Unless otherwise specified all data types are naturally aligned. Structures are aligned on boundaries equal to the largest internal datum of the structure and internal data are implicitly padded to achieve natural alignment.

Logical Boolean. 1-byte value containing a 0 for FALSE or a 1 for TRUE. Other values are undefined.

//*******************************************************
// SIMPLE_TEXT_OUTPUT_MODE
//*******************************************************
typedef struct {
  INT32 MaxMode;
// current settings
  INT32 Mode;
  INT32 Attribute;
  INT32 CursorColumn;
  INT32 CursorRow;
  BOOLEAN CursorVisible;
} SIMPLE_TEXT_OUTPUT_MODE;    

struct SIMPLE_TEXT_OUTPUT_MODE
  MaxMode               rd 1
  Mode                  rd 1
  Attribute             rd 1
  CursorColumn          rd 1
  CursorRow             rd 1
  CursorVisible         rb 1
                        rb 3    ;needed?
ends    

P S.: I'm not sure if FASM really aligns the structures or not

In the example, no padding is needed because CursorVisible is aligned to its size (1).

cod3b453 wrote:

In the example, no padding is needed because CursorVisible is aligned to its size (1).

The overall size of the structure should also be respected with proper padding since there is no guarantee that the API will not write to unused padding bytes. If you have some other data that follows the structure in the memory space of the padding bytes than it might be at risk of being corrupted.