Vol. 3B 21-7

MIXING 16-BIT AND 32-BIT CODE

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Allowing procedures in 16-bit code segments to call procedures with offsets greater than FFFFH in 32-bit code 
segments.

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Matching operand-size attributes between companion CALL and RET instructions.

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Translating parameters (data), including managing parameter strings with a variable count or an odd number 
of 16-bit words.

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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 proce-
dure 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:

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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).

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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).