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18-6 Vol. 3
MIXING 16-BIT AND 32-BIT CODE
instruction (see Figure 18-1). On a 16-bit call, the processor pushes the contents of
the 16-bit IP register and (for calls between privilege levels) the 16-bit SP register.
The matching RET instruction must also use a 16-bit operand size to pop these 16-bit
values from the stack into the 16-bit registers.
A 32-bit CALL instruction pushes the contents of the 32-bit EIP register and (for
inter-privilege-level calls) the 32-bit ESP register. Here, the matching RET instruction
must use a 32-bit operand size to pop these 32-bit values from the stack into the
32-bit registers. If the two parts of a CALL/RET instruction pair do not have matching
operand sizes, the stack will not be managed correctly and the values of the instruc
-
tion pointer and stack pointer will not be restored to correct values.
Figure 18-1. Stack after Far 16- and 32-Bit Calls
SP
After 16-bit Call
PARM 1
IP
SP
SS
PARM 2
CS
031
SS
EIP
After 32-bit Call
CS
ESP
ESP
PARM 2
PARM 1
031
With Privilege Transition
Stack
Growth
After 16-bit Call
PARM 1
IP
SP
PARM 2
CS
031
Without Privilege Transition
Stack
Growth
After 32-bit Call
PARM 1
ESP
PARM 2
CS
031
EIP
Undefined