Chapter 6

Program control instructions

Program control instructions

Direct the flow of a program and allow the flow to change

Jump

The main program control instruction, allows the programmer to skip sections of a program and branch to any part of the memory for the next instruction

conditional jump instruction

Allows the programmer to make decisions based upon numerical tests

Short jump, near jump, and far jump

Three types of unconditional jump instructions

Short jump

is a 2-byte instruction that allows jumps or branches to memory locations within +127 and -128 bytes from the address following the jump

Short jump

are called relative jumps because they can be moved, along with their related software, to any location in the current code segment without a change

Near jump

The 3-byte allows a branch or jump within ±32K bytes (or any where in the current code segment) from the instruction in the current code segment

Near jump

Is similar to the short jump, except that the distance is farther

Near jump

Is also relocatable (as was the short jump) because it is also a relative jump.

Far jump

The 5-byte. allows a jump to any memory location within the real memory system

Far jump

Instructions obtains a new segment and offset address to accomplish the jump.

Intrasegment jumps

The short and near jumps are often called

Intersegment jumps

Far jumps are often called

Far label

Another way to obtain a far jump is to define a label as

greater than and less

The terms __than refer to signed numbers

above and below

The terms __ refer to unsigned numbers

LOOP instruction

Is a combination of a decrement CX and the JNZ conditional jump

LOOPE (loop while equal)

instruction jumps if CX != 0 while an equal condition exists.

LOOPNE (loop while not equal)

Instruction jumps if CX != 0 while a not-equal condition exists

WHILE

statement is used with a condition to begin the loop

ENDW

statement ends the loop

REPEAT-UNTIL Loops

series of instructions is

repeated until some condition occurs

REPEAT

statement defines the start of the loop

UNTIL

The end is defined __with the condition statement, which contains a

UNTILCXZ

instruction uses the CX register as a counter to repeat a loop a fixed number of times.

Procedure

is an important part of any computer system's architecture. A procedure is a group of instructions that usually performs one task

Procedure

is a reusable section of the software that is stored in memory once, but used as often as necessary

PROC - ENDP

End of procedure

A procedure begins with the__directive and ends with the__

NEAR or FAR

The PROC directive is followed by the type of procedure

Far procedures

Procedures that are to be used by all software (global) should be written as

Near procedures

Procedures that are used by a given task (local) are normally defined as

CALL instruction

Transfers the flow of the program to the procedure.

Near CALL instruction

is 3 bytes long; the first byte contains the opcode, and the second and third bytes contain the displacement, or distance of ±32K in the 8086 through the 80286 processors

Far CALL instruction

is like a far jump because it can call a procedure stored in any memory location in the system

Return instruction (RET)

removes a 16-bit number (near return) from the stack and places it into IP, or removes a 32-bit number (far return) and places it into IP and CS

Interrupt

is either a hardware-generated CALL or a Software-generated CALL

Exception

An internal interrupt is called an

Hardware-generated CALL

externally derived from a hardware signal

Software-generated CALL

internally derived from the execution of an instruction or by some other internal

event

Interrupt vector

is a 4-byte number stored in the first 1024 bytes of the memory (00000H-

003FFH) when the microprocessor operates in the real mode.

Interrupt return instruction (IRET

is used only with software or hardware interrupt service procedures

INT 3

Instruction is a special software interrupt designed to function as a breakpoint

Interrupt on overflow (INTO

is a conditional software interrupt that tests the overflow flag (0).

Set interrupt flag instruction (STI)

places a 1 into the I flag bit, which enables the INTR pin. Interrupt request

Clear interrupt flag instruction (CLI)

places a 0 into the I flag bit, which disables the INTR pin.

IRETQ instruction

64-bit system uses the__ to return from an interrupt service

Carry flag (C)

Propagates the carry or borrow in multiple-word/doubleword addition and subtraction

WAIT

instruction monitors the hardware BUSY pin on the 80286 and 80386, and the

pin on the 8086/8088.

Halt instruction (HLT)

Stops the execution of software

This instruction normally appears in a program to wait for an interrupt

No operation instruction (NOP

was often used to pad software with space for future machine language instructions.

LOCK prefix

Appends an instruction and causes the pin to become a logic 0

BOUND instruction

is a comparison

instruction that may cause an interrupt (vector type number 5)

ENTER and LEAVE instructions

first made available to the 80186 microprocessor, are used with stack frames, which are mechanisms used to pass parameters to a procedure through the stack memory

ENTER instruction

creates a stack frame by pushing BP onto the stack and then loading BP with the uppermost address of the stack frame