prog rom generation using Amaranth

1 files changed, 27 insertions, 35 deletions

diff --git a/hdl/state_machine.vhd b/hdl/state_machine.vhd
index 33d2f33..1ab862e 100644
--- a/hdl/state_machine.vhd
+++ b/hdl/state_machine.vhd
@@ -16,7 +16,6 @@ entity state_machine is
     we_mem     : out std_logic                    := '0';
     re_mem     : out std_logic                    := '0';
     we_w       : out std_logic                    := '0';
-    instr_ret  : out std_logic                    := '0';
     bit_select : out std_logic_vector(2 downto 0) := (others => '0');
     data       : out std_logic_vector(7 downto 0) := (others => '0');
     pc         : out std_logic_vector(12 downto 0);
@@ -68,7 +67,6 @@ begin
       we_mem     <= '0';
       we_w       <= '0';
       we_status  <= '0';
-      instr_ret  <= '0';
       stack_push <= '0';
       stack_pop  <= '0';
       gie_clear  <= '0';
@@ -86,28 +84,28 @@ begin
 
             -- Hardware interrupt: push PC, jump to vector, clear GIE
             if irq_pending = '1' and not do_phantom_nop then
-              stack_din    <= pc_internal;
-              stack_push   <= '1';
-              pc_internal  <= "0000000000100";  -- 0x0004
-              gie_clear    <= '1';
-              instr          := instruction_decode((others => '0'));
-              is_phantom     := true;
+              stack_din       <= pc_internal;
+              stack_push      <= '1';
+              pc_internal     <= "0000000000100";  -- 0x0004
+              gie_clear       <= '1';
+              instr           := instruction_decode((others => '0'));
+              is_phantom      := true;
               is_skip_phantom := false;
-              do_phantom_nop := false;
-              state <= MRead;
+              do_phantom_nop  := false;
+              state           <= MRead;
 
-            -- if we had a branch before, this cycle is a NOP.
+              -- if we had a branch before, this cycle is a NOP.
             elsif do_phantom_nop then
               instr          := instruction_decode((others => '0'));
               do_phantom_nop := false;
               is_phantom     := true;
               -- is_skip_phantom carries over from whoever set do_phantom_nop
-              state <= MRead;
+              state          <= MRead;
             else
-              instr          := instruction_decode(opcode);
-              is_phantom     := false;
+              instr           := instruction_decode(opcode);
+              is_phantom      := false;
               is_skip_phantom := false;
-              state <= MRead;
+              state           <= MRead;
             end if;
 
             -- set use_literal based on decoded instruction, holds through MWrite
@@ -149,8 +147,8 @@ begin
               -- increment PC or change address, different for different instructions.
               if instr.op = GOTO then
 
-                pc_internal    <= "00" & instr.k;
-                do_phantom_nop := true;
+                pc_internal     <= "00" & instr.k;
+                do_phantom_nop  := true;
                 is_skip_phantom := false;
 
               elsif instr.op = CALL then
@@ -160,16 +158,16 @@ begin
                 stack_push <= '1';
 
                 -- go to the function we called
-                pc_internal    <= "00" & instr.k;
-                do_phantom_nop := true;
+                pc_internal     <= "00" & instr.k;
+                do_phantom_nop  := true;
                 is_skip_phantom := false;
 
               elsif instr.op = RETUR or instr.op = RETLW or instr.op = RETFIE then
 
                 -- pop the PC from stack.
-                pc_internal    <= stack_dout;
-                stack_pop      <= '1';
-                do_phantom_nop := true;
+                pc_internal     <= stack_dout;
+                stack_pop       <= '1';
+                do_phantom_nop  := true;
                 is_skip_phantom := false;
                 if instr.op = RETFIE then
                   gie_set <= '1';
@@ -202,8 +200,8 @@ begin
                       end if;
                       -- Write to PCL (addr 0x02): redirect PC, like a GOTO
                       if instr.f = "0000010" then
-                        pc_internal    <= pclath & alu_result;
-                        do_phantom_nop := true;
+                        pc_internal     <= pclath & alu_result;
+                        do_phantom_nop  := true;
                         is_skip_phantom := false;
                       end if;
                     else
@@ -225,23 +223,17 @@ begin
 
             -- DECFSZ/INCFSZ/BTFSS/BTFSC: skip next instruction if ALU signals skip
             if not is_phantom and
-               (instr.op = DECFSZ or instr.op = INCFSZ or instr.op = BTFSS or instr.op = BTFSC) and alu_skip = '1' then
+              (instr.op = DECFSZ or instr.op = INCFSZ or instr.op = BTFSS or instr.op = BTFSC) and alu_skip = '1' then
               do_phantom_nop  := true;
               is_skip_phantom := true;
             end if;
 
-            if not is_phantom then
+            if not is_phantom and
+              instr.op /= GOTO and instr.op /= CALL and
+              instr.op /= RETUR and instr.op /= RETLW then
               we_status <= '1';
-              -- branch instructions complete over 2 cycles; pulse instr_ret only
-              -- on the phantom NOP cycle (below), not here
-              if instr.op /= GOTO and instr.op /= CALL and
-                 instr.op /= RETUR and instr.op /= RETLW then
-                instr_ret <= '1';
-              end if;
-            elsif not is_skip_phantom then
-              -- branch phantom NOP: this is the true end of the 2-cycle instruction
-              instr_ret <= '1';
             end if;
+
             state <= IFetch;
 
         end case;