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R65Cx2.vhd
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R65Cx2.vhd
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-- -----------------------------------------------------------------------
--
-- This is a table driven 65Cx2 core by A.Daly
-- This is a derivative of the excellent FPGA64 core see below
--
-- -----------------------------------------------------------------------
-- Copyright 2005-2008 by Peter Wendrich (pwsoft@syntiac.com)
-- http://www.syntiac.com/fpga64.html
-- -----------------------------------------------------------------------
library IEEE;
use ieee.std_logic_1164.ALL;
use ieee.numeric_std.ALL;
entity R65C02 is
port (
reset : in std_logic;
clk : in std_logic;
enable : in std_logic;
nmi_n : in std_logic;
irq_n : in std_logic;
di : in unsigned(7 downto 0);
do : out unsigned(7 downto 0);
addr : out unsigned(15 downto 0);
nwe : out std_logic;
sync : out std_logic;
sync_irq : out std_logic;
-- 6502 registers (MSB) PC, SP, P, Y, X, A (LSB)
Regs : out std_logic_vector(63 downto 0)
);
end R65C02;
-- Store Zp (3) => fetch, cycle2, cycleEnd
-- Store Zp,x (4) => fetch, cycle2, preWrite, cycleEnd
-- Read Zp,x (4) => fetch, cycle2, cycleRead, cycleRead2
-- Rmw Zp,x (6) => fetch, cycle2, cycleRead, cycleRead2, cycleRmw, cycleEnd
-- Store Abs (4) => fetch, cycle2, cycle3, cycleEnd
-- Store Abs,x (5) => fetch, cycle2, cycle3, preWrite, cycleEnd
-- Rts (6) => fetch, cycle2, cycle3, cycleRead, cycleJump, cycleIncrEnd
-- Rti (6) => fetch, cycle2, stack1, stack2, stack3, cycleJump
-- Jsr (6) => fetch, cycle2, .. cycle5, cycle6, cycleJump
-- Jmp abs (-) => fetch, cycle2, .., cycleJump
-- Jmp (ind) (-) => fetch, cycle2, .., cycleJump
-- Brk / irq (6) => fetch, cycle2, stack2, stack3, stack4
-- -----------------------------------------------------------------------
architecture Behavioral of R65C02 is
-- signal counter : unsigned(27 downto 0);
-- signal mask_irq : std_logic;
-- signal mask_enable : std_logic;
-- Statemachine
type cpuCycles is (
opcodeFetch, -- New opcode is read and registers updated
cycle2,
cycle3,
cyclePreIndirect,
cycleIndirect,
cycleBranchTaken,
cycleBranchPage,
cyclePreRead, -- Cycle before read while doing zeropage indexed addressing.
cycleRead, -- Read cycle
cycleRead2, -- Second read cycle after page-boundary crossing.
cycleRmw, -- Calculate ALU output for read-modify-write instr.
cyclePreWrite, -- Cycle before write when doing indexed addressing.
cycleWrite, -- Write cycle for zeropage or absolute addressing.
cycleStack1,
cycleStack2,
cycleStack3,
cycleStack4,
cycleJump, -- Last cycle of Jsr, Jmp. Next fetch address is target addr.
cycleEnd
);
signal theCpuCycle : cpuCycles;
signal nextCpuCycle : cpuCycles;
signal updateRegisters : boolean;
signal processIrq : std_logic;
signal nmiReg: std_logic;
signal nmiEdge: std_logic;
signal irqReg : std_logic; -- Delay IRQ input with one clock cycle.
signal soReg : std_logic; -- SO pin edge detection
-- Opcode decoding
constant opcUpdateA : integer := 0;
constant opcUpdateX : integer := 1;
constant opcUpdateY : integer := 2;
constant opcUpdateS : integer := 3;
constant opcUpdateN : integer := 4;
constant opcUpdateV : integer := 5;
constant opcUpdateD : integer := 6;
constant opcUpdateI : integer := 7;
constant opcUpdateZ : integer := 8;
constant opcUpdateC : integer := 9;
constant opcSecondByte : integer := 10;
constant opcAbsolute : integer := 11;
constant opcZeroPage : integer := 12;
constant opcIndirect : integer := 13;
constant opcStackAddr : integer := 14; -- Push/Pop address
constant opcStackData : integer := 15; -- Push/Pop status/data
constant opcJump : integer := 16;
constant opcBranch : integer := 17;
constant indexX : integer := 18;
constant indexY : integer := 19;
constant opcStackUp : integer := 20;
constant opcWrite : integer := 21;
constant opcRmw : integer := 22;
constant opcIncrAfter : integer := 23; -- Insert extra cycle to increment PC (RTS)
constant opcRti : integer := 24;
constant opcIRQ : integer := 25;
constant opcInA : integer := 26;
constant opcInBrk : integer := 27;
constant opcInX : integer := 28;
constant opcInY : integer := 29;
constant opcInS : integer := 30;
constant opcInT : integer := 31;
constant opcInH : integer := 32;
constant opcInClear : integer := 33;
constant aluMode1From : integer := 34;
--
constant aluMode1To : integer := 37;
constant aluMode2From : integer := 38;
--
constant aluMode2To : integer := 40;
--
constant opcInCmp : integer := 41;
constant opcInCpx : integer := 42;
constant opcInCpy : integer := 43;
subtype addrDef is unsigned(0 to 15);
--
-- is Interrupt -----------------+
-- instruction is RTI ----------------+|
-- PC++ on last cycle (RTS) ---------------+||
-- RMW --------------+|||
-- Write -------------+||||
-- Pop/Stack up -------------+|||||
-- Branch ---------+ ||||||
-- Jump ----------+| ||||||
-- Push or Pop data -------+|| ||||||
-- Push or Pop addr ------+||| ||||||
-- Indirect -----+|||| ||||||
-- ZeroPage ----+||||| ||||||
-- Absolute ---+|||||| ||||||
-- PC++ on cycle2 --+||||||| ||||||
-- |AZI||JBXY|WM|||
constant immediate : addrDef := "1000000000000000";
constant implied : addrDef := "0000000000000000";
-- Zero page
constant readZp : addrDef := "1010000000000000";
