Add lesson 6

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+[ZoneTransfer]

+ZoneId=3

+ReferrerUrl=https://blog.csdn.net/lindahui2008/article/details/78796550

+HostUrl=https://img-blog.csdn.net/20171213204808632?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvbGluZGFodWkyMDA4/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast

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diff --git a/Lesson_6/README.md b/Lesson_6/README.md
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+Let's create `ImageInfo` app in our `UefiLessonsPkg`.
+
+First generate a random GUID.
+```
+$ uuidgen
+b68d3472-70c7-4928-841b-6566032e0a23
+```
+
+And then create *.inf and *.c file based on the code in our `HelloWorld` app:
+```
+$ cat UefiLessonsPkg/ImageInfo/ImageInfo.inf
+[Defines]
+  INF_VERSION                    = 1.25
+  BASE_NAME                      = ImageInfo
+  FILE_GUID                      = b68d3472-70c7-4928-841b-6566032e0a23
+  MODULE_TYPE                    = UEFI_APPLICATION
+  VERSION_STRING                 = 1.0
+  ENTRY_POINT                    = UefiMain
+
+[Sources]
+  ImageInfo.c
+
+[Packages]
+  MdePkg/MdePkg.dec
+
+[LibraryClasses]
+  UefiApplicationEntryPoint
+  UefiLib
+```
+```
+$ cat UefiLessonsPkg/ImageInfo/ImageInfo.c
+#include <Library/UefiBootServicesTableLib.h>
+#include <Library/UefiLib.h>
+
+EFI_STATUS
+EFIAPI
+UefiMain (
+  IN EFI_HANDLE        ImageHandle,
+  IN EFI_SYSTEM_TABLE  *SystemTable
+  )
+{
+  return EFI_SUCCESS;
+}
+```
+Add a newly created app to our `UefiLessonsPkg/UefiLessonsPkg.dsc` file:
+```
+[Components]
+  ...
++ UefiLessonsPkg/ImageInfo/ImageInfo.inf
+```
+
+Now let's get to work.
+
+Our main function in a *.c file has 2 input parameters `EFI_HANDLE ImageHandle` and `EFI_SYSTEM_TABLE *SystemTable`:
+```
+UefiMain (
+  IN EFI_HANDLE        ImageHandle,
+  IN EFI_SYSTEM_TABLE  *SystemTable
+  )
+```
+We've already started to explore the `EFI_SYSTEM_TABLE`. In this lesson let's look what `ImageHandle` is about.
+
+`ImageHandle` parameter is a handle of an image for the program itself.
+
+You can found a definition for a `EFI_HANDLE` type in a https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Uefi/UefiBaseType.h
+```
+///
+/// A collection of related interfaces.
+///
+typedef VOID                      *EFI_HANDLE;
+```
+So it is simply a pointer to a void. So it could be a pointer to anything.
+
+In a reality it is a pointer to this type of structure https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Core/Dxe/Hand/Handle.h :
+```
+#define EFI_HANDLE_SIGNATURE            SIGNATURE_32('h','n','d','l')
+
+///
+/// IHANDLE - contains a list of protocol handles
+///
+typedef struct {
+  UINTN               Signature;
+  /// All handles list of IHANDLE
+  LIST_ENTRY          AllHandles;
+  /// List of PROTOCOL_INTERFACE's for this handle
+  LIST_ENTRY          Protocols;
+  UINTN               LocateRequest;
+  /// The Handle Database Key value when this handle was last created or modified
+  UINT64              Key;
+} IHANDLE;
+```
+LIST_ENTRY is a double linked list. So with it we can traverse to any handle in the system. Sometimes it is called the Handle database.
+
+In case you wonder how LIST_ENTRY is defined look at the https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Base.h :
+```
+///
+/// LIST_ENTRY structure definition.
+///
+typedef struct _LIST_ENTRY LIST_ENTRY;
+
+///
+/// _LIST_ENTRY structure definition.
+///
+struct _LIST_ENTRY {
+  LIST_ENTRY  *ForwardLink;
+  LIST_ENTRY  *BackLink;
+};
+```
+Each handle has a list of protocols attached to it. Protocol is like an interface - a set of functions and data fields. To track the list of protocols attached to a handle this field is used:
+```
+   /// List of PROTOCOL_INTERFACE's for this handle
+   LIST_ENTRY          Protocols;
+```
+It is another double linked list that points to these structures:
+```
+#define PROTOCOL_INTERFACE_SIGNATURE  SIGNATURE_32('p','i','f','c')
+
+///
+/// PROTOCOL_INTERFACE - each protocol installed on a handle is tracked
+/// with a protocol interface structure
+///
+typedef struct {
+  UINTN                       Signature;
+  /// Link on IHANDLE.Protocols
+  LIST_ENTRY                  Link;
+  /// Back pointer
+  IHANDLE                     *Handle;
+  /// Link on PROTOCOL_ENTRY.Protocols
+  LIST_ENTRY                  ByProtocol;
