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3-92 Vol. 2A CALL—Call Procedure
INSTRUCTION SET REFERENCE, A-M
pushes the segment selector and stack pointer for the calling procedure’s stack and
the segment selector and instruction pointer for the calling procedure’s code
segment. (Parameter copy is not supported in IA-32e mode.) Finally, the processor
branches to the address of the procedure being called within the new code segment.
Near/(Far) Calls in 64-bit Mode. When the processor is operating in 64-bit mode, the
CALL instruction can be used to perform the following types of far calls:
• Far call to the same privilege level, transitioning to compatibility mode
• Far call to the same privilege level, remaining in 64-bit mode
• Far call to a different privilege level (inter-privilege level call), remaining in 64-bit
mode
Note that in this mode the CALL instruction can not be used to cause a task switch in
64-bit mode since task switches are not supported in IA-32e mode.
In 64-bit mode, the processor always uses the segment selector part of the far
address to access the corresponding descriptor in the GDT or LDT. The descriptor
type (code segment, call gate) and access rights determine the type of call operation
to be performed.
If the selected descriptor is for a code segment, a far call to a code segment at the
same privilege level is performed. (If the selected code segment is at a different priv-
ilege level and the code segment is non-conforming, a general-protection exception
is generated.) A far call to the same privilege level in 64-bit mode is very similar to
one carried out in compatibility mode. The target operand specifies an absolute far
address indirectly with a memory location (m16:16, m16:32 or m16:64). The form
of CALL with a direct specification of absolute far address is not defined in 64-bit
mode. The operand-size attribute determines the size of the offset (16, 32, or 64
bits) in the far address. The new code segment selector and its descriptor are loaded
into the CS register; the offset from the instruction is loaded into the EIP register. The
new code segment may specify entry either into compatibility or 64-bit mode, based
on the L bit value.
A 64-bit call gate (described in the next paragraph) can also be used to perform a far
call to a code segment at the same privilege level. However, using this mechanism
requires that the target code segment descriptor have the L bit set.
When executing an inter-privilege-level far call, the code segment for the procedure
being called must be accessed through a 64-bit call gate. The segment selector spec-
ified by the target operand identifies the call gate. The target operand can only
specify the call gate segment selector indirectly with a memory location (m16:16,
m16:32 or m16:64). The processor obtains the segment selector for the new code
segment and the new instruction pointer (offset) from the 16-byte call gate
descriptor. (The offset from the target operand is ignored when a call gate is used.)
On inter-privilege-level calls, the processor switches to the stack for the privilege
level of the called procedure. The segment selector for the new stack segment is set
to NULL. The new stack pointer is specified in the TSS for the currently running task.
The branch to the new code segment occurs after the stack switch.