Match secure area CRC

1 files changed, 25 insertions, 3 deletions

diff --git a/INSTALL.md b/INSTALL.md
index fe653525..43b80275 100644
--- a/INSTALL.md
+++ b/INSTALL.md
@@ -4,6 +4,7 @@ Contents:
 
 - [Prerequisites](#prerequisites)
 - [Build the ROM](#build-the-rom)
+    - [Matching the base ROM](#matching-the-base-rom)
 
 ## Prerequisites
 
@@ -19,9 +20,8 @@ python tools/setup.py
 
 ## Build the ROM
 
-This repository does not include any of the game's assets. You will need an original decrypted ROM for the version(s) you want
-to build.
-Put the original ROM in the root directory of this repository. Please verify that your dumped ROM matches one of the versions
+This repository does not include any of the game's assets, and you will need an original decrypted base ROM.  
+Put the base ROM in the root directory of this repository. Please verify that your dumped ROM matches one of the versions
 below:
 
 | Version | File name         | SHA1                                       | 
@@ -31,3 +31,25 @@ below:
 Run `make extract` to extract from the base ROM. You only need to do this once.
 
 Once you have extracted the base ROM, simply run `make` to rebuild it.
+
+### Matching the base ROM
+
+**This is optional!** You only need to follow these steps if you want a matching ROM.
+
+> [!NOTE]
+> For interested readers:  
+> Retail games are usually "encrypted," which means that the first 0x800 bytes of the secure area is encrypted using a
+4168-byte key found in the ARM7 BIOS. The secure area is 0x4000 bytes long and lives at the start of the ARM9 program at
+address 0x2000000.  
+> This encryption is optional, and games will run just fine without it. In fact, this project doesn't even produce an
+encrypted ROM. However, the ROM header includes a checksum of the secure area **after** encryption, so we must calculate it
+somehow.
+
+First, [extract the ARM7 BIOS from your DS device](https://wiki.ds-homebrew.com/ds-index/ds-bios-firmware-dump). Put the
+ARM7 BIOS in the root directory of this repository, and verify that your dumped BIOS matches the one below:
+
+| File name       | SHA1                                       | 
+| --------------- | ------------------------------------------ |
+| `arm7_bios.bin` | `6ee830c7f552c5bf194c20a2c13d5bb44bdb5c03` |
+
+Now, `make` should automatically detect the ARM7 BIOS and will build a matching ROM (soon™).