Filters and BCJ/BCJ2

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)

After BCJ filter:

E8 [absolute address]
E9 [absolute address]

This makes similar code sections more compressible.

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

Four output streams:

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

Delta parameter specifies the distance between bytes to compute differences:

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

After Delta filter (distance=1):

100, 2, 3, 2, 3, 2

The transformed data compresses much better.

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

Performance Impact

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

Cross-Platform Executables

# 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

Getting Started

Command Reference

Compression Methods

Archive Formats

Advanced Usage

Filters and BCJ/BCJ2

What are Filters?

Available Filters

BCJ Filters (Branch-Call-Jump)

Delta Filter

BCJ Filter (x86)

How BCJ Works

Using BCJ Filter

Compression Improvement

BCJ2 Filter

Advanced x86 Filter

BCJ vs BCJ2

ARM Filters

ARM (Little-Endian)

ARM64 / AArch64

ARMT (ARM Thumb)

Delta Filter

Delta Example

Filter Chains

Automatic Filter Selection

Filter Implementation

BCJ Assembly Optimization

Performance Impact

Advanced Examples

Maximum Compression for Executables

Solid Archive with Filters

Cross-Platform Executables

When to Use Filters

Disabling Automatic Filters

See Also

​What are Filters?

​Available Filters

​BCJ Filters (Branch-Call-Jump)

​Delta Filter

​BCJ Filter (x86)

​How BCJ Works

​Using BCJ Filter

​Compression Improvement

​BCJ2 Filter

​Advanced x86 Filter

​BCJ vs BCJ2

​ARM Filters

​ARM (Little-Endian)

​ARM64 / AArch64

​ARMT (ARM Thumb)

​Delta Filter

​Delta Example

​Filter Chains

​Automatic Filter Selection

​Filter Implementation

​BCJ Assembly Optimization

​Performance Impact

​Advanced Examples

​Maximum Compression for Executables

​Solid Archive with Filters

​Cross-Platform Executables

​When to Use Filters

​Disabling Automatic Filters

​See Also

What are Filters?

Available Filters

BCJ Filters (Branch-Call-Jump)

Delta Filter

BCJ Filter (x86)

How BCJ Works

Using BCJ Filter

Compression Improvement

BCJ2 Filter

Advanced x86 Filter

BCJ vs BCJ2

ARM Filters

ARM (Little-Endian)

ARM64 / AArch64

ARMT (ARM Thumb)

Delta Filter

Delta Example

Filter Chains

Automatic Filter Selection

Filter Implementation

BCJ Assembly Optimization

Performance Impact

Advanced Examples

Maximum Compression for Executables

Solid Archive with Filters

Cross-Platform Executables

When to Use Filters

Disabling Automatic Filters

See Also