Int 2F/AX=4B01h

DOS 5+ TASK SWITCHER - BUILD CALLOUT CHAIN

AX = 4B01h
CX:DX -> task switcher entry point (see #02819)
ES:BX = 0000h:0000h

Return:
ES:BX -> callback info structure (see #02817) or 0000h:0000h

Notes: Called by the task switcher. This function is hooked by clients which require notification of task switcher activities; the call must first be passed on to the prior handler with registers unchanged using a simulated interrupt. On return, the client must build a callback info structure and store the returned ES:BX in the "next" field, then return the address of its own callback info structure.. A client program must add itself to the notification chain if it provides services to other programs; before terminating, it must remove itself from the chain by calling the task switcher's entry point with AX=0005h (see #02819). The task switcher entry point should not be saved, as it is subject to change and will be provided on any notification call. The Windows 3.1 Standard Mode supports this API

See Also: AX=160Bh - AX=4B02h


Format of task switcher callback info structure:

Offset  Size    Description     (Table 02817)
00h    DWORD   pointer to next callback info structure
04h    DWORD   pointer to notification function (see #02818)
08h    DWORD   reserved
0Ch    DWORD   address of zero-terminated list of API info structures
(see #02821)


(Table 02818)
Values task switcher notification function is called with:.
AX = function
0000h switcher initialization
Return:
AX = status
0000h if OK to load
nonzero to abort task switcher
0001h query suspend
BX = session ID
Return:
AX = status
0000h if OK to switch session
0001h if not
0002h suspend session
BX = session ID
interrupts disabled
Return:
AX = 0000h if OK to switch session
= 0001h if not
0003h activate session
BX = session ID
CX = session status flags
bit 0:
Set if first activation of session
bits 1-15:
Reserved (0)
interrupts disabled
Return:
AX = 0000h
0004h session active
BX = session ID
CX = session status flags
bit 0:
Set if first activation of session
bits 1-15:
Reserved (0)
Return:
AX = 0000h
0005h create session
BX = session ID
Return:
AX = 0000h if OK to create session
= 0001h if not
0006h destroy session
BX = session ID
Return:
AX = 0000h
0007h switcher termination
BX = flags
bit 0:
Set if calling switcher is only switcher loaded
bits 1-15:
Reserved (0)
Return:
AX = 0000h.
ES:DI -> task switcher entry point (see #02819)

Notes: Function 0000h is generally called by the program which controls or invokes the task switcher, rather than by the task switcher itself; the entry point supplied to this function is not necessarily the entry point to the task switcher itself, and may be 0000h:0000h. If any client indicates that loading is not possible, all clients will be called with function 0007h; thus it is possible for a client to receive a termination notice without a corresponding initialization notice.. Except for functions 0002h and 0003h, the notification handler is called with interrupts enabled and may make any INT 21h function call; interrupts must not be enabled in functions 0002h and 0003h. Function 0007h may be called with ES:DI = 0000h:0000h if the entry point is no longer valid

Category: DOS-Based Task Switchers/Multitaskers - Int 2Fh - D

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