# ROM applications

Applications in this directory execute directly from ROM; the reset bootstrap
does not copy them. Each application must have its own non-overlapping `ORG`
below `8000` and should include `bios.inc` from the project's `include`
directory instead of accessing the UART hardware directly.

The current allocation is:

- `1400-1C0C`: Tiny BASIC
- `1C0D-2FFF`: available
- `3000-301D`: tape loader
- `301E-7FFF`: available for more applications (`3100` is the next convenient
  aligned origin)

The resident BIOS and monitor own high RAM at `C000-FFFF`. Tiny BASIC owns
`8000-BFFF` for its workspace and stack. Applications may reuse this lower-RAM
allocation because they run one at a time, but they must leave `C000-FFFF`
untouched.

Applications using the interrupt-driven console must leave interrupt mode 2,
the `I` register, and the BIOS UART/CTC configuration intact. They may return
normally if they preserve the caller's stack, or jump to `BIOS_MONITOR` for a
non-returning monitor warm start after replacing the stack pointer.

Use `BIOS_BANK`, `BIOS_ROM_DISABLE`, and `BIOS_ROM_ENABLE` for the write-only
banking latch rather than writing port `00` directly. The BIOS keeps the
ROM-disable state when the RAM bank selection changes. A ROM application must
not expect a normal `RET` after it maps out the ROM containing its return
address.

Application origins and the resident-memory layout are defined centrally in
`rom_layout.mk`. The build turns that file into C and assembly includes. To add
an application, add its address there, add its build and overlay rules to the
Makefile, and add a monitor command or use `G address` to launch it.