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diff --git a/Lessons/Lesson_17/README.md b/Lessons/Lesson_17/README.md new file mode 100644 index 0000000..6dd3ed6 --- /dev/null +++ b/Lessons/Lesson_17/README.md @@ -0,0 +1,91 @@ +In the last lesson we've added our own boot option. + +Let's boot it. + +Type `exit` in the UEFI shell to go to the BIOS menu: +``` +Shell> exit +``` +Navigate to the `Boot Manager` window. Our `HelloWorld` option would be present here: + + +If you'll try to boot it, app would print its strings and immediately return. + +Let's fix it. Let's add wait for a key input from user in our app code. +To do it we will need `WaitForEvent` blocking function: + +``` +EFI_BOOT_SERVICES.WaitForEvent() + +Summary +Stops execution until an event is signaled. + +Prototype +typedef + +EFI_STATUS +(EFIAPI *EFI_WAIT_FOR_EVENT) ( + IN UINTN NumberOfEvents, + IN EFI_EVENT *Event, + OUT UINTN *Index +); + +Parameters +NumberOfEvents The number of events in the Event array. +Event An array of EFI_EVENT. +Index Pointer to the index of the event which satisfied the wait condition. +``` + +To our task we should use `EFI_EVENT WaitForKey` from the `EFI_SIMPLE_TEXT_INPUT_PROTOCOL` (which is placed in a `gST->ConIn`) +``` +typedef struct _EFI_SIMPLE_TEXT_INPUT_PROTOCOL { + EFI_INPUT_RESET Reset; + EFI_INPUT_READ_KEY ReadKeyStroke; + EFI_EVENT WaitForKey; +} EFI_SIMPLE_TEXT_INPUT_PROTOCOL; +``` + +Now when we now all this API add this code to the end of the `HelloWorld` app main function: +``` +UINTN Index; +gBS->WaitForEvent(1, &(gST->ConIn->WaitForKey), &Index); +``` + +Compile OVMF again and try to boot our `HelloWorld` boot option through the `Boot Manager` BIOS menu. +Now output of the app stops and waits for the user keystroke. + +Everything works fine except a fact that if we choose `Enter` as our keystroke, `HelloWorld` app would be immediately launched again. + +This happens because we didn't read or clear the input buffer. As we don't need keystroke information let's simply reset it with a `Reset` function: +``` +EFI_SIMPLE_TEXT_INPUT_PROTOCOL.Reset() + +Summary: +Resets the input device hardware. + +Prototype: +typedef +EFI_STATUS +(EFIAPI *EFI_INPUT_RESET) ( + IN EFI_SIMPLE_TEXT_INPUT_PROTOCOL *This, + IN BOOLEAN ExtendedVerification + ); + +Parameters: +This A pointer to the EFI_SIMPLE_TEXT_INPUT_PROTOCOL instance. +ExtendedVerification Indicates that the driver may perform a more exhaustive verification + operation of the device during reset. + +Description: +The Reset() function resets the input device hardware. +The implementation of Reset is required to clear the contents of any input queues resident in memory +used for buffering keystroke data and put the input stream in a known empty state +``` + +Add this to the end of our app: +``` +gST->ConIn->Reset(gST->ConIn, FALSE); +``` + +Now if we boot our app from the `Boot Manager` BIOS menu it would work correctly for any keystroke. + |