downto 3); -- ddd reg code
p_field <= IR(5 downto 4); -- pp 16-bit reg pair code   
 
 
--##############################################################################
-- Microcode sequencer
 
process(clk)    -- do_reset is reset delayed 1 cycle
begin
  if clk'event and clk='1' then
    do_reset <= reset;
  end if;
end process;
 
uc_flags1 <= ucode(31 downto 29);
uc_flags2 <= ucode(28 downto 26);
 
-- microcode address control flags are gated by do_reset (reset has priority)
uc_do_ret <= '1' when uc_flags2 = "011" and do_reset = '0' else '0';
uc_jsr    <= '1' when uc_flags2 = "010" and do_reset = '0' else '0';  
uc_tjsr   <= '1' when uc_flags2 = "100" and do_reset = '0' else '0';    
uc_decode <= '1' when uc_flags1 = "001" and do_reset = '0' else '0';  
uc_end    <= '1' when (uc_flags2 = "001" or (uc_tjsr='1' and condition_reg='0'))
                  and do_reset = '0' else '0';  
 
-- other microinstruction flags are decoded
uc_halt_flag  <= '1' when uc_flags1 = "111" else '0';
uc_halt   <= '1' when uc_halt_flag='1' and inta_reg='0' else '0';  
uc_ei     <= '1' when uc_flags1 = "011" else '0';  
uc_di     <= '1' when uc_flags1 = "010" or inta_reg='1' else '0'; 
-- clr_t1/2 clears T1/T2 when explicitly commanded; T2 and T1 clear implicitly 
-- at the end of each instruction (by uc_decode)
clr_t2    <= '1' when uc_flags2 = "001" else '0';
clr_t1    <= '1' when uc_flags1 = "110" else '0';
use_aux   <= '1' when uc_flags1 = "101" else '0';  
set_aux   <= '1' when uc_flags2 = "111" else '0';
 
load_al <= ucode(24);
load_addr <= ucode(25);
 
do_cy_op_d <= '1' when ucode(5 downto 2)="1011" else '0'; -- decode CY ALU op
do_cpc_d <= ucode(0); -- decode CPC ALU op
 
-- uinst jump command, either unconditional or on a given condition
uc_do_jmp <= uc_jsr or (uc_tjsr and condition_reg);
 
vma <= load_addr;  -- addr is valid, either for memmory or io
 
-- assume the only uinst that does memory access in the range 0..f is 'fetch'
fetch <= '1' when uc_addr(7 downto 4)=X"0" and load_addr='1' else '0';
 
-- external bus interface control signals
io <= '1' when uc_flags1="100" else '0'; -- IO access (vs. memory)
rd <= '1' when uc_flags2="101" else '0'; -- RD access
wr <= '1' when uc_flags2="110" else '0'; -- WR access  
 
uc_jmp_addr <= ucode(11 downto 10) & ucode(5 downto 0);
 
uc_addr_sel <= uc_do_ret & uc_do_jmp & uc_decode & uc_end;
 
addr_plus_1 <= uc_addr + 1;
 
-- TODO simplify this!!
 
-- NOTE: when end='1' we jump either to the FETCH ucode ot to the HALT ucode
-- depending on the value of the halt signal.
-- We use the unregistered uc_halt instead of halt_reg because otherwise #end
-- should be on the cycle following #halt, wasting a cycle.
-- This means that the flag #halt has to be used with #end or will be ignored. 
 
with uc_addr_sel select
  next_uc_addr <= '0'&uc_ret_addr when "1000", -- ret
                  '0'&uc_jmp_addr when "0100", -- jsr/tjsr
                  '0'&addr_plus_1 when "0000", -- uaddr++
                  "000000"&uc_halt&"11" 
                                  when "0001", -- end: go to fetch/halt uaddr
                  '1'&DI          when others; -- decode fetched address 
 
-- Note how we used DI (containing instruction opcode) as a microcode address
 
-- read microcode rom 
process (clk)
begin
  if clk'event and clk='1' then
    ucode <= rom(conv_integer(next_uc_addr));
  end if;
end process;
 
