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# Oxford Pascal (C64, 1984) — Copy-Protection Analysis & De-Protector

A preservation project: a full reverse-engineering write-up of the two copy
protections on **Oxford Pascal v1.0** (© O.C.S.S. & D. Goodman, 1984) for the
Commodore 64, plus **`CRACK.prg`**, a one-step tool that produces a clean,
freely-copyable archival version of a disk you own — on real hardware, with a
single 1541/1541-II drive.

> Done strictly for preservation of software I legally own, on original
> hardware. Oxford Pascal has been out of print for decades. See
> [the legal/preservation note](#preservation--legal-note) below.

---

## The protection, in brief

There are two independent layers:

**1. The boot loader (`LD`) reads a hidden sector chain.**
Instead of loading the Pascal system as a normal directory file, `LD` reads it
from an absolute, undirectoried **Track 13 / Sector 5** sector-chain, following
the links by hand. Because the data is located by *physical geometry* rather
than by filename, an ordinary file copy never reproduces it — so a copied disk
loads garbage and crashes. (`LD` also redirects the NMI vector to an `RTI` so
RUN/STOP-RESTORE can't break the load.)

**2. The loaded Pascal program checks for a deliberately bad sector.**
Once `LD` hands control to the Pascal system image, right after the title
banner it reads **Track 13 / Sector 9** and *requires*
the drive to return **DOS error 23 (READ ERROR)** — that sector is written with
a bad checksum on the master. A clean copy returns error 00, the check fails,
and the machine drops into an infinite loop (banner-then-freeze).

Full disassembly-level detail is in **[docs/writeup.md](docs/writeup.md)**.

---

## How the bypass works

Both layers insist the data live in a specific physical place that normal
copying can't reproduce; the fix is to stop depending on geometry.

- **Layer 1:** reuse the loader's own routine to follow the T13/S5 chain and
  pull the image into normal memory.
- **Layer 2:** NOP the two branch instructions of the error-23 check (4 bytes:
  `$0881` and `$0888`).
- **Repackage:** write out a single, self-contained `LD` that loads at `$0100`
  like the original, embeds the patched image at `$0801`, and jumps straight
  into it — no hidden chain, no bad sector, no second file. The output is
  emitted byte-by-byte through a write channel (synthesising the auto-start
  stack page cleanly) rather than `SAVE`d from live memory.

## Using `CRACK.prg` (single 1541/1541-II)

1. Put `CRACK.prg` on a formatted work disk and `LOAD"CRACK",8,1` — it
   **auto-runs**.
2. **Insert the ORIGINAL**, press a key — it reads the hidden image and patches
   the bad-sector check in RAM. *Nothing is written to the original.*
3. **Insert the WORK disk**, press a key — it writes a finished de-protected
   `LD` and prints `DONE. CRACKED 'LD' WRITTEN.`
4. File-copy the rest of the original's normal files onto the work disk.

The work disk now boots straight into Pascal and copies with any file copier.
Step-by-step details: **[docs/PROCEDURE.md](docs/PROCEDURE.md)**.

---

## Repository layout

```
CRACK.prg                 the one-step de-protector (C64 PRG, load $0100)
src/CRACK.asm             commented source for CRACK.prg
disasm/LD.asm             annotated disassembly of the original boot loader
disasm/PASCAL-STARTUP.asm annotated disassembly of the Pascal startup +
                          the second-layer bad-sector check

docs/PROCEDURE.md         step-by-step de-protection procedure
```

The canonical binary is `CRACK.prg` (it was produced by direct, byte-verified
patching of the original loader). The `.asm` files are commented
disassembly/source for study; `src/CRACK.asm` uses generic 6502 syntax
(`* =`, `.byte`, `.text`) and documents exactly what `CRACK.prg` contains.

---

## Preservation & legal note

This repository exists for **software preservation, interoperability research,
and education**. **Oxford Pascal is © O.C.S.S. & D. Goodman, 1984**; all rights
to the original software remain with its authors/owners. No original Oxford
Pascal program files are distributed here — only original analysis, commentary,
and a tool that operates on a disk you already own.

The reverse-engineering write-up, the annotated disassembly comments, and the
de-protection tool are my own work, shared freely for the retrocomputing
community. I'm not a lawyer; copy-protection-circumvention and abandonware law
vary by country. Use this only with media you lawfully own, for your own
archival/preservation purposes.

---

## Credits

*Long live the breadbox.*

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