The typebuf_T struct is used for the typeahead buffer.
An «instance» of typebuf_T is stored in the global variable typebuf (declared in globals.h).
A static array of typebuf_T is found in getchar.c:
static typebuf_T saved_typebuf[NSCRIPT];
The head of the buffer may contain the result of mappings, abbreviations and @a commands. The length of this part is typebuf.tb_maplen.
typedef struct
{
char_u *tb_buf; // buffer for typed characters (that are not consumed yet?)
//
// tb_buf has three parts:
// - room in front (for result of mappings)
// - the middle for typeahead and
// - room for new characters (which needs to be 3 * (MAXMAPLEN+4)) for the Amiga
//
char_u *tb_noremap; // mapping flags for characters in tb_buf[]
int tb_buflen; // size of tb_buf[]
int tb_off; // current position in tb_buf[] (the first valid character)0
int tb_len; // number of valid bytes in tb_buf[]
//(typebuf.tb_buf[typebuf.tb_off + typebuf.tb_len - 1] is the last valid char,
// typebuf.tb_buf[typebuf.tb_off + typebuf.tb_len] must be NUL)
int tb_maplen; // nr of mapped bytes in tb_buf[]
int tb_silent; // nr of silently mapped bytes in tb_buf[] (The part where <silent> applies)
int tb_no_abbr_cnt; // nr of bytes without abbrev. in tb_buf[]
int tb_change_cnt; // nr of time tb_buf was changed; never zero
} typebuf_T;