Intel IA-32 - Manual (Page 588)

16-8 Vol. 3A
MIXING 16-BIT AND 32-BIT CODE
16.4.3 Interrupt Control Transfers
A program-control transfer caused by an exception or interrupt is always carried out through an
interrupt or trap gate (located in the IDT). Here, the type of the gate (16-bit or 32-bit) determines
the operand-size attribute used in the implicit call to the exception or interrupt handler procedure
in another code segment.
A 32-bit interrupt or trap gate provides a safe interface to a 32-bit exception or interrupt handler
when the exception or interrupt occurs in either a 32-bit or a 16-bit code segment. It is sometimes
impractical, however, to place exception or interrupt handlers in 16-bit code segments, because
only 16-bit return addresses are saved on the stack. If an exception or interrupt occurs in a 32-bit
code segment when the EIP was greater than FFFFH, the 16-bit handler procedure cannot
provide the correct return address.
16.4.4 Parameter Translation
When segment offsets or pointers (which contain segment offsets) are passed as parameters
between 16-bit and 32-bit procedures, some translation is required. If a 32-bit procedure passes
a pointer to data located beyond 64 KBytes to a 16-bit procedure, the 16-bit procedure cannot
use it. Except for this limitation, interface code can perform any format conversion between
32-bit and 16-bit pointers that may be needed.
Parameters passed by value between 32-bit and 16-bit code also may require translation between
32-bit and 16-bit formats. The form of the translation is application-dependent.
16.4.5 Writing Interface Procedures
Placing interface code between 32-bit and 16-bit procedures can be the solution to the following
interface problems:
•
Allowing procedures in 16-bit code segments to call procedures with offsets greater than
FFFFH in 32-bit code segments.
•
Matching operand-size attributes between companion CALL and RET instructions.
•
Translating parameters (data), including managing parameter strings with a variable count
or an odd number of 16-bit words.
•
The possible invalidation of the upper bits of the ESP register.
The interface procedure is simplified where these rules are followed.
1. The interface procedure must reside in a 32-bit code segment (the D flag for the code-
segment descriptor is set).
2. All procedures that may be called by 16-bit procedures must have offsets not greater than
FFFFH.
3. All return addresses saved by 16-bit procedures must have offsets not greater than FFFFH.