7-Zip filters are preprocessing algorithms that transform data before compression to achieve better compression ratios for specific file types.
What are Filters?
Filters preprocess data to make it more compressible:
- Filter - Transform data (e.g., convert relative jumps in executables)
- Compress - Apply LZMA or other compression
- Result - Better compression ratio
Filters are lossless and reversible transformations.
Available Filters
BCJ Filters (Branch-Call-Jump)
Designed for executable files:
| Filter | Method ID | Architecture | Files |
|---|
| BCJ | 04 | x86 (32-bit) | .exe, .dll, .sys |
| BCJ2 | 03 03 01 1B | x86 (advanced) | .exe, .dll (better compression) |
| ARM | 07 | ARM (little-endian) | ARM executables |
| ARMT | 08 | ARM Thumb | ARM Thumb binaries |
| ARM64 | 0A | ARM64 / AArch64 | 64-bit ARM |
| PPC | 05 | PowerPC (big-endian) | PowerPC executables |
| IA64 | 06 | Itanium | IA-64 binaries |
| SPARC | 09 | SPARC | SPARC binaries |
| RISCV | 0B | RISC-V | RISC-V executables |
Delta Filter
Method ID: 03
For data with small numeric differences:
- BMP images
- WAV audio
- Numeric data arrays
- Database dumps
BCJ Filter (x86)
How BCJ Works
Converts relative jumps and calls in x86 code to absolute addresses:
Original x86 code:
E8 45 00 00 00 call +0x45 (relative)
E9 10 00 00 00 jmp +0x10 (relative)
E8 [absolute address]
E9 [absolute address]
Using BCJ Filter
# Automatic - 7-Zip detects .exe/.dll files
7z a archive.7z program.exe
# Manual - force BCJ filter
7z a -m0=BCJ -m1=LZMA archive.7z program.exe
# With maximum compression
7z a -m0=BCJ -m1=LZMA2:d=64m archive.7z *.exe *.dll
7-Zip automatically applies BCJ filter to .exe and .dll files when compressing to 7z format.
Compression Improvement
Typical compression improvement with BCJ:
| File Type | Without BCJ | With BCJ | Improvement |
|---|
| .exe (native) | 45% | 35% | 22% better |
| .dll (system) | 50% | 38% | 24% better |
| .sys (driver) | 48% | 36% | 25% better |
BCJ2 Filter
Advanced x86 Filter
BCJ2 is a more sophisticated filter that separates executable code into multiple streams:
# Use BCJ2 for maximum compression
7z a -m0=BCJ2 -m1=LZMA2 archive.7z program.exe
- Main data stream
- CALL/JMP targets
- JUMP targets
- Control bytes
BCJ vs BCJ2
| Feature | BCJ | BCJ2 |
|---|
| Compression | Good | Better (2-5% more) |
| Speed | Fast | Slower |
| Complexity | Simple (1 stream) | Complex (4 streams) |
| Streams | 1 | 4 |
| Use case | General | Maximum compression |
# Compare compression
# BCJ (faster)
7z a -m0=BCJ -m1=LZMA2 -mx=9 bcj.7z program.exe
# BCJ2 (smaller)
7z a -m0=BCJ2 -m1=LZMA2 -mx=9 bcj2.7z program.exe
BCJ2 provides 2-5% better compression than BCJ but is slower to compress and decompress.
ARM Filters
ARM (Little-Endian)
# For ARM executables
7z a -m0=ARM -m1=LZMA2 archive.7z arm_program
ARM64 / AArch64
# For 64-bit ARM
7z a -m0=ARM64 -m1=LZMA2 archive.7z aarch64_program
ARMT (ARM Thumb)
# For ARM Thumb mode
7z a -m0=ARMT -m1=LZMA2 archive.7z thumb_program
Delta Filter
For data with small differences between consecutive bytes:
# Delta filter for BMP images
7z a -m0=Delta:1 -m1=LZMA2 archive.7z image.bmp
# Delta filter for 16-bit WAV
7z a -m0=Delta:2 -m1=LZMA2 archive.7z audio.wav
# Delta filter for 32-bit data
7z a -m0=Delta:4 -m1=LZMA2 archive.7z data.raw
1 - Adjacent bytes (8-bit images)2 - 16-bit values (stereo audio)4 - 32-bit values (RGB images, int arrays)
Delta Example
Original data:
100, 102, 105, 107, 110, 112
Filter Chains
Combine multiple filters:
# BCJ + LZMA2 (common for executables)
7z a -m0=BCJ -m1=LZMA2 archive.7z program.exe
# Delta + LZMA2 (for images)
7z a -m0=Delta:1 -m1=LZMA2 archive.7z image.bmp
# BCJ2 + LZMA2 + LZMA2 + LZMA2 + LZMA2 (for 4 BCJ2 streams)
7z a -m0=BCJ2 -m1=LZMA2 -m2=LZMA2 -m3=LZMA2 -m4=LZMA2 archive.7z program.exe
Automatic Filter Selection
7-Zip automatically selects filters based on file extension:
| Extension | Filter Applied |
|---|
| .exe, .dll, .sys | BCJ (x86) |
| .arm | ARM |
| .bmp | Delta |
| .wav | Delta |
| (others) | None |
# Automatic - 7-Zip chooses filter
7z a archive.7z *.exe *.dll *.bmp *.wav
Filter Implementation
Filter source code locations:
- BCJ:
C/Bra86.c (lines 1-124)
- BCJ2:
C/Bcj2.c (lines 1-274)
- ARM:
C/Bra.c (ARM filter section)
- ARM64:
C/Bra.c (ARM64 filter section)
- Delta:
C/Delta.c (lines 1-93)
BCJ Assembly Optimization
Optimized versions with x86-64 assembly:
Asm/x86/7zCrcOpt.asm - CRC with BCJ preprocessing- Uses SIMD instructions for faster filtering
| Filter | Compression Speed | Decompression Speed | Compression Gain |
|---|
| BCJ | -5% | -5% | +20-25% |
| BCJ2 | -15% | -15% | +23-30% |
| ARM | -5% | -5% | +20-25% |
| Delta | -2% | -2% | +10-40% |
Filters have minimal speed impact but significant compression improvements for appropriate file types.
Advanced Examples
Maximum Compression for Executables
7z a -t7z -m0=BCJ2 -m1=LZMA2:d=64m:fb=273 -m2=LZMA2:d=64m:fb=273 \
-m3=LZMA2:d=64m:fb=273 -m4=LZMA2:d=64m:fb=273 \
-mx=9 -ms=on -mmt=on archive.7z program.exe
Solid Archive with Filters
# Multiple executables with BCJ
7z a -ms=on -m0=BCJ -m1=LZMA2 archive.7z *.exe *.dll
# Archive with mixed architectures
7z a archive.7z \
-m0=BCJ x86/*.exe \
-m0=ARM64 arm64/*.bin \
-m0=ARM arm/*.elf
When to Use Filters
Use filters for:
- Executable files (.exe, .dll, .so)
- ARM/ARM64 binaries
- BMP images
- WAV audio files
- Raw numeric data
Don’t use filters for:
- Already compressed files (JPEG, PNG, MP3)
- Encrypted data
- Random data
- Text files (no benefit)
Disabling Automatic Filters
# Disable automatic filter selection
7z a -m0=Copy -m1=LZMA2 archive.7z program.exe
See Also