Intel ARCHITECTURE IA-32 User Manual download pdf (Page 584)

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16-4 Vol. 3A

MIXING 16-BIT AND 32-BIT CODE

A stack that spans less than 64 KBytes can be shared by both 16- and 32-bit code segments. This

class of stacks includes:

• Stacks in expand-up segments with the G (granularity) and B (big) flags in the stack-

segment descriptor clear.

• Stacks in expand-down segments with the G and B flags clear.

• Stacks in expand-up segments with the G flag set and the B flag clear and where the stack

is contained completely within the lower 64 KBytes. (Offsets greater than FFFFH can be

used for data, other than the stack, which is not shared.)

See Section 3.4.5, “Segment Descriptors,” for a description of the G and B flags and the expand-

down stack type.

The B flag cannot, in general, be used to change the size of stack used by a 16-bit code segment.

This flag controls the size of the stack pointer only for implicit stack references such as those

caused by interrupts, exceptions, and the PUSH, POP, CALL, and RET instructions. It does not

control explicit stack references, such as accesses to parameters or local variables. A 16-bit code

segment can use a 32-bit stack only if the code is modified so that all explicit references to the

stack are preceded by the 32-bit address-size prefix, causing those references to use 32-bit

addressing and explicit writes to the stack pointer are preceded by a 32-bit operand-size prefix.

In 32-bit, expand-down segments, all offsets may be greater than 64 KBytes; therefore, 16-bit

code cannot use this kind of stack segment unless the code segment is modified to use 32-bit

addressing.

16.4 TRANSFERRING CONTROL AMONG MIXED-SIZE CODE

SEGMENTS

There are three ways for a procedure in a 16-bit code segment to safely make a call to a 32-bit

code segment:

• Make the call through a 32-bit call gate.

• Make a 16-bit call to a 32-bit interface procedure. The interface procedure then makes a

32-bit call to the intended destination.

• Modify the 16-bit procedure, inserting an operand-size prefix before the call, to change it

to a 32-bit call.

Likewise, there are three ways for procedure in a 32-bit code segment to safely make a call to a

16-bit code segment:

• Make the call through a 16-bit call gate. Here, the EIP value at the CALL instruction

cannot exceed FFFFH.

• Make a 32-bit call to a 16-bit interface procedure. The interface procedure then makes a

16-bit call to the intended destination.

• Modify the 32-bit procedure, inserting an operand-size prefix before the call, changing it to

a 16-bit call. Be certain that the return offset does not exceed FFFFH.

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Intel ARCHITECTURE IA-32 User Manual Page 584

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