6-23
INSTRUCTION SET SUMMARY
reflect the result that would have been obtained by subtracting the destination operand from the
value in the EAX register.
The CMPXCHG instruction is commonly used for testing and modifying semaphores. It checks
to see if a semaphore is free. If the semaphore is free it is marked allocated, otherwise it gets the
ID of the current owner. This is all done in one uninterruptible operation. In a single-processor
system, the CMPXCHG instruction eliminates the need to switch to protection level 0 (to disable
interrupts) before executing multiple instructions to test and modify a semaphore. For multiple
processor systems, CMPXCHG can be combined with the LOCK prefix to perform the compare
and exchange operation atomically. (Refer to Section 7.1., Locked Atomic Operations of the
Intel Architecture Software Developers Manual, Volume 3, for more information on atomic
operations.)
The CMPXCHG8B instruction also requires three operands: a 64-bit value in EDX:EAX, a
64-bit value in ECX:EBX, and a destination operand in memory. The instruction compares the
64-bit value in the EDX:EAX registers with the destination operand. If they are equal, the 64-bit
value in the ECX:EBX register is stored in the destination operand. If the EDX:EAX register
and the destination are not equal, the destination is loaded in the EDX:EAX register. The
CMPXCHG8B instruction can be combined with the LOCK prefix to perform the operation
atomically.
6.3.2.Stack Manipulation Instructions
The PUSH, POP, PUSHA (push all registers), and POPA (pop all registers) instructions move
data to and from the stack. The PUSH instruction decrements the stack pointer (contained in the
ESP register), then copies the source operand to the top of stack (refer to Figure 6-1). It operates
on memory operands, immediate operands, and register operands (including segment registers).
The PUSH instruction is commonly used to place parameters on the stack before calling a proce-
dure. It can also be used to reserve space on the stack for temporary variables.
The PUSHA instruction saves the contents of the eight general-purpose registers on the stack
(refer to Figure 6-2). This instruction simplifies procedure calls by reducing the number of
instructions required to save the contents of the general-purpose registers. The registers are
pushed on the stack in the following order: EAX, ECX, EDX, EBX, the initial value of ESP
before EAX was pushed, EBP, ESI, and EDI.
Figure 6-1. Operation of the PUSH Instruction
0
Stack31
Before Pushing Doubleword
Growth
ESP
n
?
4
n
?
8
n
Stack
0
31
After Pushing Doubleword
ESP
Doubleword Value