constant writeZp : addrDef := "1010000000010000";
constant rmwZp : addrDef := "1010000000001000";
-- Zero page indexed
constant readZpX : addrDef := "1010000010000000";
constant writeZpX : addrDef := "1010000010010000";
constant rmwZpX : addrDef := "1010000010001000";
constant readZpY : addrDef := "1010000001000000";
constant writeZpY : addrDef := "1010000001010000";
constant rmwZpY : addrDef := "1010000001001000";
-- Zero page indirect
constant readIndX : addrDef := "1001000010000000";
constant writeIndX : addrDef := "1001000010010000";
constant rmwIndX : addrDef := "1001000010001000";
constant readIndY : addrDef := "1001000001000000";
constant writeIndY : addrDef := "1001000001010000";
constant rmwIndY : addrDef := "1001000001001000";
constant rmwInd : addrDef := "1001000000001000";
constant readInd : addrDef := "1001000000000000";
constant writeInd : addrDef := "1001000000010000";
-- |AZI||JBXY|WM||
-- Absolute
constant readAbs : addrDef := "1100000000000000";
constant writeAbs : addrDef := "1100000000010000";
constant rmwAbs : addrDef := "1100000000001000";
constant readAbsX : addrDef := "1100000010000000";
constant writeAbsX : addrDef := "1100000010010000";
constant rmwAbsX : addrDef := "1100000010001000";
constant readAbsY : addrDef := "1100000001000000";
constant writeAbsY : addrDef := "1100000001010000";
constant rmwAbsY : addrDef := "1100000001001000";
-- PHA PHP
constant push : addrDef := "0000010000000000";
-- PLA PLP
constant pop : addrDef := "0000010000100000";
-- Jumps
constant jsr : addrDef := "1000101000000000";
constant jumpAbs : addrDef := "1000001000000000";
constant jumpInd : addrDef := "1100001000000000";
constant jumpIndX : addrDef := "1100001010000000";
constant relative : addrDef := "1000000100000000";
-- Specials
constant rts : addrDef := "0000101000100100";
constant rti : addrDef := "0000111000100010";
constant brk : addrDef := "1000111000000001";
-- constant irq : addrDef := "0000111000000001";
-- constant : unsigned(0 to 0) := "0";
constant xxxxxxxx : addrDef := "----------0---00";
-- A = accu
-- X = index X
-- Y = index Y
-- S = Stack pointer
-- H = indexH
--
-- AEXYSTHc
constant aluInA : unsigned(0 to 7) := "10000000";
constant aluInBrk : unsigned(0 to 7) := "01000000";
constant aluInX : unsigned(0 to 7) := "00100000";
constant aluInY : unsigned(0 to 7) := "00010000";
constant aluInS : unsigned(0 to 7) := "00001000";
constant aluInT : unsigned(0 to 7) := "00000100";
constant aluInClr : unsigned(0 to 7) := "00000001";
constant aluInSet : unsigned(0 to 7) := "00000000";
constant aluInXXX : unsigned(0 to 7) := "--------";
-- Most of the aluModes are just like the opcodes.
-- aluModeInp -> input is output. calculate N and Z
-- aluModeCmp -> Compare for CMP, CPX, CPY
-- aluModeFlg -> input to flags needed for PLP, RTI and CLC, SEC, CLV
-- aluModeInc -> for INC but also INX, INY
-- aluModeDec -> for DEC but also DEX, DEY
subtype aluMode1 is unsigned(0 to 3);
subtype aluMode2 is unsigned(0 to 2);
subtype aluMode is unsigned(0 to 9);
-- Logic/Shift ALU
constant aluModeInp : aluMode1 := "0000";
constant aluModeP : aluMode1 := "0001";
constant aluModeInc : aluMode1 := "0010";
constant aluModeDec : aluMode1 := "0011";
constant aluModeFlg : aluMode1 := "0100";
constant aluModeBit : aluMode1 := "0101";
-- 0110
-- 0111
constant aluModeLsr : aluMode1 := "1000";
constant aluModeRor : aluMode1 := "1001";
constant aluModeAsl : aluMode1 := "1010";
constant aluModeRol : aluMode1 := "1011";
constant aluModeTSB : aluMode1 := "1100";
constant aluModeTRB : aluMode1 := "1101";
-- 1110
-- 1111;
-- Arithmetic ALU
constant aluModePss : aluMode2 := "000";
constant aluModeCmp : aluMode2 := "001";
constant aluModeAdc : aluMode2 := "010";
constant aluModeSbc : aluMode2 := "011";
constant aluModeAnd : aluMode2 := "100";
constant aluModeOra : aluMode2 := "101";
constant aluModeEor : aluMode2 := "110";
constant aluModeNoF : aluMode2 := "111";
--aluModeBRK
--constant aluBrk : aluMode := aluModeBRK & aluModePss & "---";
--constant aluFix : aluMode := aluModeInp & aluModeNoF & "---";
constant aluInp : aluMode := aluModeInp & aluModePss & "---";
constant aluP : aluMode := aluModeP & aluModePss & "---";
constant aluInc : aluMode := aluModeInc & aluModePss & "---";
constant aluDec : aluMode := aluModeDec & aluModePss & "---";
constant aluFlg : aluMode := aluModeFlg & aluModePss & "---";
constant aluBit : aluMode := aluModeBit & aluModeAnd & "---";
constant aluRor : aluMode := aluModeRor & aluModePss & "---";
constant aluLsr : aluMode := aluModeLsr & aluModePss & "---";
constant aluRol : aluMode := aluModeRol & aluModePss & "---";
constant aluAsl : aluMode := aluModeAsl & aluModePss & "---";
constant aluTSB : aluMode := aluModeTSB & aluModePss & "---";
constant aluTRB : aluMode := aluModeTRB & aluModePss & "---";
constant aluCmp : aluMode := aluModeInp & aluModeCmp & "100";
constant aluCpx : aluMode := aluModeInp & aluModeCmp & "010";
constant aluCpy : aluMode := aluModeInp & aluModeCmp & "001";
constant aluAdc : aluMode := aluModeInp & aluModeAdc & "---";
constant aluSbc : aluMode := aluModeInp & aluModeSbc & "---";
constant aluAnd : aluMode := aluModeInp & aluModeAnd & "---";
constant aluOra : aluMode := aluModeInp & aluModeOra & "---";
constant aluEor : aluMode := aluModeInp & aluModeEor & "---";
