ROM Software for the SBC

In order to get minimum access to our 6502 based computer, we need some basic software. Here is a set of tools designed to provide basic I/Os and programming capability to the computer.

Where to store this software ?

Multiple options:

1. EEPROM: Burn it to an EEPROM to have it permanently available.
2. Emulator: ROM image to be used by an emulator.
3. RAM: it can be uploaded to RAM after each boot if the computer supports it.

Constraint: The reset Vector needs to be at address $FFFC

Which software is included ?

The ROM image can be compiled to contain three different software.

• BIOS: Entrypoint at startup or Reset, along with I/O Handlers for 6850 or 6551 ACIA. The menu can displays[B]asic and/or [M]onitor depending on what is compiled.
• Monitor: Jeff Tranter's JMON adapted to compile with Merlin32
• [REMOVED] BASIC: ehBasic to be compiled with Merlin32

BIOS must be part of the ROM image, but Basic and Monitor are optional. The default memory map with both Basic and Monitor is:

$0000-$BFFF		RAM
$C100-$C1FF		ACIA
$C200-$C2FF		VIA
$D000-$FCFF		Monitor
$FD00-$FFFF		BIOS

This requires a 16k EEPROM to run and associated decoding logic.

Compile

Required:

• Merlin32 Assembler: You can get it for Windows, Mac or Linux from Brutal Deluxe Software. Merlin32 should be in the PATH and all the macros from its Library in /usr/local/includes/merlin32.

Optional:

• SRecord EEPROM utilities: In order to convert the ROM image to Intel HEX format, you will need SRecord. For Mac users, you can get it through brew.

• In order to test the ROM without burning it each time, I used SYMON, with a small modification to the Multicomp setup to simulate the SBC.

Here the modified code from src/main/java/com/loomcom/symon/machines/MulticompMachine.java

public class MulticompMachine implements Machine {

  private final static Logger logger = Logger.getLogger(MulticompMachine.class.getName());
  
  // Constants used by the simulated system. These define the memory map.
  private static final int BUS_BOTTOM = 0x0000;
  private static final int BUS_TOP    = 0xffff;

  // 40K of RAM from $0000 - $9FFF
  private static final int MEMORY_BASE = 0x0000;
  private static final int MEMORY_SIZE = 0xC000;

  // ACIA at $A000-$BFFF
  private static final int ACIA_BASE = 0xC100;

  // SD controller at $FFD8-$FFDF
  //private static final int SD_BASE = 0xFFD8;

  // 16KB ROM at $C000-$FFFF
  private static final int ROM_BASE = 0xD000;
  private static final int ROM_SIZE = 0x3000;


      // The simulated peripherals
  private final Bus    bus;
  private final Cpu    cpu;
  private final Acia   acia;
  private final Memory ram;
  private       Memory rom;


  public MulticompMachine() throws Exception {
      this.bus = new Bus(BUS_BOTTOM, BUS_TOP);
      this.cpu = new Cpu();
      this.ram = new Memory(MEMORY_BASE, MEMORY_BASE + MEMORY_SIZE - 1, false);
      this.acia = new Acia6551(ACIA_BASE);
      this.acia.setBaudRate(0);

      bus.addCpu(cpu);
      bus.addDevice(ram);
      bus.addDevice(acia, 1);
      //bus.addDevice(new SdController(SD_BASE), 1);
      
      // TODO: Make this configurable, of course.
      File romImage = new File("rom.bin");
      if (romImage.canRead()) {
          logger.info("Loading ROM image from file " + romImage);
          this.rom = Memory.makeROM(ROM_BASE, ROM_BASE + ROM_SIZE - 1, romImage);
      } else {
          logger.info("Default ROM file " + romImage +
                      " not found, loading empty R/W memory image.");
          this.rom = Memory.makeRAM(ROM_BASE, ROM_BASE + ROM_SIZE - 1);
      }

      bus.addDevice(rom);
      
  } 
}   

Building the ROM:

First, you need to chose memory locations and adjust it in the ROM.s file. If you don't want to use Monitor or Basic, simply edit the BASICSTART or MONITORSTART to be $0000.

Among other settings, you can select the type of ACIA to use by setting the proper valuer for ACIATYPE.

Finally, choose a name for your ROM by editing the value of DSK directive

Then, simply:
make clean ; make

This will produce 2 files :

1. <YOURROMNAME>.bin: pure binary file to be used with 6502 Simulator (SYMON)
2. <YOURROMNAME>.hex: Intel HEX file format, easier to manipulate with EEPROM burner.

Now you just need to burn it to EEPROM or any emulator software.