Intel ARCHITECTURE IA-32 User Manual download pdf (Page 589)

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Vol. 3A 16-9

MIXING 16-BIT AND 32-BIT CODE

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).

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Intel ARCHITECTURE IA-32 User Manual Page 589

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