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-rw-r--r--Lessons/Lesson_22/README.md127
1 files changed, 62 insertions, 65 deletions
diff --git a/Lessons/Lesson_22/README.md b/Lessons/Lesson_22/README.md
index 5280e72..2d23674 100644
--- a/Lessons/Lesson_22/README.md
+++ b/Lessons/Lesson_22/README.md
@@ -1,117 +1,114 @@
-Let's explore another PCD type - feature flag PCD.
+Let's explore another PCD type - feature flag PCD. This is basically a boolean value.
-Add PCD to DEC file UefiLessonsPkg/UefiLessonsPkg.dec:
+Add this PCD to DEC file `UefiLessonsPkg/UefiLessonsPkg.dec`:
```
[PcdsFeatureFlag]
- gUefiLessonsPkgTokenSpaceGuid.PcdMyFeatureFlagVar|FALSE|BOOLEAN|0x20000001
+ gUefiLessonsPkgTokenSpaceGuid.PcdFeatureFlag|TRUE|BOOLEAN|0x16DD586E
```
-Populate it to INF UefiLessonsPkg/PCDLesson/PCDLesson.inf:
+And populate it to the INF file `UefiLessonsPkg/PCDLesson/PCDLesson.inf`. In the INF file PCDs of this type goes to `[FeaturePcd]` section:
```
[FeaturePcd]
- gUefiLessonsPkgTokenSpaceGuid.PcdMyFeatureFlagVar
+ gUefiLessonsPkgTokenSpaceGuid.PcdFeatureFlag
```
-To get PCD value in a *.c file either `FeaturePcdGet` or `PcdGetBool` can be used which are practically the same https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Library/PcdLib.h:
+To get the PCD value in a `*.c` file either `FeaturePcdGet` or `PcdGetBool` can be used which are practically the same [https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Library/PcdLib.h](https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Library/PcdLib.h):
```
#define FeaturePcdGet(TokenName) _PCD_GET_MODE_BOOL_##TokenName
+...
#define PcdGetBool(TokenName) _PCD_GET_MODE_BOOL_##TokenName
```
-Let's add both to our UefiLessonsPkg/PCDLesson/PCDLesson.c
+Let's use both methonds in our `UefiLessonsPkg/PCDLesson/PCDLesson.c` to print the flag value:
```
-Print(L"PcdMyFeatureFlagVar=%d\n", FeaturePcdGet(PcdMyFeatureFlagVar));
-Print(L"PcdMyFeatureFlagVar=%d\n", PcdGetBool(PcdMyFeatureFlagVar));
+Print(L"PcdFeatureFlag=%d\n", FeaturePcdGet(PcdFeatureFlag));
+Print(L"PcdFeatureFlag=%d\n", PcdGetBool(PcdFeatureFlag));
```
-Ok, we are ready to build. Build your module and check out AutoGen files.
-
-Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/AutoGen.h
-```
-#define _PCD_TOKEN_PcdMyFeatureFlagVar 0U
-#define _PCD_SIZE_PcdMyFeatureFlagVar 1
-#define _PCD_GET_MODE_SIZE_PcdMyFeatureFlagVar _PCD_SIZE_PcdMyFeatureFlagVar
-#define _PCD_VALUE_PcdMyFeatureFlagVar ((BOOLEAN)0U)
-extern const BOOLEAN _gPcd_FixedAtBuild_PcdMyFeatureFlagVar;
-#define _PCD_GET_MODE_BOOL_PcdMyFeatureFlagVar _gPcd_FixedAtBuild_PcdMyFeatureFlagVar
-//#define _PCD_SET_MODE_BOOL_PcdMyFeatureFlagVar ASSERT(FALSE) // It is not allowed to set value for a FIXED_AT_BUILD PCD
-```
-
-Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/AutoGen.c
+Ok, we are ready to build. Build your module and check out `AutoGen.h` file `Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/AutoGen.h`:
```