constant aluXXX : aluMode := (others => '-');
-- Stack operations. Push/Pop/None
constant stackInc : unsigned(0 to 0) := "0";
constant stackDec : unsigned(0 to 0) := "1";
constant stackXXX : unsigned(0 to 0) := "-";
subtype decodedBitsDef is unsigned(0 to 43);
type opcodeInfoTableDef is array(0 to 255) of decodedBitsDef;
constant opcodeInfoTable : opcodeInfoTableDef := (
-- +------- Update register A
-- |+------ Update register X
-- ||+----- Update register Y
-- |||+---- Update register S
-- |||| +-- Update Flags
-- |||| |
-- |||| _|__
-- |||| / \
-- AXYS NVDIZC addressing aluInput aluMode
-- AXYS NVDIZC addressing aluInput aluMode
"0000" & "001100" & brk & aluInBrk & aluP, -- 00 BRK
"1000" & "100010" & readIndX & aluInT & aluOra, -- 01 ORA (zp,x)
"0000" & "000000" & immediate & aluInXXX & aluXXX, -- 02 NOP ------- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 03 NOP ------- 65C02
"0000" & "000010" & rmwZp & aluInT & aluTSB, -- 04 TSB zp ----------- 65C02
"1000" & "100010" & readZp & aluInT & aluOra, -- 05 ORA zp
"0000" & "100011" & rmwZp & aluInT & aluAsl, -- 06 ASL zp
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 07 NOP ------- 65C02
"0000" & "000000" & push & aluInXXX & aluP, -- 08 PHP
"1000" & "100010" & immediate & aluInT & aluOra, -- 09 ORA imm
"1000" & "100011" & implied & aluInA & aluAsl, -- 0A ASL accu
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 0B NOP ------- 65C02
"0000" & "000010" & rmwAbs & aluInT & aluTSB, -- 0C TSB abs ---------- 65C02
"1000" & "100010" & readAbs & aluInT & aluOra, -- 0D ORA abs
"0000" & "100011" & rmwAbs & aluInT & aluAsl, -- 0E ASL abs
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 0F NOP ------- 65C02
"0000" & "000000" & relative & aluInXXX & aluXXX, -- 10 BPL
"1000" & "100010" & readIndY & aluInT & aluOra, -- 11 ORA (zp),y
"1000" & "100010" & readInd & aluInT & aluOra, -- 12 ORA (zp) --------- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 13 NOP ------- 65C02
"0000" & "000010" & rmwZp & aluInT & aluTRB, -- 14 TRB zp ~---------- 65C02
"1000" & "100010" & readZpX & aluInT & aluOra, -- 15 ORA zp,x
"0000" & "100011" & rmwZpX & aluInT & aluAsl, -- 16 ASL zp,x
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 17 NOP ------- 65C02
"0000" & "000001" & implied & aluInClr & aluFlg, -- 18 CLC
"1000" & "100010" & readAbsY & aluInT & aluOra, -- 19 ORA abs,y
"1000" & "100010" & implied & aluInA & aluInc, -- 1A INC accu --------- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 1B NOP ------- 65C02
"0000" & "000010" & rmwAbs & aluInT & aluTRB, -- 1C TRB abs ~----- --- 65C02
"1000" & "100010" & readAbsX & aluInT & aluOra, -- 1D ORA abs,x
"0000" & "100011" & rmwAbsX & aluInT & aluAsl, -- 1E ASL abs,x
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 1F NOP ------- 65C02
-- AXYS NVDIZC addressing aluInput aluMode
"0000" & "000000" & jsr & aluInXXX & aluXXX, -- 20 JSR
"1000" & "100010" & readIndX & aluInT & aluAnd, -- 21 AND (zp,x)
"0000" & "000000" & immediate & aluInXXX & aluXXX, -- 22 NOP ------- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 23 NOP ------- 65C02
"0000" & "110010" & readZp & aluInT & aluBit, -- 24 BIT zp
"1000" & "100010" & readZp & aluInT & aluAnd, -- 25 AND zp
"0000" & "100011" & rmwZp & aluInT & aluRol, -- 26 ROL zp
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 27 NOP ------- 65C02
"0000" & "111111" & pop & aluInT & aluFlg, -- 28 PLP
"1000" & "100010" & immediate & aluInT & aluAnd, -- 29 AND imm
"1000" & "100011" & implied & aluInA & aluRol, -- 2A ROL accu
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 2B NOP ------- 65C02
"0000" & "110010" & readAbs & aluInT & aluBit, -- 2C BIT abs
"1000" & "100010" & readAbs & aluInT & aluAnd, -- 2D AND abs
"0000" & "100011" & rmwAbs & aluInT & aluRol, -- 2E ROL abs
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 2F NOP ------- 65C02
"0000" & "000000" & relative & aluInXXX & aluXXX, -- 30 BMI
"1000" & "100010" & readIndY & aluInT & aluAnd, -- 31 AND (zp),y
"1000" & "100010" & readInd & aluInT & aluAnd, -- 32 AND (zp) -------- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 33 NOP ------- 65C02
"0000" & "110010" & readZpX & aluInT & aluBit, -- 34 BIT zp,x -------- 65C02
"1000" & "100010" & readZpX & aluInT & aluAnd, -- 35 AND zp,x
"0000" & "100011" & rmwZpX & aluInT & aluRol, -- 36 ROL zp,x
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 37 NOP ------- 65C02
"0000" & "000001" & implied & aluInSet & aluFlg, -- 38 SEC
"1000" & "100010" & readAbsY & aluInT & aluAnd, -- 39 AND abs,y
"1000" & "100010" & implied & aluInA & aluDec, -- 3A DEC accu -------- 65C12
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 3B NOP ------- 65C02
"0000" & "110010" & readAbsX & aluInT & aluBit, -- 3C BIT abs,x ------- 65C02
"1000" & "100010" & readAbsX & aluInT & aluAnd, -- 3D AND abs,x
"0000" & "100011" & rmwAbsX & aluInT & aluRol, -- 3E ROL abs,x
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 3F NOP ------- 65C02
-- AXYS NVDIZC addressing aluInput aluMode
"0000" & "111111" & rti & aluInT & aluFlg, -- 40 RTI
"1000" & "100010" & readIndX & aluInT & aluEor, -- 41 EOR (zp,x)
"0000" & "000000" & immediate & aluInXXX & aluXXX, -- 42 NOP ------- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 43 NOP ------- 65C02
