Intel 253668-032US ,

Vol. 3 18-9

MIXING 16-BIT AND 32-BIT CODE

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.

The interface procedure becomes more complex if any of these rules are violated. For

example, if a 16-bit procedure calls a 32-bit procedure with an entry point beyond

FFFFH, the interface procedure will need to provide the offset to the entry point. The

mapping between 16- and 32-bit addresses is only performed automatically when a

call gate is used, because the gate descriptor for a call gate contains a 32-bit

address. When a call gate is not used, the interface code must provide the 32-bit

address.

The structure of the interface procedure depends on the types of calls it is going to

support, as follows:

•Calls from 16-bit procedures to 32-bit procedures — Calls to the interface

procedure from a 16-bit code segment are made with 16-bit CALL instructions

(by default, because the D flag for the calling code-segment descriptor is clear),

and 16-bit operand-size prefixes are used with RET instructions to return from

the interface procedure to the calling procedure. Calls from the interface

procedure to 32-bit procedures are performed with 32-bit CALL instructions (by

default, because the D flag for the interface procedure’s code segment is set),

and returns from the called procedures to the interface procedure are performed

with 32-bit RET instructions (also by default).

•Calls from 32-bit procedures to 16-bit procedures — Calls to the interface

procedure from a 32-bit code segment are made with 32-bit CALL instructions

(by default), and returns to the calling procedure from the interface procedure

are made with 32-bit RET instructions (also by default). Calls from the interface

procedure to 16-bit procedures require the CALL instructions to have the

operand-size prefixes, and returns from the called procedures to the interface

procedure are performed with 16-bit RET instructions (by default).