Intel ARCHITECTURE IA-32 User Manual download pdf (Page 588)

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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.

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