"0000" & "000000" & immediate & aluInXXX & aluXXX, -- 44 NOP ------- 65C02
"1000" & "100010" & readZp & aluInT & aluEor, -- 45 EOR zp
"0000" & "100011" & rmwZp & aluInT & aluLsr, -- 46 LSR zp
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 47 NOP ------- 65C02
"0000" & "000000" & push & aluInA & aluInp, -- 48 PHA
"1000" & "100010" & immediate & aluInT & aluEor, -- 49 EOR imm
"1000" & "100011" & implied & aluInA & aluLsr, -- 4A LSR accu -------- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 4B NOP ------- 65C02
"0000" & "000000" & jumpAbs & aluInXXX & aluXXX, -- 4C JMP abs
"1000" & "100010" & readAbs & aluInT & aluEor, -- 4D EOR abs
"0000" & "100011" & rmwAbs & aluInT & aluLsr, -- 4E LSR abs
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 4F NOP ------- 65C02
"0000" & "000000" & relative & aluInXXX & aluXXX, -- 50 BVC
"1000" & "100010" & readIndY & aluInT & aluEor, -- 51 EOR (zp),y
"1000" & "100010" & readInd & aluInT & aluEor, -- 52 EOR (zp) -------- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 53 NOP ------- 65C02
"0000" & "000000" & immediate & aluInXXX & aluXXX, -- 54 NOP ------- 65C02
"1000" & "100010" & readZpX & aluInT & aluEor, -- 55 EOR zp,x
"0000" & "100011" & rmwZpX & aluInT & aluLsr, -- 56 LSR zp,x
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 57 NOP ------- 65C02
"0000" & "000100" & implied & aluInClr & aluXXX, -- 58 CLI
"1000" & "100010" & readAbsY & aluInT & aluEor, -- 59 EOR abs,y
"0000" & "000000" & push & aluInY & aluInp, -- 5A PHY ------------- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 5B NOP ------- 65C02
"0000" & "000000" & readAbs & aluInXXX & aluXXX, -- 5C NOP ------- 65C02
"1000" & "100010" & readAbsX & aluInT & aluEor, -- 5D EOR abs,x
"0000" & "100011" & rmwAbsX & aluInT & aluLsr, -- 5E LSR abs,x
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 5F NOP ------- 65C02
-- AXYS NVDIZC addressing aluInput aluMode
"0000" & "000000" & rts & aluInXXX & aluXXX, -- 60 RTS
"1000" & "110011" & readIndX & aluInT & aluAdc, -- 61 ADC (zp,x)
"0000" & "000000" & immediate & aluInXXX & aluXXX, -- 62 NOP ------- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 63 NOP ------- 65C02
"0000" & "000000" & writeZp & aluInClr & aluInp, -- 64 STZ zp ---------- 65C02
"1000" & "110011" & readZp & aluInT & aluAdc, -- 65 ADC zp
"0000" & "100011" & rmwZp & aluInT & aluRor, -- 66 ROR zp
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 67 NOP ------- 65C02
"1000" & "100010" & pop & aluInT & aluInp, -- 68 PLA
"1000" & "110011" & immediate & aluInT & aluAdc, -- 69 ADC imm
"1000" & "100011" & implied & aluInA & aluRor, -- 6A ROR accu
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 6B NOP ------ 65C02
"0000" & "000000" & jumpInd & aluInXXX & aluXXX, -- 6C JMP indirect
"1000" & "110011" & readAbs & aluInT & aluAdc, -- 6D ADC abs
"0000" & "100011" & rmwAbs & aluInT & aluRor, -- 6E ROR abs
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 6F NOP ------ 65C02
"0000" & "000000" & relative & aluInXXX & aluXXX, -- 70 BVS
"1000" & "110011" & readIndY & aluInT & aluAdc, -- 71 ADC (zp),y
"1000" & "110011" & readInd & aluInT & aluAdc, -- 72 ADC (zp) -------- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 73 NOP ------ 65C02
"0000" & "000000" & writeZpX & aluInClr & aluInp, -- 74 STZ zp,x -------- 65C02
"1000" & "110011" & readZpX & aluInT & aluAdc, -- 75 ADC zp,x
"0000" & "100011" & rmwZpX & aluInT & aluRor, -- 76 ROR zp,x
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 77 NOP ----- 65C02
"0000" & "000100" & implied & aluInSet & aluXXX, -- 78 SEI
"1000" & "110011" & readAbsY & aluInT & aluAdc, -- 79 ADC abs,y
"0010" & "100010" & pop & aluInT & aluInp, -- 7A PLY ------------- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 7B NOP ----- 65C02
"0000" & "000000" & jumpIndX & aluInXXX & aluXXX, -- 7C JMP indirect,x -- 65C02
--"0000" & "000000" & jumpInd & aluInXXX & aluXXX, -- 6C JMP indirect
"1000" & "110011" & readAbsX & aluInT & aluAdc, -- 7D ADC abs,x
"0000" & "100011" & rmwAbsX & aluInT & aluRor, -- 7E ROR abs,x
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 7F NOP ----- 65C02
-- AXYS NVDIZC addressing aluInput aluMode
"0000" & "000000" & relative & aluInXXX & aluXXX, -- 80 BRA ----------- 65C02
"0000" & "000000" & writeIndX & aluInA & aluInp, -- 81 STA (zp,x)
"0000" & "000000" & immediate & aluInXXX & aluXXX, -- 82 NOP ----- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 83 NOP ----- 65C02
"0000" & "000000" & writeZp & aluInY & aluInp, -- 84 STY zp
"0000" & "000000" & writeZp & aluInA & aluInp, -- 85 STA zp
"0000" & "000000" & writeZp & aluInX & aluInp, -- 86 STX zp
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 87 NOP ----- 65C02
"0010" & "100010" & implied & aluInY & aluDec, -- 88 DEY
"0000" & "000010" & immediate & aluInT & aluBit, -- 89 BIT imm ------- 65C02
"1000" & "100010" & implied & aluInX & aluInp, -- 8A TXA
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 8B NOP ----- 65C02
"0000" & "000000" & writeAbs & aluInY & aluInp, -- 8C STY abs ------- 65C02
"0000" & "000000" & writeAbs & aluInA & aluInp, -- 8D STA abs
"0000" & "000000" & writeAbs & aluInX & aluInp, -- 8E STX abs
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 8F NOP ----- 65C02
"0000" & "000000" & relative & aluInXXX & aluXXX, -- 90 BCC