-- microcode address register
process (clk)
begin
  if clk'event and clk='1' then
    if reset = '1' then
      uc_addr <= X"00";
    else
      uc_addr <= next_uc_addr(7 downto 0);  
    end if;
  end if;
end process;
 
-- ucode address 1-level 'return stack'
process (clk)
begin
  if clk'event and clk='1' then
    if reset = '1' then
      uc_ret_addr <= X"00";
    elsif uc_do_jmp='1' then
      uc_ret_addr <= addr_plus_1;
    end if;  
  end if;
end process;    
 
 
alu_op <= ucode(3 downto 0); 
 
-- pipeline uinst field2 for 1-cycle delayed execution.
-- note the same rbank addr field is used in cycles 1 and 2; this enforces
-- some constraints on uinst programming but simplifies the system.
process(clk)
begin
  if clk'event and clk='1' then
    ucode_field2 <= do_cy_op_d & do_cpc_d & clr_t2 & clr_t1 & 
                    set_aux & use_aux & rbank_rd_addr & 
                    ucode(14 downto 4) & alu_op;
  end if;
end process;
 
--#### HALT logic
process(clk)
begin
  if clk'event and clk='1' then
    if reset = '1' or int_pending = '1' then --inta_reg
      halt_reg <= '0';
    else 
      if uc_halt = '1' then
        halt_reg <= '1';
      end if;
    end if;
  end if;
end process;
 
halt <= halt_reg;
 
--#### INTE logic -- inte_reg = '1' means interrupts ENABLED
process(clk)
begin
  if clk'event and clk='1' then
    if reset = '1' then
      inte_reg <= '0';
    else 
      if uc_di='1' or uc_ei='1' then
        inte_reg <= uc_ei;
      end if;
    end if;
  end if;
end process;
 
inte <= inte_reg;
 
-- interrupts are ignored when inte='0'
process(clk)
begin
  if clk'event and clk='1' then
    if reset = '1' then
      int_pending <= '0';
    else 
      if intr = '1' and inte_reg = '1' and int_pending = '0' then
        int_pending <= '1';
      else 
        if inte_reg = '1' and uc_end='1' then
          int_pending <= '0';
        end if;
      end if;
    end if;
  end if;
end process;
 
 
--#### INTA logic
-- INTA goes high from END to END, that is for the entire time the instruction
-- takes to fetch and execute; in the original 8080 it was asserted only for 
-- the M1 cycle.
-- All instructions can be used in an inta cycle, including XTHL which was
-- forbidden in the original 8080. 
-- It's up to you figuring out which cycle is which in multibyte instructions.
process(clk)
begin
  if clk'event and clk='1' then
    if reset = '1' then
      inta_reg <= '0';
    else 
      if int_pending = '1' and uc_end='1' then
        -- enter INTA state
        inta_reg <= '1';
      else  
        -- exit INTA state
        -- NOTE: don't reset inta when exiting halt state (uc_halt_flag='1').
        -- If we omit this condition, when intr happens on halt state, inta
        -- will only last for 1 cycle, because in halt state uc_end is 
        -- always asserted.
        if uc_end = '1' and uc_halt_flag='0' then
          inta_reg <= '0';
        end if;
      end if;
    end if;
  end if;
end process;    
 
inta <= inta_reg;
 
 
--##############################################################################
-- Datapath
 
-- extract pipelined microcode fields
ra_field <= ucode(18 downto 15);
load_t1 <= ucode(23);  
load_t2 <= ucode(22);  
mux_in <= ucode(21);
rb_addr_sel <= ucode(20 downto 19);  
load_do <= ucode_field2(7);
set_aux_cy <= ucode_field2(20); 
do_clr_t1 <= ucode_field2(21); 
do_clr_t2 <= ucode_field2(22); 
 
