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Ben Eater 6502 Kit — Day 11

krischikPosted for Everyone to comment on, 2 years ago3 min read

Add reset vectors to ROM

Screenshot at Nov 11 192533.png

Up until now the 65C02 was only supplied with NOP operation. Including the reset and interrupt vectors making the CPU start execution at EAEA₁₆ while the first address of the ROM is 8000₁₆ which would also be the correct place to start the program.

So I updated the Ada program which generates the ROM image with so that the reset and interrupt vectors are all filled with 8000₁₆. Do read the comments for more informations on this over engendered code.

with Ada.Sequential_IO;
with Ada.IO_Exceptions;
with Ada.Command_Line;
with Ada.Text_IO;
with Interfaces;

procedure Create_Reset is

  type Byte is new Interfaces.Unsigned_8;
  type Address is new Interfaces.Unsigned_16;

  --  One of the cool feature of Ada is it's ability to set the lower and
  --  upper bound of an array to any value you want. So we can set use the
  --  actual ROM addresses as array index an save ourself the work and
  --  remove one potential error source.
  type ROM_Type is
     array (Address range 16#8000# .. 16#FFFF#)
     of Byte;

  package ROM_IO is new Ada.Sequential_IO (ROM_Type);

  ROM_Output : ROM_IO.File_Type;

  --  65C02 related constants as named in the original
  --  WDC design document
  NOP      : constant Byte    := 16#EA#;
  NMIB     : constant Address := 16#FFFA#;
  RESB     : constant Address := 16#FFFC#;
  BRK_IRQB : constant Address := 16#FFFE#;

  --  Another interesting feature Ada arrays is specifying the index of
  --  elements when initialising the array. As well specifying an value
  --  for all not explicitly initialised values. Makes setting up the
  --  ROM array super easy.
  ROM : constant ROM_Type :=
     (NMIB + 0     => 16#00#,
      NMIB + 1     => 16#80#,
      RESB + 0     => 16#00#,
      RESB + 1     => 16#80#,
      BRK_IRQB + 0 => 16#00#,
      BRK_IRQB + 1 => 16#80#,
      others       => NOP);

  if Ada.Command_Line.Argument_Count < 1 then
     Ada.Text_IO.Put_Line ("Syntax: Create_Reset <output file>");
        File_Name : constant String := Ada.Command_Line.Argument (1);
        ROM_IO.Create (ROM_Output, ROM_IO.Out_File, File_Name);
        when Ada.IO_Exceptions.Name_Error =>
           Ada.Text_IO.Put_Line ("File “" & File_Name & "” couldn't be created.");
           Ada.Text_IO.Put_Line ("See stack trace below for more details.");

     --  Just like Python Ada can write the whole array in one go.
     ROM_IO.Write (ROM_Output, ROM);
     ROM_IO.Close (ROM_Output);
  end if;
end Create_Reset;

Write the ROM image

Test using the Arduino

You find the full source code with makefiles, project files and source on GitLab: 6502Tutorial — Tools/Create_Reset

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