-GLOBAL_REMOVE_IF_UNREFERENCED const BOOLEAN _gPcd_FixedAtBuild_PcdMyFeatureFlagVar = _PCD_VALUE_PcdMyFeatureFlagVar;
+#define _PCD_TOKEN_PcdFeatureFlag 0U
+#define _PCD_SIZE_PcdFeatureFlag 1
+#define _PCD_GET_MODE_SIZE_PcdFeatureFlag _PCD_SIZE_PcdFeatureFlag
+#define _PCD_VALUE_PcdFeatureFlag ((BOOLEAN)1U)
+extern const BOOLEAN _gPcd_FixedAtBuild_PcdFeatureFlag;
+#define _PCD_GET_MODE_BOOL_PcdFeatureFlag _gPcd_FixedAtBuild_PcdFeatureFlag
+//#define _PCD_SET_MODE_BOOL_PcdFeatureFlag ASSERT(FALSE) // It is not allowed to set value for a FIXED_AT_BUILD PCD
```
So both calls for our PCD would translate to:
```
-FeaturePcdGet/PcdGetBool(PcdMyFeatureFlagVar) --> _PCD_GET_MODE_BOOL_PcdMyFeatureFlagVar --> _gPcd_FixedAtBuild_PcdMyFeatureFlagVar
+FeaturePcdGet/PcdGetBool(PcdFeatureFlag) --> _PCD_GET_MODE_BOOL_PcdFeatureFlag --> _gPcd_FixedAtBuild_PcdFeatureFlag
```
-And this variable is declared in AutoGen.c file:
+
+And this `_gPcd_FixedAtBuild_PcdFeatureFlag` variable is a constant from the `AutoGen.c` file `Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/AutoGen.c`:
```
-GLOBAL_REMOVE_IF_UNREFERENCED const BOOLEAN _gPcd_FixedAtBuild_PcdMyFeatureFlagVar = ((BOOLEAN)0U);
+GLOBAL_REMOVE_IF_UNREFERENCED const BOOLEAN _gPcd_FixedAtBuild_PcdFeatureFlag; // = ((BOOLEAN)1U);
```
-If you execute it under OVMF these print statements would get you:
+If you execute the application under OVMF the print statements would get you:
```
-PcdMyFeatureFlagVar=0
-PcdMyFeatureFlagVar=0
+FS0:\> PCDLesson.efi
+...
+PcdFeatureFlag=1
+PcdFeatureFlag=1
```
-________________________
-
-So as you can see a feature PCD is similar to a BOOLEAN PCD defined under `PcdsFixedAtBuild` section
+# Comparision to `[PcdsFixedAtBuild]` BOOLEAN datum type PCD
-Let's create a fixed bool PCD and compare results:
+As you can see a feature PCD is similar to a BOOLEAN PCD defined under `PcdsFixedAtBuild` section.
-UefiLessonsPkg/UefiLessonsPkg.dec:
+Compare how they are declared in the DEC file `UefiLessonsPkg/UefiLessonsPkg.dec`:
```
[PcdsFixedAtBuild]
...
- gUefiLessonsPkgTokenSpaceGuid.PcdMyVarBool|FALSE|BOOLEAN|0x00000004
+ gUefiLessonsPkgTokenSpaceGuid.PcdBool|TRUE|BOOLEAN|0x69E88A63
[PcdsFeatureFlag]
- gUefiLessonsPkgTokenSpaceGuid.PcdMyFeatureFlagVar|FALSE|BOOLEAN|0x20000001
+ gUefiLessonsPkgTokenSpaceGuid.PcdFeatureFlag|TRUE|BOOLEAN|0x16DD586E
```
-UefiLessonsPkg/PCDLesson/PCDLesson.inf:
+
+And in the INF file `UefiLessonsPkg/PCDLesson/PCDLesson.inf`:
```
[FixedPcd]
...
- gUefiLessonsPkgTokenSpaceGuid.PcdMyVarBool
+ gUefiLessonsPkgTokenSpaceGuid.PcdBool
[FeaturePcd]
- gUefiLessonsPkgTokenSpaceGuid.PcdMyFeatureFlagVar
+ gUefiLessonsPkgTokenSpaceGuid.PcdFeatureFlag
```
-UefiLessonsPkg/PCDLesson/PCDLesson.c:
+
+As for the `UefiLessonsPkg/PCDLesson/PCDLesson.c` in case of a FeatureFlag PCD you should use `FeaturePcdGet` instead of `FixedPcdGetBool`. But at the same time in both cases you can use `PcdGetBool` function:
```
-Print(L"PcdMyFeatureFlagVar=%d\n", FeaturePcdGet(PcdMyFeatureFlagVar));
-Print(L"PcdMyFeatureFlagVar=%d\n", PcdGetBool(PcdMyFeatureFlagVar));