 
-- T1 register 
process (clk)
begin
  if clk'event and clk='1' then
    if reset = '1' or uc_decode = '1' or do_clr_t1='1' then 
      T1 <= X"00";
    else
      if load_t1 = '1' then
        T1 <= alu_input;
      end if;
    end if;    
  end if;
end process;
 
-- T2 register
process (clk)
begin
  if clk'event and clk='1' then
    if reset = '1' or uc_decode = '1' or do_clr_t2='1' then 
      T2 <= X"00";
    else
      if load_t2 = '1' then
        T2 <= alu_input;
      end if;
    end if;    
  end if;
end process;
 
-- T1/T2 input data mux
alu_input <= rbank_data when mux_in = '1' else DI;
 
-- register bank address mux logic
 
rbh <= '1' when p_field = "11" else '0';
 
with rb_addr_sel select 
  rbank_rd_addr <=  ra_field    when "00",
                    "0"&s_field when "01",
                    "0"&d_field when "10",
                    rbh&p_field&ra_field(0) when others;   
 
-- RBank writes are inhibited in INTA state, but only for PC increments.
inhibit_pc_increment <= '1' when inta_reg='1' and use_aux_cy='1' 
                                 and rbank_wr_addr(3 downto 1) = "100" 
                                 else '0';
we_rb <= ucode_field2(6) and not inhibit_pc_increment;
 
-- Register bank logic
-- NOTE: read is asynchronous, while write is synchronous; but note also
-- that write phase for a given uinst happens the cycle after the read phase.
-- This way we give the ALU time to do its job.
rbank_wr_addr <= ucode_field2(18 downto 15);
process(clk)
begin
  if clk'event and clk='1' then
    if we_rb = '1' then
      rbank(conv_integer(rbank_wr_addr)) <= alu_output;
    end if;
  end if;
end process;
rbank_data <= rbank(conv_integer(rbank_rd_addr));
 
-- should we read F register or ALU output?
use_psw <= '1' when ucode_field2(5 downto 4)="11" else '0';
data_output <= flag_reg when use_psw = '1' else alu_output;
 
 
process (clk)
begin
  if clk'event and clk='1' then
    if load_do = '1' then
        DO <= data_output;
    end if;
  end if;
end process;
 
--##############################################################################
-- ALU 
 
alu_fn <= ucode_field2(1 downto 0);
use_logic <= ucode_field2(2);
mux_fn <= ucode_field2(4 downto 3);
--#### make sure this is "00" in the microcode when no F updates should happen!
flag_pattern <=  ucode_field2(9 downto 8);
use_aux_cy <= ucode_field2(19);
do_cpc <= ucode_field2(23);
do_cy_op <= ucode_field2(24);
do_daa <= '1' when ucode_field2(5 downto 2) = "1010" else '0';    
 
aux_cy_in <= reg_aux_cy when set_aux_cy = '0' else '1';
 
-- carry input selection: normal or aux (for 16 bit increments)?
cy_in <= flag_reg(0) when use_aux_cy = '0' else aux_cy_in;
 
-- carry is not used (0) in add/sub operations
cy_in_gated <= cy_in and alu_fn(0);
 
--##### Adder/substractor
 
-- zero extend adder operands to 9 bits to ease CY output synthesis
-- use zero extension because we're only interested in cy from 7 to 8
arith_op1 <= '0' & T2;
arith_op2 <= '0' & T1;
 
-- The adder/substractor is done in 2 stages to help XSL synth it properly
-- Other codings result in 1 adder + a substractor + 1 mux
 
-- do 2nd op 2's complement if substracting...
arith_op2_sgn <=  arith_op2 when alu_fn(1) = '0' else not arith_op2;
-- ...and complement cy input too
cy_in_sgn <= cy_in_gated when alu_fn(1) = '0' else not cy_in_gated;
 
-- once 2nd operand has been negated (or not) add operands normally
arith_res <= arith_op1 + arith_op2_sgn + cy_in_sgn;
 
-- take only 8 bits; 9th bit of adder is cy output
arith_res8 <= arith_res(7 downto 0);
cy_adder <= arith_res(8);
 
--##### DAA dedicated logic
-- Note a DAA takes 2 cycles to complete! 
 
