31.5.1 POST Card Debug

A POST card is an add-in card that displays the hex value of an 8-bit I/O write cycle to address 0x80 Some POST cards support more than 8-bits and use additional I/O port addresses such as 0x81 The EDK II MdePkg provides a library class called PostCodeLib that includes the POST_CODE() macro that may be used to abstract access to a POST card. When a UEFI Driver is built, it can be configured in the DSC file to map the PostCodeLib class to the MdePkg/Library/BasePostCodeLibPort80 instance that performs 8-bit writes to I/O port 0x80. If a platform has the equivalent POST card functionality, but it is not located at I/O port 0x80, an alternate implementation of the PostCodeLib instance can be provided that allows a UEFI Driver to send POST code values to the alternate POST card device without any source code changes to the UEFI Driver itself. This example shows an example usage of the POST_CODE() macro to send POST code values of 0x10 and 0x11 as a UEFI Driver enters and leaves the driver entry point.

Example 269-UEFI Driver Entry Point with POST_CODE() Macros

#include <Uefi.h>
#include <Library/PostCodeLib.h>
#include <Library/UefiLib.h>

EFI_STATUS
EFIAPI
AbcDriverEntryPoint (
  IN EFI_HANDLE        ImageHandle,
  IN EFI_SYSTEM_TABLE  *SystemTable
  )
{
  EFI_STATUS  Status;

  POST_CODE (0x10);

  //
  // Install driver model protocol(s) on ImageHandle
  //
  Status = EfiLibInstallDriverBinding (
             ImageHandle, // ImageHandle
             SystemTable, // SystemTable
             &gAbcDriverBinding, // DriverBinding
             ImageHandle // DriverBindingHandle
             );
  ASSERT_EFI_ERROR (Status);

  POST_CODE (0x11);

  return Status;
}

The PostCodeLib uses PCDs to enable and disable the POST_CODE() macros. This means that POST_CODE() macros can be enabled during UEFI Driver development and debug when a platform with a POST card is being used, and can be easily disabled for production builds of UEFI Drivers. The example below contains a portion of the DSC file that shows how to enable POST_CODE() macros in a UEFI Driver.

Example 270-Enable POST_CODE() macros from DSC file

[PcdsFixedAtBuild]
  #
  # Set POST_CODE_PROPERTY_POST_CODE_ENABLED bit (0x8) in
  # PcdPostCodePropertyMask to enable POST_CODE() macros
  #
  gEfiMdePkgTokenSpaceGuid. PcdPostCodePropertyMask |0x08