Tandy 1000 Programmer's Reference Manual page 21

Chawter 1 I System Calls
Notes:
This exit is taken only for disk errors occurring during
an Interrupt 21H. It is not used for errors during In-
terrupts 25H or 26H.
This routine is entered in a disabled state.
0
The SS, SP, DS, ES, BX, CX, and DX registers must
be reserved.
This interrupt handler should not use MS-DOS function
calls. If necessary, it can use calls OIH through OCH.
Use of any other call will destroy the MS-DOS stack
and will leave MS-DOS in an unpredictable state.
0
The interrupt handler must not change the contents of
the device header.
If the interrupt handler has its own error management
routine, rather than returning to MS-DOS, it should re-
store the application program's registers from the stack,
remove all but the last 3 words on the stack, and then
issue an IRET. This will cause a return t o the pro-
gram immediately after the INT 21H that experienced
the error. Note that if this is done, MS-DOS will be in
an unstable state until a function call higher than OCH
is issued.
19