--daa_test1a='1' when daa_res9(7 downto 4) > 0x06
daa_test1a <= arith_op2(3) and (arith_op2(2) or arith_op2(1) or arith_op2(0));  
daa_test1 <= '1' when flag_reg(4)='1' or daa_test1a='1' else '0';
 
process(clk)
begin
  if clk'event and clk='1' then
    if reset='1' then
      daa_res9 <= "000000000";
    else
      if daa_test1='1' then
        daa_res9 <= arith_op2 + "000000110";
      else
        daa_res9 <= arith_op2;
      end if;
    end if;
  end if;
end process;
 
--daa_test2a='1' when daa_res9(7 downto 4) > 0x06 FIXME unused?
daa_test2a <= daa_res9(7) and (daa_res9(6) or daa_res9(5) or daa_res9(4));
daa_test2 <= '1' when flag_reg(0)='1' or daa_test1a='1' else '0';
 
daa_res <= '0'&daa_res9(7 downto 0) + "01100000" when daa_test2='1' 
           else daa_res9;
 
cy_daa <= daa_res(8);
 
-- DAA vs. adder mux
arith_daa_res <= daa_res(7 downto 0) when do_daa='1' else arith_res8;  
 
-- DAA vs. adder CY mux
cy_arith <= cy_daa when do_daa='1' else cy_adder;
 
--##### Logic operations block
logic_res <=  T1 and T2 when alu_fn = "00" else
              T1 xor T2 when alu_fn = "01" else
              T1 or  T2 when alu_fn = "10" else
              not T1;
 
--##### Shifter
shifter:
for i in 1 to 6 generate
begin
  shift_res(i) <= T1(i-1) when alu_fn(0) = '0' else T1(i+1); 
end generate;
shift_res(0) <= T1(7) when alu_fn = "00" else -- rot left 
                cy_in when alu_fn = "10" else -- rot left through carry
                T1(1); -- rot right
shift_res(7) <= T1(0) when alu_fn = "01" else -- rot right
                cy_in when alu_fn = "11" else -- rot right through carry
                T1(6); -- rot left
 
cy_shifter   <= T1(7) when alu_fn(0) = '0' else -- left
                T1(0);                          -- right
 
alu_mux1 <= logic_res when use_logic = '1' else shift_res;
 
 
with mux_fn select
  alu_output <= alu_mux1      when "00",
                arith_daa_res when "01",
                not alu_mux1  when "10",
                "00"&d_field&"000" when others; -- RST  
 
--###### flag computation 
 
flag_s <= alu_output(7);
flag_p <= not(alu_output(7) xor alu_output(6) xor alu_output(5) xor alu_output(4) xor
         alu_output(3) xor alu_output(2) xor alu_output(1) xor alu_output(0));
flag_z <= '1' when alu_output=X"00" else '0';
flag_ac <= (arith_op1(4) xor arith_op2_sgn(4) xor alu_output(4));
 
flag_cy_1 <= cy_arith when use_logic = '1' else cy_shifter;
flag_cy_2 <= not flag_reg(0) when do_cpc='0' else '1'; -- cmc, stc
flag_cy <= flag_cy_1 when do_cy_op='0' else flag_cy_2;
 
flag_aux_cy <= cy_adder;
 
-- auxiliary carry reg
process(clk)
begin
  if clk'event and clk='1' then
    if reset='1' or uc_decode = '1' then
      reg_aux_cy <= '1'; -- inits to 0 every instruction
    else
      reg_aux_cy <= flag_aux_cy;
    end if;
  end if;
end process;              
 
-- load PSW from ALU (i.e. POP AF) or from flag signals
load_psw <= '1' when we_rb='1' and rbank_wr_addr="0110" else '0';
 
-- The F register has been split in two separate groupt that always update
-- together (C and all others).
 
