Vol. 1 7-17

PROGRAMMING WITH GENERAL-PURPOSE INSTRUCTIONS

operand is a relative address (that is, an offset relative to the contents of the EIP register), and it generally points 
to the first instruction in the block of code that is to be executed in the loop. When the count in the ECX register 
reaches zero, program control is transferred to the instruction immediately following the LOOP instruction, 
which terminates the loop. If the count in the ECX register is zero when the LOOP instruction is first executed, the 
register is pre-decremented to FFFFFFFFH, causing the loop to be executed 2

32

 times.

The LOOPE and LOOPZ instructions perform the same operation (they are mnemonics for the same instruction). 
These instructions operate the same as the LOOP instruction, except that they also test the ZF flag. 
If the count in the ECX register is not zero and the ZF flag is set, program control is transferred to the destination 
operand. When the count reaches zero or the ZF flag is clear, the loop is terminated by transferring program control 
to the instruction immediately following the LOOPE/LOOPZ instruction.
The LOOPNE and LOOPNZ instructions (mnemonics for the same instruction) operate the same as the 
LOOPE/LOOPZ instructions, except that they terminate the loop if the ZF flag is set. 
Jump if zero instructions — The JECXZ (jump if ECX zero) instruction jumps to the location specified in the desti-
nation operand if the ECX register contains the value zero. This instruction can be used in combination with a loop 
instruction (LOOP, LOOPE, LOOPZ, LOOPNE, or LOOPNZ) to test the ECX register prior to beginning a loop. As 
described in “Loop instructions” on page 7-16, the loop instructions decrement the contents of the ECX register 
before testing for zero. If the value in the ECX register is zero initially, it will be decremented to FFFFFFFFH on the 
first loop instruction, causing the loop to be executed 2

32

 times. To prevent this problem, a JECXZ instruction can 

be inserted at the beginning of the code block for the loop, causing a jump out of the loop if the ECX register count 
is initially zero. When used with repeated string scan and compare instructions, the JECXZ instruction can deter-
mine whether the loop terminated because the count reached zero or because the scan or compare conditions were 
satisfied.
The JCXZ (jump if CX is zero) instruction operates the same as the JECXZ instruction when the 16-bit address-size 
attribute is used. Here, the CX register is tested for zero.

7.3.8.3  

Control Transfer Instructions in 64-Bit Mode

In 64-bit mode, the operand size for all near branches (CALL, RET, JCC, JCXZ, JMP, and LOOP) is forced to 64 bits. 
The listed instructions update the 64-bit RIP without need for a REX operand-size prefix. 
Near branches in the following operations are forced to 64-bits (regardless of operand size prefixes):

•

Truncation of the size of the instruction pointer

•

Size of a stack pop or push, due to CALL or RET

•

Size of a stack-pointer increment or decrement, due to CALL or RET

•

Indirect-branch operand size

Note that the displacement field for relative branches is still limited to 32 bits and the address size for near 
branches is not forced.
Address size determines the register size (CX/ECX/RCX) used for JCXZ and LOOP. It also impacts the address 
calculation for memory indirect branches. Addresses size is 64 bits by default, although it can be over-ridden to 32 
bits (using a prefix).

7.3.8.4  

Software Interrupt Instructions

The INT n (software interrupt), INTO (interrupt on overflow), and BOUND (detect value out of range) instructions 
allow a program to explicitly raise a specified interrupt or exception, which in turn causes the handler routine for 
the interrupt or exception to be called.
The INT n instruction can raise any of the processor’s interrupts or exceptions by encoding the vector of the inter-
rupt or exception in the instruction. This instruction can be used to support software generated interrupts or to test 
the operation of interrupt and exception handlers.
The IRET (return from interrupt) instruction returns program control from an interrupt handler to the interrupted 
procedure. The IRET instruction performs a similar operation to the RET instruction.