Add lesson 20

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+The Platform Configuration Database (PCD) is a database that contains a variety of current platform settings or directives that can be accessed by a driver or application.
+
+You can checkout edk2 specification https://edk2-docs.gitbook.io/edk-ii-pcd-specification/ or https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/edkii-platform-config-database-entries-paper.pdf for more explanation on PCD.
+
+PCD entry is also called PCD, so we will use this term further.
+
+The PCD entry is defined in a DEC file in a format:
+```
+<TokenSpaceGuidCName>.<PcdCName>|<DefaultValue>|<DatumType>|<Token>
+```
+`<TokenSpaceGuidCName>` is a GUID value, `<Token>` is a 32-bit value. Together they are used to uniqely identify PCD.
+
+First let's declare a Token Space that would contain all our PCDs.
+Usually in is defined as a `g<PackageName>TokenSpaceGuid`, so add this to our `UefiLessonsPkg/UefiLessonsPkg.dec`:
+```
+[Guids]
+  ...
+  gUefiLessonsPkgTokenSpaceGuid = {0x150cab53, 0xad47, 0x4385, {0xb5, 0xdd, 0xbc, 0xfc, 0x76, 0xba, 0xca, 0xf0}}
+```
+
+Now we can define our PCDs in the same *.dec file. Let's start with `UINT32 PcdMyVar32 = 42`:
+```
+[PcdsFixedAtBuild]
+  gEfiUefiLessonsPkgTokenSpaceGuid.PcdMyVar32|42|UINT32|0x00000001
+```
+
+Now create an app `PCDLesson` with the following code in its entry point function:
+```
+Print(L"PcdMyVar32=%d\n", FixedPcdGet32(PcdMyVar32));
+```
+To use `FixedPcdGet32` in our code we need to add the necessary include:
+```
+#include <Library/PcdLib.h>
+```
+If you check out this file you'll see that `FixedPcdGet32` is simply a define statement:
+```
+#define FixedPcdGet32(TokenName)            _PCD_VALUE_##TokenName
+```
+
+If we try to build our app now, build will fail, as we don't have such define in our app:
+```
+/home/kostr/tiano/edk2/MdePkg/Include/Library/PcdLib.h:97:45: error: ‘_PCD_VALUE_PcdMyVar32’ undeclared (first use in this function)
+   97 | #define FixedPcdGet32(TokenName)            _PCD_VALUE_##TokenName
+      |                                             ^~~~~~~~~~~
+```
+
+To fix this we need to add this PCD to our app `*.inf` file:
+```
+[FixedPcd]
+  gUefiLessonsPkgTokenSpaceGuid.PcdMyVar32
+```
+
+Also we need to include "dec" file that defines this PCD:
+```
+[Packages]
+  ...
+  UefiLessonsPkg/UefiLessonsPkg.dec
+```
+
+Now compilation would succeed.
+
+If you check out the content of autogenerated files `AutoGen.h`/`AutoGen.c`, you'll see, that our PCD is there.
+
+Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/PCDLesson/PCDLesson/DEBUG/AutoGen.h
+```
+// Definition of PCDs used in this module
+
+#define _PCD_TOKEN_PcdMyVar32  0U
+#define _PCD_SIZE_PcdMyVar32 4
+#define _PCD_GET_MODE_SIZE_PcdMyVar32  _PCD_SIZE_PcdMyVar32
+#define _PCD_VALUE_PcdMyVar32  42U
+extern const  UINT32  _gPcd_FixedAtBuild_PcdMyVar32;
+#define _PCD_GET_MODE_32_PcdMyVar32  _gPcd_FixedAtBuild_PcdMyVar32
+//#define _PCD_SET_MODE_32_PcdMyVar32  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
+```
+// Definition of PCDs used in this module
+GLOBAL_REMOVE_IF_UNREFERENCED const UINT32 _gPcd_FixedAtBuild_PcdMyVar32 = _PCD_VALUE_PcdMyVar32;
+```
+
+If you execute app code under OVMF:
+```
+FS0:\> PCDLesson.efi
+PcdMyVar32=42
+```