-- F register, flags S,Z,AC,P
process(clk)
begin
  if clk'event and clk='1' then
    if reset='1' then
      flag_reg(7) <= '0';
      flag_reg(6) <= '0';
      flag_reg(4) <= '0';
      flag_reg(2) <= '0';
    elsif flag_pattern(1) = '1' then
      if load_psw = '1' then
        flag_reg(7) <= alu_output(7);
        flag_reg(6) <= alu_output(6);
        flag_reg(4) <= alu_output(4);
        flag_reg(2) <= alu_output(2);      
      else
        flag_reg(7) <= flag_s;
        flag_reg(6) <= flag_z;
        flag_reg(4) <= flag_ac;
        flag_reg(2) <= flag_p;      
      end if;
    end if;
  end if;
end procesS;
 
-- F register, flag C
process(clk)
begin
  if clk'event and clk='1' then
    if reset = '1' then
      flag_reg(0) <= '0';
    elsif flag_pattern(0) = '1' then
      if load_psw = '1' then
        flag_reg(0) <= alu_output(0);  
      else
        flag_reg(0) <= flag_cy;
      end if;
    end if;
  end if;
end procesS;
 
flag_reg(5) <= '0'; -- constant flag
flag_reg(3) <= '0'; -- constant flag
flag_reg(1) <= '1'; -- constant flag
 
--##### Condition computation
 
condition_sel <= d_field(2 downto 0);
with condition_sel select 
  condition <=  
            not flag_reg(6) when "000", -- NZ
                flag_reg(6) when "001", -- Z
            not flag_reg(0) when "010", -- NC
                flag_reg(0) when "011", -- C
            not flag_reg(2) when "100", -- PO
                flag_reg(2) when "101", -- PE  
            not flag_reg(7) when "110", -- P  
                flag_reg(7) when others;-- M                  
 
 
-- condition is registered to shorten the delay path; the extra 1-cycle
-- delay is not relevant because conditions are tested in the next instruction
-- at the earliest, and there's at least the fetch uinsts intervening.                
process(clk)
begin
  if clk'event and clk='1' then
    if reset = '1' then 
      condition_reg <= '0';
    else
      condition_reg <= condition;
    end if;
  end if;
end process;                            
 
-- low byte address register
process(clk)
begin
  if clk'event and clk='1' then
    if reset = '1' then
      addr_low <= X"00";
    elsif load_al = '1' then
      addr_low <= rbank_data;
    end if;
  end if;
end process;
 
-- note external address registers (high byte) are loaded directly from rbank
addr_out <= rbank_data & addr_low;
 
data_out <= DO;
 
end microcoded;
 
--------------------------------------------------------------------------------
-- Timing diagram 1: RD and WR cycles
--------------------------------------------------------------------------------
--            1     2     3     4     5     6     7     8     
--             __    __    __    __    __    __    __    __   
-- clk      __/  \__/  \__/  \__/  \__/  \__/  \__/  \__/  \__
--
-- addr_o   xxxxxxxxxxxxxx< ADR >xxxxxxxxxxx< ADR >xxxxxxxxxxx
--
-- data_i   xxxxxxxxxxxxxxxxxxxx< Din >xxxxxxxxxxxxxxxxxxxxxxx
--
-- data_o   xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx< Dout>xxxxxxxxxxx
--                         _____             _____
-- vma_o    ______________/     \___________/     \___________
--                         _____
-- rd_o     ______________/     \_____________________________
--                                           _____
-- wr_o     ________________________________/     \___________
--
-- (functional diagram, actual time delays not shown)
--------------------------------------------------------------------------------
-- This diagram shows a read cycle and a write cycle back to back.
-- In clock edges (4) and (7), the address is loaded into the external 
-- synchronous RAM address register. 
-- In clock edge (5), read data is loaded into the CPU.
-- In clock edge (7), write data is loaded into the external synchronous RAM.
-- In actual operation, the CPU does about 1 rd/wr cycle for each 5 clock 
-- cycles, which is a waste of RAM bandwidth.
--
 

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