"0000" & "000000" & writeIndY & aluInA & aluInp, -- 91 STA (zp),y
"0000" & "000000" & writeInd & aluInA & aluInp, -- 92 STA (zp) ------ 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 93 NOP ----- 65C02
"0000" & "000000" & writeZpX & aluInY & aluInp, -- 94 STY zp,x
"0000" & "000000" & writeZpX & aluInA & aluInp, -- 95 STA zp,x
"0000" & "000000" & writeZpY & aluInX & aluInp, -- 96 STX zp,y
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 97 NOP ----- 65C02
"1000" & "100010" & implied & aluInY & aluInp, -- 98 TYA
"0000" & "000000" & writeAbsY & aluInA & aluInp, -- 99 STA abs,y
"0001" & "000000" & implied & aluInX & aluInp, -- 9A TXS
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 9B NOP ----- 65C02
"0000" & "000000" & writeAbs & aluInClr & aluInp, -- 9C STZ Abs ------- 65C02
"0000" & "000000" & writeAbsX & aluInA & aluInp, -- 9D STA abs,x
"0000" & "000000" & writeAbsX & aluInClr & aluInp, -- 9C STZ Abs,x ----- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- 9F NOP ----- 65C02
-- AXYS NVDIZC addressing aluInput aluMode
"0010" & "100010" & immediate & aluInT & aluInp, -- A0 LDY imm
"1000" & "100010" & readIndX & aluInT & aluInp, -- A1 LDA (zp,x)
"0100" & "100010" & immediate & aluInT & aluInp, -- A2 LDX imm
"0000" & "000000" & implied & aluInXXX & aluXXX, -- A3 NOP ----- 65C02
"0010" & "100010" & readZp & aluInT & aluInp, -- A4 LDY zp
"1000" & "100010" & readZp & aluInT & aluInp, -- A5 LDA zp
"0100" & "100010" & readZp & aluInT & aluInp, -- A6 LDX zp
"0000" & "000000" & implied & aluInXXX & aluXXX, -- A7 NOP ----- 65C02
"0010" & "100010" & implied & aluInA & aluInp, -- A8 TAY
"1000" & "100010" & immediate & aluInT & aluInp, -- A9 LDA imm
"0100" & "100010" & implied & aluInA & aluInp, -- AA TAX
"0000" & "000000" & implied & aluInXXX & aluXXX, -- AB NOP ----- 65C02
"0010" & "100010" & readAbs & aluInT & aluInp, -- AC LDY abs
"1000" & "100010" & readAbs & aluInT & aluInp, -- AD LDA abs
"0100" & "100010" & readAbs & aluInT & aluInp, -- AE LDX abs
"0000" & "000000" & implied & aluInXXX & aluXXX, -- AF NOP ----- 65C02
"0000" & "000000" & relative & aluInXXX & aluXXX, -- B0 BCS
"1000" & "100010" & readIndY & aluInT & aluInp, -- B1 LDA (zp),y
"1000" & "100010" & readInd & aluInT & aluInp, -- B2 LDA (zp) ------ 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- B3 NOP ----- 65C02
"0010" & "100010" & readZpX & aluInT & aluInp, -- B4 LDY zp,x
"1000" & "100010" & readZpX & aluInT & aluInp, -- B5 LDA zp,x
"0100" & "100010" & readZpY & aluInT & aluInp, -- B6 LDX zp,y
"0000" & "000000" & implied & aluInXXX & aluXXX, -- B7 NOP ----- 65C02
"0000" & "010000" & implied & aluInClr & aluFlg, -- B8 CLV
"1000" & "100010" & readAbsY & aluInT & aluInp, -- B9 LDA abs,y
"0100" & "100010" & implied & aluInS & aluInp, -- BA TSX
"0000" & "000000" & implied & aluInXXX & aluXXX, -- BB NOP ----- 65C02
"0010" & "100010" & readAbsX & aluInT & aluInp, -- BC LDY abs,x
"1000" & "100010" & readAbsX & aluInT & aluInp, -- BD LDA abs,x
"0100" & "100010" & readAbsY & aluInT & aluInp, -- BE LDX abs,y
"0000" & "000000" & implied & aluInXXX & aluXXX, -- BF NOP ----- 65C02
-- AXYS NVDIZC addressing aluInput aluMode
"0000" & "100011" & immediate & aluInT & aluCpy, -- C0 CPY imm
"0000" & "100011" & readIndX & aluInT & aluCmp, -- C1 CMP (zp,x)
"0000" & "000000" & immediate & aluInXXX & aluXXX, -- C2 NOP ----- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- C3 NOP ----- 65C02
"0000" & "100011" & readZp & aluInT & aluCpy, -- C4 CPY zp
"0000" & "100011" & readZp & aluInT & aluCmp, -- C5 CMP zp
"0000" & "100010" & rmwZp & aluInT & aluDec, -- C6 DEC zp
"0000" & "000000" & implied & aluInXXX & aluXXX, -- C7 NOP ----- 65C02
"0010" & "100010" & implied & aluInY & aluInc, -- C8 INY
"0000" & "100011" & immediate & aluInT & aluCmp, -- C9 CMP imm
"0100" & "100010" & implied & aluInX & aluDec, -- CA DEX
"0000" & "000000" & implied & aluInXXX & aluXXX, -- CB NOP ----- 65C02
"0000" & "100011" & readAbs & aluInT & aluCpy, -- CC CPY abs
"0000" & "100011" & readAbs & aluInT & aluCmp, -- CD CMP abs
"0000" & "100010" & rmwAbs & aluInT & aluDec, -- CE DEC abs
"0000" & "000000" & implied & aluInXXX & aluXXX, -- CF NOP ----- 65C02
"0000" & "000000" & relative & aluInXXX & aluXXX, -- D0 BNE
"0000" & "100011" & readIndY & aluInT & aluCmp, -- D1 CMP (zp),y
"0000" & "100011" & readInd & aluInT & aluCmp, -- D2 CMP (zp) ------ 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- D3 NOP ----- 65C02
"0000" & "000000" & immediate & aluInXXX & aluXXX, -- D4 NOP ----- 65C02
"0000" & "100011" & readZpX & aluInT & aluCmp, -- D5 CMP zp,x
"0000" & "100010" & rmwZpX & aluInT & aluDec, -- D6 DEC zp,x
"0000" & "000000" & implied & aluInXXX & aluXXX, -- D7 NOP ----- 65C02
"0000" & "001000" & implied & aluInClr & aluXXX, -- D8 CLD
"0000" & "100011" & readAbsY & aluInT & aluCmp, -- D9 CMP abs,y
"0000" & "000000" & push & aluInX & aluInp, -- DA PHX ----------- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- DB NOP ----- 65C02
"0000" & "000000" & readAbs & aluInXXX & aluXXX, -- DC NOP ----- 65C02
"0000" & "100011" & readAbsX & aluInT & aluCmp, -- DD CMP abs,x
"0000" & "100010" & rmwAbsX & aluInT & aluDec, -- DE DEC abs,x
"0000" & "000000" & implied & aluInXXX & aluXXX, -- DF NOP ----- 65C02
-- AXYS NVDIZC addressing aluInput aluMode
