From 8bd8dd6ce07fb8e247f651b3c61c94bb1f00895a Mon Sep 17 00:00:00 2001
From: Väinö Kauppila <vaino@vke.fi>
Date: Sun, 19 Apr 2026 00:33:08 +0300
Subject: starting files and example programs

---
 PIC/decoder.vhd | 155 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 155 insertions(+)
 create mode 100644 PIC/decoder.vhd

(limited to 'PIC/decoder.vhd')

diff --git a/PIC/decoder.vhd b/PIC/decoder.vhd
new file mode 100644
index 0000000..5a1eb2c
--- /dev/null
+++ b/PIC/decoder.vhd
@@ -0,0 +1,155 @@
+library ieee;
+use ieee.std_logic_1164.all;
+use work.alu_types.all;
+
+package decoder is
+  type instruction_class_t is (
+    BIT_OP,
+    BYTE_OP,
+    LITERAL_OP,
+    CONTROL_OP,
+    UNKNOWN
+    );
+
+  type instruction_t is record
+    class : instruction_class_t;
+    op    : alu_op;
+    f     : std_logic_vector(6 downto 0);
+    b     : std_logic_vector(2 downto 0);
+    d     : std_logic;
+    k     : std_logic_vector(10 downto 0);
+  end record;
+
+  pure function instruction_get_class(opc : std_logic_vector(13 downto 0)) return instruction_class_t;
+
+  pure function instruction_get_operation(
+    opc         : std_logic_vector(13 downto 0);
+    instr_class : instruction_class_t)
+    return alu_op;
+
+  pure function instruction_decode(opc : std_logic_vector(13 downto 0)) return instruction_t;
+
+end package;
+
+package body decoder is
+
+  pure function instruction_decode(opc : std_logic_vector(13 downto 0))
+    return instruction_t is
+    variable instr : instruction_t;
+  begin
+    instr.class := instruction_get_class(opc);
+    instr.op    := instruction_get_operation(opc, instr.class);
+
+    instr.f := (others => '0');
+    instr.b := (others => '0');
+    instr.d := '0';
+    instr.k := (others => '0');
+
+    case instr.class is
+      when BIT_OP =>
+        instr.b := opc(9 downto 7);
+        instr.f := opc(6 downto 0);
+      when BYTE_OP =>
+        instr.f := opc(6 downto 0);
+        instr.d := opc(7);
+      when LITERAL_OP | CONTROL_OP =>
+        instr.k := opc(10 downto 0);
+      when UNKNOWN =>
+    end case;
+
+    return instr;
+  end function;
+
+  pure function instruction_get_class(opc : std_logic_vector(13 downto 0))
+    return instruction_class_t is
+  begin
+    -- RETURN (0x0008) has bits[13:12]="00" but must be treated as LITERAL_OP
+    if opc = "00000000001000" then return LITERAL_OP; end if;
+    case opc(13 downto 12) is
+      -- byte-oriented file-register operations
+      when "00" =>
+        return BYTE_OP;
+      when "01" =>
+        return BIT_OP;
+      when "10" =>
+        return CONTROL_OP;
+      when "11" =>
+        return LITERAL_OP;
+      when others =>
+        return UNKNOWN;
+    end case;
+  end function;
+
+  pure function instruction_get_operation(
+    opc         : std_logic_vector(13 downto 0);
+    instr_class : instruction_class_t)
+    return alu_op is
+  begin
+    case instr_class is
+
+      when BYTE_OP =>
+        case opc(11 downto 8) is
+          when "0000" =>
+            if opc(7) = '1' then return MOVWF;
+            else return NOP;
+            end if;
+          when "0001" =>
+            if opc(7) = '1' then return CLRF;
+            else return CLRW;
+            end if;
+          when "0010" => return SUBWF;
+          when "0011" => return DECF;
+          when "0100" => return IORWF;
+          when "0101" => return ANDWF;
+          when "0110" => return XORWF;
+          when "0111" => return ADDWF;
+          when "1000" => return MOVF;
+          when "1001" => return COMF;
+          when "1010" => return INCF;
+          when "1011" => return DECFSZ;
+          when "1100" => return RRF;
+          when "1101" => return RLF;
+          when "1110" => return SWAPF;
+          when "1111" => return INCFSZ;
+          when others => return NOP;
+        end case;
+
+      when BIT_OP =>
+        case opc(11 downto 10) is
+          when "00"   => return BCF;
+          when "01"   => return BSF;
+          when "10"   => return BTFSC;
+          when "11"   => return BTFSS;
+          when others => return NOP;
+        end case;
+
+      when CONTROL_OP =>
+        if opc(11) = '1' then return GOTO;
+        else return CALL;
+        end if;
+
+      when LITERAL_OP =>
+        -- fixed full-word patterns first (fall inside 00xx space)
+        -- if opc(11 downto 0) = "000000000001" then return RETFIE;
+        if opc(11 downto 0) = "000000001000" then return RETUR;
+        -- elsif opc(11 downto 0) = "000000000011" then return SLEEP;
+        -- elsif opc(11 downto 0) = "000000000100" then return CLRWDT;
+        else
+          case? opc(11 downto 8) is
+            when "00--" => return MOVLW;
+            when "01--" => return RETLW;
+            when "1000" => return IORLW;
+            when "1001" => return ANDLW;
+            when "1010" => return XORLW;
+            when "110-" => return SUBLW;
+            when "111-" => return ADDLW;
+            when others => return NOP;
+          end case?;
+        end if;
+
+      when others => return NOP;
+
+    end case;
+  end function;
+
+end decoder;
-- 
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