CSO file format: parts

The contents of the parts of a shader container, as Microsoft’s DirectXShaderCompiler headers and the LLVM project’s DirectX Container document describe them. All numbers are little-endian. Parts written only by the older fxc compiler (RDEF, ISGN, OSGN, OSG5, PCSG, SHDR, SHEX) and the STAT, RDAT and PDBI parts are not described in those sources and are not covered here.

DXIL and ILDB: program header

Offset	Size	Field	Meaning
`0`	4	`ProgramVersion`	Shader kind in bits 16–31, shader model major version in bits 4–7, minor in bits 0–3.
`4`	4	`SizeInUint32`	Size of the program in 32-bit words, including this header.
`8`	4	`DxilMagic`	`0x4C495844`, the text `DXIL`.
`12`	4	`DxilVersion`	DXIL version.
`16`	4	`BitcodeOffset`	Offset of the LLVM bitcode, counted from the start of `DxilMagic` (the bitcode header).
`20`	4	`BitcodeSize`	Size of the LLVM bitcode.

The bitcode is an LLVM 3.7 IR module (see LLVM bitcode). ILDB has the same layout but keeps debug information, which the stripped DXIL part leaves out.

Shader kind	Value	Shader kind	Value
Pixel	0	Intersection	8
Vertex	1	AnyHit	9
Geometry	2	ClosestHit	10
Hull	3	Miss	11
Domain	4	Callable	12
Compute	5	Mesh	13
Library	6	Amplification	14
RayGeneration	7	Node	15

Value 16 is Invalid. For example a shader model 6.5 compute shader has ProgramVersion 0x00050065.

ISG1, OSG1 and PSG1: signatures

A signature part starts with a header, then an array of 32-byte elements, then a string table holding the semantic names.

Offset	Size	Field	Meaning
`0`	4	`ParamCount`	Number of elements.
`4`	4	`ParamOffset`	Offset of the first element from the start of the part data.

Offset	Size	Field	Meaning
`0`	4	`Stream`	Stream index; elements appear in non-decreasing stream order.
`4`	4	`SemanticName`	Offset of the name string from the start of the part data; 0 means no name.
`8`	4	`SemanticIndex`	Semantic index.
`12`	4	`SystemValue`	System-value semantic; see below.
`16`	4	`CompType`	Component type; see below.
`20`	4	`Register`	Register (row) index.
`24`	1	`Mask`	Components (columns) the element uses.
`25`	1	`NeverWrites_Mask` / `AlwaysReads_Mask`	For an output signature, components the shader never writes; for an input signature, components it always reads.
`26`	2	`Pad`	Padding.
`28`	4	`MinPrecision`	Minimum precision; see below.

SystemValue	Name	SystemValue	Name
0	Undefined	14	FinalTriInsideTessfactor
1	Position	15	FinalLineDetailTessfactor
2	ClipDistance	16	FinalLineDensityTessfactor
3	CullDistance	23	Barycentrics
4	RenderTargetArrayIndex	24	ShadingRate
5	ViewPortArrayIndex	25	CullPrimitive
6	VertexID	64	Target
7	PrimitiveID	65	Depth
8	InstanceID	66	Coverage
9	IsFrontFace	67	DepthGE
10	SampleIndex	68	DepthLE
11	FinalQuadEdgeTessfactor	69	StencilRef
12	FinalQuadInsideTessfactor	70	InnerCoverage
13	FinalTriEdgeTessfactor

The header file notes that these values correspond to Direct3D’s D3D_NAME and D3D10_SB_NAME codes.

CompType	Name	MinPrecision	Name
0	Unknown	`0x00`	Default
1	UInt32	`0x01`	Float16
2	SInt32	`0x02`	Float2_8
3	Float32	`0x03`	Reserved
4	UInt16	`0x04`	SInt16
5	SInt16	`0x05`	UInt16
6	Float16	`0xF0`	Any16
7	UInt64	`0xF1`	Any10
8	SInt64
9	Float64

SFI0: feature flags

One 64-bit mask of optional features the shader needs.