"0000" & "100011" & immediate & aluInT & aluCpx, -- E0 CPX imm
"1000" & "110011" & readIndX & aluInT & aluSbc, -- E1 SBC (zp,x)
"0000" & "000000" & immediate & aluInXXX & aluXXX, -- E2 NOP ----- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- E3 NOP ----- 65C02
"0000" & "100011" & readZp & aluInT & aluCpx, -- E4 CPX zp
"1000" & "110011" & readZp & aluInT & aluSbc, -- E5 SBC zp
"0000" & "100010" & rmwZp & aluInT & aluInc, -- E6 INC zp
"0000" & "000000" & implied & aluInXXX & aluXXX, -- E7 NOP ----- 65C02
"0100" & "100010" & implied & aluInX & aluInc, -- E8 INX
"1000" & "110011" & immediate & aluInT & aluSbc, -- E9 SBC imm
"0000" & "000000" & implied & aluInXXX & aluXXX, -- EA NOP
"0000" & "000000" & implied & aluInXXX & aluXXX, -- EB NOP ----- 65C02
"0000" & "100011" & readAbs & aluInT & aluCpx, -- EC CPX abs
"1000" & "110011" & readAbs & aluInT & aluSbc, -- ED SBC abs
"0000" & "100010" & rmwAbs & aluInT & aluInc, -- EE INC abs
"0000" & "000000" & implied & aluInXXX & aluXXX, -- EF NOP ----- 65C02
"0000" & "000000" & relative & aluInXXX & aluXXX, -- F0 BEQ
"1000" & "110011" & readIndY & aluInT & aluSbc, -- F1 SBC (zp),y
"1000" & "110011" & readInd & aluInT & aluSbc, -- F2 SBC (zp) ------ 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- F3 NOP ----- 65C02
"0000" & "000000" & immediate & aluInXXX & aluXXX, -- F4 NOP ----- 65C02
"1000" & "110011" & readZpX & aluInT & aluSbc, -- F5 SBC zp,x
"0000" & "100010" & rmwZpX & aluInT & aluInc, -- F6 INC zp,x
"0000" & "000000" & implied & aluInXXX & aluXXX, -- F7 NOP ---- 65C02
"0000" & "001000" & implied & aluInSet & aluXXX, -- F8 SED
"1000" & "110011" & readAbsY & aluInT & aluSbc, -- F9 SBC abs,y
"0100" & "100010" & pop & aluInT & aluInp, -- FA PLX ----------- 65C02
"0000" & "000000" & implied & aluInXXX & aluXXX, -- FB NOP ----- 65C02
"0000" & "000000" & readAbs & aluInXXX & aluXXX, -- FC NOP ----- 65C02
"1000" & "110011" & readAbsX & aluInT & aluSbc, -- FD SBC abs,x
"0000" & "100010" & rmwAbsX & aluInT & aluInc, -- FE INC abs,x
"0000" & "000000" & implied & aluInXXX & aluXXX -- FF NOP ----- 65C02
);
signal opcInfo : decodedBitsDef;
signal nextOpcInfo : decodedBitsDef; -- Next opcode (decoded)
signal nextOpcInfoReg : decodedBitsDef; -- Next opcode (decoded) pipelined
signal theOpcode : unsigned(7 downto 0);
signal nextOpcode : unsigned(7 downto 0);
-- Program counter
signal PC : unsigned(15 downto 0); -- Program counter
-- Address generation
type nextAddrDef is (
nextAddrHold,
nextAddrIncr,
nextAddrIncrL, -- Increment low bits only (zeropage accesses)
nextAddrIncrH, -- Increment high bits only (page-boundary)
nextAddrDecrH, -- Decrement high bits (branch backwards)
nextAddrPc,
nextAddrIrq,
nextAddrReset,
nextAddrAbs,
nextAddrAbsIndexed,
nextAddrZeroPage,
nextAddrZPIndexed,
nextAddrStack,
nextAddrRelative
);
signal nextAddr : nextAddrDef;
signal myAddr : unsigned(15 downto 0);
signal myAddrIncr : unsigned(15 downto 0);
signal myAddrIncrH : unsigned(7 downto 0);
signal myAddrDecrH : unsigned(7 downto 0);
signal theWe : std_logic;
signal irqActive : std_logic;
-- Output register
signal doReg : unsigned(7 downto 0);
-- Buffer register
signal T : unsigned(7 downto 0);
-- General registers
signal A: unsigned(7 downto 0); -- Accumulator
signal X: unsigned(7 downto 0); -- Index X
signal Y: unsigned(7 downto 0); -- Index Y
signal S: unsigned(7 downto 0); -- stack pointer
-- Status register
signal C: std_logic; -- Carry
signal Z: std_logic; -- Zero flag
signal I: std_logic; -- Interrupt flag
signal D: std_logic; -- Decimal mode
signal B: std_logic; -- Break software interrupt
signal R: std_logic; -- always 1
signal V: std_logic; -- Overflow
signal N: std_logic; -- Negative
-- ALU
-- ALU input
signal aluInput : unsigned(7 downto 0);
signal aluCmpInput : unsigned(7 downto 0);
-- ALU output
signal aluRegisterOut : unsigned(7 downto 0);
signal aluRmwOut : unsigned(7 downto 0);
signal aluC : std_logic;
signal aluZ : std_logic;
signal aluV : std_logic;
signal aluN : std_logic;
-- Indexing
signal indexOut : unsigned(8 downto 0);
signal realbrk : std_logic;
begin
processAluInput: process(clk, opcInfo, A, X, Y, T, S)
variable temp : unsigned(7 downto 0);
begin
temp := (others => '1');
if opcInfo(opcInA) = '1' then
temp := temp and A;
end if;
if opcInfo(opcInX) = '1' then
temp := temp and X;
end if;
if opcInfo(opcInY) = '1' then
temp := temp and Y;
end if;
if opcInfo(opcInS) = '1' then
temp := temp and S;
end if;
if opcInfo(opcInT) = '1' then
temp := temp and T;
end if;
if opcInfo(opcInBrk) = '1' then
temp := temp and "11100111"; -- also DMB clear D (bit 3)
end if;
if opcInfo(opcInClear) = '1' then
temp := (others => '0');
end if;
aluInput <= temp;
end process;
processCmpInput: process(clk, opcInfo, A, X, Y)
variable temp : unsigned(7 downto 0);
begin
temp := (others => '1');
if opcInfo(opcInCmp) = '1' then
temp := temp and A;
end if;
if opcInfo(opcInCpx) = '1' then
temp := temp and X;
end if;
if opcInfo(opcInCpy) = '1' then
temp := temp and Y;
end if;
aluCmpInput <= temp;
end process;
-- ALU consists of two parts
-- Read-Modify-Write or index instructions: INC/DEC/ASL/LSR/ROR/ROL
-- Accumulator instructions: ADC, SBC, EOR, AND, EOR, ORA
-- Some instructions are both RMW and accumulator so for most
-- instructions the rmw results are routed through accu alu too.
