Ben Eater 6502 Kit — Day 11

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);

begin
  if Ada.Command_Line.Argument_Count < 1 then
     Ada.Text_IO.Put_Line ("Syntax: Create_Reset <output file>");
  else
     declare
        File_Name : constant String := Ada.Command_Line.Argument (1);
     begin
        ROM_IO.Create (ROM_Output, ROM_IO.Out_File, File_Name);
     exception
        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.");
           raise;
     end;

     --  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;

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