-Print(L"PcdMyVarBool=%d\n", FixedPcdGetBool(PcdMyVarBool));
-Print(L"PcdMyVarBool=%d\n", PcdGetBool(PcdMyVarBool));
+Print(L"PcdFeatureFlag=%d\n", FeaturePcdGet(PcdFeatureFlag));
+Print(L"PcdFeatureFlag=%d\n", PcdGetBool(PcdFeatureFlag));
+Print(L"PcdBool=%d\n", FixedPcdGetBool(PcdBool));
+Print(L"PcdBool=%d\n", PcdGetBool(PcdBool));
```
-Ok, now let's build and look at AutoGen files:
+Ok, now let's compare the code for the PCDs in the AutoGen files:
-AutoGen.c
+`AutoGen.c`:
```
-GLOBAL_REMOVE_IF_UNREFERENCED const BOOLEAN _gPcd_FixedAtBuild_PcdMyVarBool = _PCD_VALUE_PcdMyVarBool;
-GLOBAL_REMOVE_IF_UNREFERENCED const BOOLEAN _gPcd_FixedAtBuild_PcdMyFeatureFlagVar = _PCD_VALUE_PcdMyFeatureFlagVar;
+GLOBAL_REMOVE_IF_UNREFERENCED const BOOLEAN _gPcd_FixedAtBuild_PcdBool = _PCD_VALUE_PcdBool;
+GLOBAL_REMOVE_IF_UNREFERENCED const BOOLEAN _gPcd_FixedAtBuild_PcdFeatureFlag = _PCD_VALUE_PcdFeatureFlag;
```
-AutoGen.h
+`AutoGen.h`:
```
-#define _PCD_TOKEN_PcdMyVarBool 0U
-#define _PCD_SIZE_PcdMyVarBool 1
-#define _PCD_GET_MODE_SIZE_PcdMyVarBool _PCD_SIZE_PcdMyVarBool
-#define _PCD_VALUE_PcdMyVarBool 0U
-extern const BOOLEAN _gPcd_FixedAtBuild_PcdMyVarBool;
-#define _PCD_GET_MODE_BOOL_PcdMyVarBool _gPcd_FixedAtBuild_PcdMyVarBool
-//#define _PCD_SET_MODE_BOOL_PcdMyVarBool ASSERT(FALSE) // It is not allowed to set value for a FIXED_AT_BUILD PCD
+#define _PCD_TOKEN_PcdBool 0U
+#define _PCD_SIZE_PcdBool 1
+#define _PCD_GET_MODE_SIZE_PcdBool _PCD_SIZE_PcdBool
+#define _PCD_VALUE_PcdBool 1U
+extern const BOOLEAN _gPcd_FixedAtBuild_PcdBool;
+#define _PCD_GET_MODE_BOOL_PcdBool _gPcd_FixedAtBuild_PcdBool
+//#define _PCD_SET_MODE_BOOL_PcdBool ASSERT(FALSE) // It is not allowed to set value for a FIXED_AT_BUILD PCD
-#define _PCD_TOKEN_PcdMyFeatureFlagVar 0U
-#define _PCD_SIZE_PcdMyFeatureFlagVar 1
-#define _PCD_GET_MODE_SIZE_PcdMyFeatureFlagVar _PCD_SIZE_PcdMyFeatureFlagVar
-#define _PCD_VALUE_PcdMyFeatureFlagVar ((BOOLEAN)0U)
-extern const BOOLEAN _gPcd_FixedAtBuild_PcdMyFeatureFlagVar;
-#define _PCD_GET_MODE_BOOL_PcdMyFeatureFlagVar _gPcd_FixedAtBuild_PcdMyFeatureFlagVar
-//#define _PCD_SET_MODE_BOOL_PcdMyFeatureFlagVar ASSERT(FALSE) // It is not allowed to set value for a FIXED_AT_BUILD PCD
+#define _PCD_TOKEN_PcdFeatureFlag 0U
+#define _PCD_SIZE_PcdFeatureFlag 1
+#define _PCD_GET_MODE_SIZE_PcdFeatureFlag _PCD_SIZE_PcdFeatureFlag
+#define _PCD_VALUE_PcdFeatureFlag ((BOOLEAN)1U)
+extern const BOOLEAN _gPcd_FixedAtBuild_PcdFeatureFlag;
+#define _PCD_GET_MODE_BOOL_PcdFeatureFlag _gPcd_FixedAtBuild_PcdFeatureFlag
+//#define _PCD_SET_MODE_BOOL_PcdFeatureFlag ASSERT(FALSE) // It is not allowed to set value for a FIXED_AT_BUILD PCD
```
-As you see the differences are subtle. `FeaturePcd` is simply a syntactic sugar for BOOLEAN `FixedAtBuild` PCD.
+So as you see the differences are subtle. Therefore the `FeaturePcd` is simply a syntactic sugar for BOOLEAN `FixedAtBuild` PCD.
generated by cgit v1.2.3-18-g5258 (git 2.32.0) at 2024-12-22 05:10:58 +0000