Bit value	Name	Since
`0x0001`	Doubles
`0x0002`	ComputeShadersPlusRawAndStructuredBuffersViaShader4X
`0x0004`	UAVsAtEveryStage
`0x0008`	64UAVs
`0x0010`	MinimumPrecision
`0x0020`	11_1_DoubleExtensions
`0x0040`	11_1_ShaderExtensions
`0x0080`	LEVEL9ComparisonFiltering
`0x0100`	TiledResources
`0x0200`	StencilRef
`0x0400`	InnerCoverage
`0x0800`	TypedUAVLoadAdditionalFormats
`0x1000`	ROVs
`0x2000`	ViewportAndRTArrayIndexFromAnyShaderFeedingRasterizer
`0x4000`	WaveOps
`0x8000`	Int64Ops
`0x10000`	ViewID	SM 6.1
`0x20000`	Barycentrics	SM 6.1
`0x40000`	NativeLowPrecision	SM 6.2
`0x80000`	ShadingRate	SM 6.4
`0x100000`	Raytracing_Tier_1_1	SM 6.5
`0x200000`	SamplerFeedback	SM 6.5
`0x400000`	AtomicInt64OnTypedResource	SM 6.6
`0x800000`	AtomicInt64OnGroupShared	SM 6.6
`0x1000000`	DerivativesInMeshAndAmpShaders	SM 6.6
`0x2000000`	ResourceDescriptorHeapIndexing	SM 6.6
`0x4000000`	SamplerDescriptorHeapIndexing	SM 6.6
`0x8000000`	Reserved	experimental SM 6.9
`0x10000000`	AtomicInt64OnHeapResource	SM 6.6
`0x20000000`	AdvancedTextureOps	SM 6.7
`0x40000000`	WriteableMSAATextures	SM 6.7
`0x80000000`	SampleCmpGradientOrBias	SM 6.8
`0x100000000`	ExtendedCommandInfo	SM 6.8
`0x200000000`	LinearAlgebra	SM 6.10

These are the container’s feature bits; the DXIL module itself carries a separate 64-bit “shader flags” value in its metadata, documented in DXC’s DXIL.rst.

HASH, ILDN and VERS

Part	Offset	Size	Field	Meaning
`HASH`	`0`	4	`Flags`	0 none; 1 `IncludesSource`: the hash takes source information into account (`-Zss`).
`HASH`	`4`	16	`Digest`	MD5 hash of the shader.
`ILDN`	`0`	2	`Flags`	Reserved, zero.
	`2`	2	`NameLength`	Length of the name, without its NUL.
	`4`	`NameLength` + 1	name	UTF-8 name of the debug file (by default `<MD5>.pdb`) and a NUL, then 0–3 zero bytes to a 4-byte boundary.
`VERS`	`0`	2	`Major`	Compiler major version.
	`2`	2	`Minor`	Compiler minor version.
	`4`	4	`Flags`	0 default, 1 debug, 2 internal.
	`8`	4	`CommitCount`	LLVM always writes 0.
	`12`	4	`ContentSizeInBytes`	Size of the strings that follow.
	`16`	—	strings	The commit SHA and a custom version string, each ending in NUL.

SRCI: source information

A header (a 32-bit aligned size, 16-bit flags and a 16-bit section count, which must be 3), then three sections, each with its own header and 4-byte aligned entries:

Section type	Contents
0	Source contents; may be compressed with zlib (compression type 1).
1	Source file names.
2	Compiler arguments, as NUL-terminated name and value string pairs.

RTS0: root signature

Structure	Size	Fields
Header	24 bytes	Six 32-bit values: `Version`, `NumParameters`, `ParametersOffset`, `NumStaticSamplers`, `StaticSamplerOffset`, `Flags`.
Parameter header	12 bytes	`ParameterType`, `ShaderVisibility`, `ParameterOffset`.
Root constants	12 bytes	—
Root descriptor	8 bytes (version 1.0), 12 bytes (1.1, adds flags)	—
Descriptor range	20 bytes (1.0), 24 bytes (1.1)	—
Static sampler	52 bytes	—

The field lists of the smaller structures and the code values they use are defined in LLVM’s DXContainerConstants.def and Direct3D 12’s root signature documentation, which were not among the sources read for this page.

PSV0: pipeline state validation

A versioned set of structures, told apart by their encoded size rather than a version number. In order: a RuntimeInfo structure; the resource bindings (a count, then the element size when the count is not zero); the signature elements (a string table, an index table and lists per kind); and mask vectors, which pack 8 elements into each 32-bit word starting from the least significant bit. Output, patch-constant and input-to-output dependency tables follow depending on the shader stage and whether it uses the view ID. The inner layouts are not given in the sources read.

PRIV

Opaque bytes, such as an embedded PDB. It must be the last part, there may be only one, and its size need not be a multiple of four.

Sources

DxilContainer.h – Microsoft: program and bitcode headers, signature structures and their enums, HASH and ILDN structures, version helpers
DxilConstants.h – Microsoft: ShaderFeatureInfo flags and the ShaderKind values
DirectX Container – LLVM: VERS, SRCI, RTS0, PSV0, PRIV and the bitcode version
DXIL.rst – Microsoft: the DXIL module’s own shader flags

.CSO File Format – Parts