-- The B flag
------------
--No actual "B" flag exists inside the 6502's processor status register. The B
--flag only exists in the status flag byte pushed to the stack. Naturally,
--when the flags are restored (via PLP or RTI), the B bit is discarded.
--
--Depending on the means, the B status flag will be pushed to the stack as
--either 0 or 1.
--
--software instructions BRK & PHP will push the B flag as being 1.
--hardware interrupts IRQ & NMI will push the B flag as being 0.
processAlu: process(clk, opcInfo, aluInput, aluCmpInput, A, T, irqActive, N, V, R, D, I, Z, C)
variable lowBits: unsigned(5 downto 0);
variable nineBits: unsigned(8 downto 0);
variable rmwBits: unsigned(8 downto 0);
variable tsxBits: unsigned(8 downto 0);
variable varC : std_logic;
variable varZ : std_logic;
variable varV : std_logic;
variable varN : std_logic;
begin
lowBits := (others => '-');
nineBits := (others => '-');
rmwBits := (others => '-');
tsxBits := (others => '-');
R <= '1';
-- Shift unit
case opcInfo(aluMode1From to aluMode1To) is
when aluModeInp => rmwBits := C & aluInput;
when aluModeP => rmwBits := C & N & V & R & (not irqActive) & D & I & Z & C; -- irqActive
when aluModeInc => rmwBits := C & (aluInput + 1);
when aluModeDec => rmwBits := C & (aluInput - 1);
when aluModeAsl => rmwBits := aluInput & "0";
when aluModeTSB => rmwBits := "0" & (aluInput(7 downto 0) or A); -- added by alan for 65c02
tsxBits := "0" & (aluInput(7 downto 0) and A);
when aluModeTRB => rmwBits := "0" & (aluInput(7 downto 0) and (not A)); -- added by alan for 65c02
tsxBits := "0" & (aluInput(7 downto 0) and A);
when aluModeFlg => rmwBits := aluInput(0) & aluInput;
when aluModeLsr => rmwBits := aluInput(0) & "0" & aluInput(7 downto 1);
when aluModeRol => rmwBits := aluInput & C;
when aluModeRoR => rmwBits := aluInput(0) & C & aluInput(7 downto 1);
when others => rmwBits := C & aluInput;
end case;
-- ALU
case opcInfo(aluMode2From to aluMode2To) is
when aluModeAdc => lowBits := ("0" & A(3 downto 0) & rmwBits(8)) + ("0" & rmwBits(3 downto 0) & "1");
ninebits := ("0" & A) + ("0" & rmwBits(7 downto 0)) + (B"00000000" & rmwBits(8));
when aluModeSbc => lowBits := ("0" & A(3 downto 0) & rmwBits(8)) + ("0" & (not rmwBits(3 downto 0)) & "1");
ninebits := ("0" & A) + ("0" & (not rmwBits(7 downto 0))) + (B"00000000" & rmwBits(8));
when aluModeCmp => ninebits := ("0" & aluCmpInput) + ("0" & (not rmwBits(7 downto 0))) + "000000001";
when aluModeAnd => ninebits := rmwBits(8) & (A and rmwBits(7 downto 0));
when aluModeEor => ninebits := rmwBits(8) & (A xor rmwBits(7 downto 0));
when aluModeOra => ninebits := rmwBits(8) & (A or rmwBits(7 downto 0));
when aluModeNoF => ninebits := "000110000";
when others => ninebits := rmwBits;
end case;
varV := aluInput(6); -- Default for BIT / PLP / RTI
if (opcInfo(aluMode1From to aluMode1To) = aluModeFlg) then
varZ := rmwBits(1);
elsif (opcInfo(aluMode1From to aluMode1To) = aluModeTSB) or (opcInfo(aluMode1From to aluMode1To) = aluModeTRB) then
if tsxBits(7 downto 0) = X"00" then
varZ := '1';
else
varZ := '0';
end if;
elsif ninebits(7 downto 0) = X"00" then
varZ := '1';
else
varZ := '0';
end if;
if (opcInfo(aluMode1From to aluMode1To) = aluModeBit) or (opcInfo(aluMode1From to aluMode1To) = aluModeFlg) then
varN := rmwBits(7);
else
varN := nineBits(7);
end if;
varC := ninebits(8);
case opcInfo(aluMode2From to aluMode2To) is
-- Flags Affected: n v — — — — z c
-- n Set if most significant bit of result is set; else cleared.
-- v Set if signed overflow; cleared if valid signed result.
-- z Set if result is zero; else cleared.
