Samsung Firmware Files Explained: BL, AP, CP, CSC and HOME_CSC

The five files at a glance

You never flash CSC and HOME_CSC together. You pick one of them for the CSC slot. More on that below.

BL: the bootloader

BL is the smallest file, but it’s the one that starts everything. It holds the code that runs before Android loads, including the part that shows Download Mode.

On the Galaxy A05, the BL file contains preloader.img, lk-verified.img, param.bin, up_param.bin, efuse.img and vbmeta.img, all compressed as .lz4. The names are different on phones with Snapdragon or Exynos chips, but the job is the same.

The BL file also sets the bootloader revision. This is why you can’t downgrade some firmware. I’ll show you how to spot the revision in the firmware name further down.

AP: the operating system

AP is the big one. It’s the actual Android and One UI build, and on the Galaxy A05 it’s 7.8 GB on its own.

Inside, you’ll find boot.img, init_boot.img, vendor_boot.img, recovery.img, dtbo.img and a few security images. The biggest part is super.img, which packs the system, vendor and product partitions into one file. On the A05, that single image is 5.3 GB even when compressed.

The A05’s AP file also carries a userdata.img. Don’t worry, that doesn’t mean your data gets wiped every time. I explain how HOME_CSC handles it below.

If you’re rooting with Magisk, AP is the file you patch.

CP: the modem

CP holds the modem firmware. That’s what handles calls, mobile data and your SIM.

On the A05, it’s a single image called md1img.img.lz4. Wi-Fi-only tablets usually don’t come with a CP file at all, and that’s normal. Just leave the CP slot empty.

CSC: region settings and a full wipe

CSC is short for Consumer Software Customization. It sets things like your region’s apps, carrier settings, network features and the default language list.

Here’s the part most guides skip. The CSC file on the A05 also carries the phone’s partition table, a file called A05M_EUR_OPEN.pit, and it has no download list. So Odin writes every image in the package, including the empty userdata image from AP. That’s why a CSC flash erases everything.

So CSC gives you a clean start, like a new phone out of the box.

Use it when you’re fixing a bootloop, changing to a different CSC, selling the phone, or when a guide asks for a clean flash.

HOME_CSC: keep your data

HOME_CSC has the same region files as CSC, but no .pit file. Instead, it has a small text file called meta-data/download-list.txt that lists every image Odin should write.

I checked the list on the A05. userdata.img isn’t on it. That’s how HOME_CSC updates the phone and leaves your apps, photos and settings alone.

Use HOME_CSC when you’re updating to a newer build and want to keep your data. It’s also the safer choice when you’re flashing the same firmware again to fix a small software problem.

I still recommend taking a backup first. If something goes wrong in the middle of a flash, you may have to wipe anyway. I wrote a detailed comparison of the two on SamUpdater: CSC or HOME_CSC, which one should you flash.

Which file goes in which Odin slot

1. BL file in the BL slot.
2. AP file in the AP slot.
3. CP file in the CP slot.
4. CSC or HOME_CSC in the CSC slot. Never both.
5. Leave USERDATA empty unless your firmware actually came with a USERDATA file.

The same slots exist in Brokkr, and odin4 and samloader use the same letters on the command line. My Odin tutorial walks you through the full flash, step by step.

How to read a Samsung firmware name

Every file name looks like a random string, but each part means something. Here’s the BL file name from the A05 firmware.

BL_A055FXXSHDZF1_A055FXXSHDZF1_MQB110774676_REV00_user_low_ship_MULTI_CERT.tar.md5

• BL: the file type.
• A055FXXSHDZF1: the build version. It shows up twice.
• MQB110774676: Samsung’s internal build ID. You can ignore it.
• user_low_ship: a normal retail build, the kind that ships on phones in stores.
• .tar.md5: a regular tar archive with an MD5 checksum added at the end. Odin checks it before flashing.

The CSC file names show the CSC code too. The A05 file is CSC_ODM_A055FODMHDZF1, so ODM is the CSC package this firmware carries for India.

Decoding the build version

The build version is where the useful stuff is. Let’s take S928BXXS6DZH2, the latest build for the Galaxy S24 Ultra in Europe (EUX) when I checked Samsung’s update server.

Letter meanings are based on SamMobile’s firmware number guide. The S24 Ultra build is from Samsung’s update server, checked on September 28, 2026.

So S928BXXS6DZH2 is a security update from August 2026, with bootloader revision 6.

Once the bootloader revision goes past 9, Samsung switches to letters. That’s why the A05 build, A055FXXSHDZF1, has an H where you’d expect a number.

Why the bootloader revision matters

You can’t flash firmware with a lower bootloader revision than the one on your phone. If your S24 Ultra is on revision 6, a build with revision 5 will fail in Odin, usually with a SW REV. CHECK FAIL message.

So before you download older firmware, compare that one character. If it’s lower, that build won’t go on your phone. I explain that error and others in my Odin errors guide.

How to find your current firmware and CSC

1. Open Settings > About phone > Software information.
2. Look at Build number for your current build version, like S928BXXS6DZH2.
3. Look at Service provider software version for your CSC codes.

Then check if there’s a newer build for your model and region on SamUpdater. To download it, use Bifrost, which gets the files straight from Samsung’s servers.