MySQL Internals Manual  /  ...  /  In-memory Record Layout

21.3.4 In-memory Record Layout

null bits:   One bit per column that can be NULL.
              The "null bits" are rounded up to the next byte boundary.
              the number of "null bytes" are also referred as data_offset.
fields:      One field per column. No separators, length fields, etc.
              Length (pack_length) and layout of the fields depend on the
              field type:
 -- to be added --

Note: The "in-memory record layout" is used by the SQL layer. It is independent from storage engines. All storage engines have to accept and produce this in-memory format at the handler interface.