+  /// The protocol ID
+  PROTOCOL_ENTRY              *Protocol;
+  /// The interface value
+  VOID                        *Interface;
+  /// OPEN_PROTOCOL_DATA list
+  LIST_ENTRY                  OpenList;
+  UINTN                       OpenListCount;
+
+} PROTOCOL_INTERFACE;
+```
+
+Effectively PROTOCOL_INTERFACE structure is only a proxy that points to an actual PROTOCOL_ENTRY structrure which are exist in a separate protocol database.
+```
+#define PROTOCOL_ENTRY_SIGNATURE        SIGNATURE_32('p','r','t','e')
+
+///
+/// PROTOCOL_ENTRY - each different protocol has 1 entry in the protocol
+/// database.  Each handler that supports this protocol is listed, along
+/// with a list of registered notifies.
+///
+typedef struct {
+  UINTN               Signature;
+  /// Link Entry inserted to mProtocolDatabase
+  LIST_ENTRY          AllEntries;
+  /// ID of the protocol
+  EFI_GUID            ProtocolID;
+  /// All protocol interfaces
+  LIST_ENTRY          Protocols;
+  /// Registerd notification handlers
+  LIST_ENTRY          Notify;
+} PROTOCOL_ENTRY;
+```
+It all sounds a little bit complicated so it is better to see it in a picture:
+![Handle_Protocol_databases_scheme](Handle_Protocol_databases_scheme.jpg?raw=true "Handle/Protocol databases scheme")
+Important information from the picture:
+- HANDLE 0 has PROTOCOL_INTERFACE 0 (->PROTOCOL_ENTRY 0) and PROTOCOL_INTERFACE 1 (->PROTOCOL_ENTRY 1)
+- HANDLE 1 has PROTOCOL_INTERFACE 2 (->PROTOCOL_ENTRY 2) and PROTOCOL_INTERFACE 3 (->PROTOCOL_ENTRY 1)
+- HANDLE 2 has PROTOCOL_INTERFACE 4 (->PROTOCOL_ENTRY 1)
+Besides that:
+- All handles are interconnected with the help of `AllHandles` field in each `IHANDLE` structure
+- All protocols are interconnected with the help of `AllEntries` field in each `PROTOCOL_ENTRY` structure
+Red lines:
+- Each handle has a double linked list of all its PROTOCOL_INTERFACES, which internally connected to each other through the `Link` field
+Green lines:
+- Each protocol entry effectively knows all the users (=PROTOCOL_INTERFACES) of it.
+
+Let's check that `EFI_HANDLE ImageHandle` effectively is the `IHANDLE` structure.
+
+To check this we will cast `ImageHandle` to `IHANDLE` and print its signature.
+
+```
+#include <Library/UefiBootServicesTableLib.h>
+#include <Library/UefiLib.h>
+
+
+typedef struct {
+  UINTN               Signature;
+  /// All handles list of IHANDLE
+  LIST_ENTRY          AllHandles;
+  /// List of PROTOCOL_INTERFACE's for this handle
+  LIST_ENTRY          Protocols;
+  UINTN               LocateRequest;
+  /// The Handle Database Key value when this handle was last created or modified
+  UINT64              Key;
+} IHANDLE;
+
+EFI_STATUS
+EFIAPI
+UefiMain (
+  IN EFI_HANDLE        ImageHandle,
+  IN EFI_SYSTEM_TABLE  *SystemTable
+  )
+{
+  IHANDLE* MyHandle = ImageHandle;
+  Print(L"Signature: %c %c %c %c\n", (MyHandle->Signature >>  0) & 0xff,
+                                     (MyHandle->Signature >>  8) & 0xff,
+                                     (MyHandle->Signature >> 16) & 0xff,
+                                     (MyHandle->Signature >> 24) & 0xff);
+
+  return EFI_SUCCESS;
+}
+
+
+Don't blame me for redefinition of `IHANDLE` structure. These structures (IHANDLE/PROTOCOL_ENTRY/PROTOCOL_INTERFACE) are private and not intended to be used outside of the `MdeModulePkg`. Even https://github.com/tianocore/edk2/blob/master/StandaloneMmPkg/Core/StandaloneMmCore.h redifines IHANDLE/PROTOCOL_ENTRY/PROTOCOL_INTERFACE.
+
+```
+$ . edksetup.sh
+$ build
+$ cp Build/UefiLessonsPkg/RELEASE_GCC5/X64/ImageInfo.efi ~/UEFI_disk/
+$ qemu-system-x86_64 -drive if=pflash,format=raw,file=Build/OvmfX64/RELEASE_GCC5/FV/OVMF.fd \
+                     -drive format=raw,file=fat:rw:~/UEFI_disk \
+                     -nographic \
+                     -net none
+```
+
+```
+UEFI Interactive Shell v2.2
+EDK II
+UEFI v2.70 (EDK II, 0x00010000)
+Mapping table
+      FS0: Alias(s):HD0a1:;BLK1:
+          PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)/HD(1,MBR,0xBE1AFDFA,0x3F,0xFBFC1)
+     BLK0: Alias(s):
+          PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
+     BLK2: Alias(s):
+          PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
+Press ESC in 4 seconds to skip startup.nsh or any other key to continue.
+Shell> fs0:
+FS0:\> ImageInfo.efi
+Signature: h n d l
+```
+
+
+