Since a display grabber is a binary-image file, the grabber cannot export its standard functions. To give Windows access to the functions, the grabber includes a standard-function dispatch table, consisting of a series of jump instructions, at the start (offset 0) of its code segment. There is one 3-byte jump instruction for each grabber function. Windows transfers control to a grabber function by computing the offset of the desired function's dispatch table entry and by making a FAR call to that entry. Although each dispatch table entry is a NEAR jump instruction to the corresponding function, the function itself must execute a FAR return instruction when it returns to Windows.
The standard-function dispatch table has the following form:
org 0
StdFuncTable label word
jmp InquireGrab ;function 00001h
jmp Obsolete ;function 00002h
jmp Obsolete ;function 00003h
jmp Obsolete ;function 00004h
jmp InquireSave ;function 00005h
jmp SaveScreen ;function 00006h
jmp RestoreScreen ;function 00007h
jmp InitScreen ;function 00008h
The grabber must not modify or move the dispatch table after it has been loaded.
The dispatch-table entry for the InitScreen function, an optional function, is at offset 015h. Windows checks for a jump instruction at offset 015h to determine whether the InitScreen function is present. If InitScreen is present, Windows calls the function whenever a non-Windows application starts up, and subsequently after every video-context switch to that application.