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rom4x

★ 91 Open GitHub ↗

Improved Apple //c (ROM 4X) and Apple IIc Plus (ROM 5X) firmware.

// repository documentation

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# ROM 4X and 5X by MG

ROM 4X and 5X are enhancements to Apple //c version 4 and Apple IIc Plus
firmware ROMs.

It adds the following features to the Apple //c and IIc Plus firmware:

- Identifies and reinstates a *bootable* (it must have something that looks
  like a boot block!) RAM disk from battery-backed expansion memory (see below),
  such as the [RAM Express II+](http://a2heaven.com/webshop/index.php?rt=product/product&product_id=144) from A2Heaven.
- Provides a menu of various tools upon pressing Ctrl+Closed-Apple+Reset (or
  holding Closed-Apple when powering up), that let you:
  - Enter the monitor unconditionally.
  - Reboot the machine (enter the standard boot sequence).
  - Zero the RAM card, in case it is corrupted.
  - Execute the machine and RAM card diagnostics.
  - Tell the machine to boot the SmartPort, the internal floppy drive, or an
    external floppy drive.
- IIc only:
  - The system drops to BASIC if no bootable device is found (this is the
    default behavior in the IIc Plus).
  - Configure the default boot device by saving a file on the RAM Disk.
  - [XModem-CRC](rom4x/xmodem.md) support.
- IIc Plus only:
  - Menu control for the built-in accelerator.
  - Accelerator settings persist across resets.
  - Build option for defaulting the system to 1 MHz.

For 09/12/25 or later versions:

- Systems with a JD Micro ROMXc (ROM 4X only for now):
  - Enter the ROMXc menu from the ROM4X menu.
- Systems with a JD Micro Xdrive IIC (ROM 4X only for now):
  - Enter the Xdrive configuration menu from the ROM4x Menu.
  - Boot the Xdrive from the ROM4X menu.

The first feature listed above is the *raison d'être* for this project.  The
larger story is down below, but in short: The Apple //c memory card driver keeps
certain information in the "screen holes" in main memory, which are required to
use the memory card as a RAM disk.  Should these screen hole values disappear,
the card is re-initialized to empty when ProDOS boots.  This happens even when
the card is battery-backed and already has a RAM disk.  The card data is not
damaged until ProDOS boots, but if you attempt to manually boot the RAM disk, it
will say "UNABLE TO START FROM MEMORY CARD" because the screen hole values are
not initialized.

This firmware enhancement identifies a ProDOS boot block on the RAM disk and, if
found, restores the appropriate screen holes to make the RAM disk bootable and
prevent firmware or ProDOS from re-initializing it.

For Apple //c systems, ROM 4X identifies the JD Micro ROMXc and Xdrive IIc and
adjusts its behavior for these devices.  JD Micro device features are not yet
available on the Apple IIc Plus, and will require separate ROM images to support
the devices due to a lack of available free space in the ROM.

## Build / Install / Documentation

### ROM 4X

See the [ROM 4X README.md](rom4x/README.md).

### ROM 5X

See the [ROM 5X README.md](rom5x/README.md).

## The Whole Story

Why did I do this?  See [story.md](story.md).

rom4x

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