-- c Set if unsigned overflow; cleared if valid unsigned result
when aluModeAdc =>
-- decimal mode low bits correction, is done after setting Z flag.
if D = '1' then
if lowBits(5 downto 1) > 9 then
ninebits(3 downto 0) := ninebits(3 downto 0) + 6;
if lowBits(5) = '0' then
ninebits(8 downto 4) := ninebits(8 downto 4) + 1;
end if;
end if;
end if;
when others => null;
end case;
case opcInfo(aluMode2From to aluMode2To) is
when aluModeAdc =>
-- decimal mode high bits correction, is done after setting Z and N flags
varV := (A(7) xor ninebits(7)) and (rmwBits(7) xor ninebits(7));
if D = '1' then
if ninebits(8 downto 4) > 9 then
ninebits(8 downto 4) := ninebits(8 downto 4) + 6;
varC := '1';
end if;
end if;
when aluModeSbc =>
varV := (A(7) xor ninebits(7)) and ((not rmwBits(7)) xor ninebits(7));
if D = '1' then
-- Check for borrow (lower 4 bits)
if lowBits(5) = '0' then
ninebits(7 downto 0) := ninebits(7 downto 0) - 6;
end if;
-- Check for borrow (upper 4 bits)
if ninebits(8) = '0' then
ninebits(8 downto 4) := ninebits(8 downto 4) - 6;
end if;
end if;
when others => null;
end case;
-- fix n and z flag for 65c02 adc sbc instructions in decimal mode
case opcInfo(aluMode2From to aluMode2To) is
when aluModeAdc =>
if D = '1' then
if ninebits(7 downto 0) = X"00" then
varZ := '1';
else
varZ := '0';
end if;
varN := ninebits(7);
end if;
when aluModeSbc =>
if D = '1' then
if ninebits(7 downto 0) = X"00" then
varZ := '1';
else
varZ := '0';
end if;
varN := ninebits(7);
end if;
when others => null;
end case;
-- DMB Remove Pipelining
-- if rising_edge(clk) then
aluRmwOut <= rmwBits(7 downto 0);
aluRegisterOut <= ninebits(7 downto 0);
aluC <= varC;
aluZ <= varZ;
aluV <= varV;
aluN <= varN;
-- end if;
end process;
calcInterrupt: process(clk)
begin
if rising_edge(clk) then
if enable = '1' then
if theCpuCycle = cycleStack4 or reset = '0' then
nmiReg <= '1';
end if;
if nextCpuCycle /= cycleBranchTaken and nextCpuCycle /= opcodeFetch then
irqReg <= irq_n;
nmiEdge <= nmi_n;
if (nmiEdge = '1') and (nmi_n = '0') then
nmiReg <= '0';
end if;
end if;
-- The 'or opcInfo(opcSetI)' prevents NMI immediately after BRK or IRQ.
-- Presumably this is done in the real 6502/6510 to prevent a double IRQ.
processIrq <= not ((nmiReg and (irqReg or I)) or opcInfo(opcIRQ));
end if;
end if;
end process;
--pipeirq: process(clk)
-- begin
-- if rising_edge(clk) then
-- if enable = '1' then
-- if (reset = '0') or (theCpuCycle = opcodeFetch) then
-- -- The 'or opcInfo(opcSetI)' prevents NMI immediately after BRK or IRQ.
-- -- Presumably this is done in the real 6502/6510 to prevent a double IRQ.
-- processIrq <= not ((nmiReg and (irqReg or I)) or opcInfo(opcIRQ));
-- end if;
-- end if;
-- end if;
-- end process;
calcNextOpcode: process(clk, di, reset, processIrq)
variable myNextOpcode : unsigned(7 downto 0);
begin
-- Next opcode is read from input unless a reset or IRQ is pending.
myNextOpcode := di;
if reset = '0' then
myNextOpcode := X"4C";
elsif processIrq = '1' then
myNextOpcode := X"00";
end if;
nextOpcode <= myNextOpcode;
end process;
nextOpcInfo <= opcodeInfoTable(to_integer(nextOpcode));
-- DMB Remove Pipelining
-- process(clk)
-- begin
-- if rising_edge(clk) then
nextOpcInfoReg <= nextOpcInfo;
-- end if;
-- end process;
-- Read bits and flags from opcodeInfoTable and store in opcInfo.
-- This info is used to control the execution of the opcode.
calcOpcInfo: process(clk)
begin
if rising_edge(clk) then
if enable = '1' then
if (reset = '0') or (theCpuCycle = opcodeFetch) then
opcInfo <= nextOpcInfo;
end if;
end if;
end if;
end process;
calcTheOpcode: process(clk)
begin
if rising_edge(clk) then
if enable = '1' then
if theCpuCycle = opcodeFetch then
irqActive <= '0';
if processIrq = '1' then
irqActive <= '1';
end if;
-- Fetch opcode
theOpcode <= nextOpcode;
end if;
end if;
end if;
end process;
-- -----------------------------------------------------------------------
-- State machine
-- -----------------------------------------------------------------------
process(enable, theCpuCycle, opcInfo)
begin
updateRegisters <= false;
if enable = '1' then
if opcInfo(opcRti) = '1' then
if theCpuCycle = cycleRead then
updateRegisters <= true;
end if;
elsif theCpuCycle = opcodeFetch then
updateRegisters <= true;
end if;
end if;
end process;
process(clk)
begin
if rising_edge(clk) then
if enable = '1' then
theCpuCycle <= nextCpuCycle;
end if;
if reset = '0' then
theCpuCycle <= cycle2;
end if;
end if;
end process;
-- Determine the next cpu cycle. After the last cycle we always
-- go to opcodeFetch to get the next opcode.
calcNextCpuCycle: process(theCpuCycle, opcInfo, theOpcode, indexOut, T, N, V, C, Z)
begin
nextCpuCycle <= opcodeFetch;
case theCpuCycle is
when opcodeFetch => nextCpuCycle <= cycle2;
when cycle2 => if opcInfo(opcBranch) = '1' then
if (N = theOpcode(5) and theOpcode(7 downto 6) = "00")
or (V = theOpcode(5) and theOpcode(7 downto 6) = "01")
or (C = theOpcode(5) and theOpcode(7 downto 6) = "10")
or (Z = theOpcode(5) and theOpcode(7 downto 6) = "11")
or (theOpcode(7 downto 0) = x"80") then -- Branch condition is true
nextCpuCycle <= cycleBranchTaken;
end if;
elsif (opcInfo(opcStackUp) = '1') then
nextCpuCycle <= cycleStack1;
elsif opcInfo(opcStackAddr) = '1' and opcInfo(opcStackData) = '1' then
nextCpuCycle <= cycleStack2;
elsif opcInfo(opcStackAddr) = '1' then