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Flag Registers

Flag register contains information reflecting the current status of microprocessor. Italso contains information which controls the operation of the microprocessor.

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Test for Sign Flags , OV Flag for the following syntax:mov al, ff , mov ax, ff, mov ax, ffff

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Flag Control Instruction : Bit Positions

ex: LAHF and SAHF

-read or change flags.-data transfer must be between AH register and the flagregister- Other instruction to clear or reset Carry and interrupt flags.

ExampleExample:Write an instruction sequence to save the current contents of the 8088s flags in thememory location at offset MEM1 of the current data segment and then reload the

flags with the contents of the storage location at offset MEM2.

Solution:LAHF ; Read from Flag to AHMOV [MEM1], AH ; Copy to Memory 1MOV AH, [MEM2] ; Copy from MEM1 to AHSAHF ; Set the flags from AH

Compare Instructions

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TO DETERMINE THE RELATIONSHIP BETWEEN TWO NUMBERS

:- equal, larger or smallerSubtraction operationThe result is not savedBased on the result, appropriate flag are set or reset.Examples:Given a sequence of number, find a minumum numberMOV AL, 7CMP AL, 5MOV AL, 99H; -103CMP AL, 1BH; +27Compares the destination operand to the source operand. Destination operand is

not changes

CMP D,SExample 1:MOV AL,5

CMP AL,5 ;Zero flag is set

Example 2:MOV AL,4

CMP AL,5 ;Carry flag is set

Example:Describe what happens to the status flags as the sequence of instructionsthat follows is executed.All flags initially reset.MOV AX,1234H

MOV BX.ABCDH

CMP AX,BX

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CONTROL FLOW AND THE JUMP INSTRUCTIONS

Control flow relates to altering the execution path of instructions in aprogram.It may cause a sequence of instructions to be repeated or a group of

instructions to not be executed at all.For a change in the control flow, we have to change the contents of thecode segment register and instruction pointer.

So, to control the flow of a program, we usejump instruction A jump may be conditional or unconditional Common conditional jumps are related to the conditions of flags such asCARRY, OVERFLOW, SIGN OR ZERO.When a jump is executed the program is sent to a new location by loading theprogram counter (CS:IP) with a new value .

If the branch address is in the same CS memory then the jump is called as anIntrasegment jump. If the branch address is in different CS memory then the jump is called anIntersegment jump.

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As the instruction is executed, the jump always take place to change theexecution sequence.- No status requirement for the jump to occur

Jump Instructions

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UNCONDITIONAL JUMP

It moves microprocessor to execute another part of the program Target can be represented by a label, immediate data, registers, ormemory locations

It does not affect flagsAssembly language couldnt exist without unconditional jumps.

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Intrasegment transfer v.s. Intersegment transfer

Intrasegment transfer: the microprocessor jumps to an address withinthe same segment we can provide only new IP value in JMP instructions.For Example: JMP 1000H Intersegment transfer: the microprocessor jumps to an address in adifference segmentwe need to provide both new CS and IP values in JMP instructions

For Example: JMP 2000H : 1000H

Direct Jump v.s. Indirect Jump

Direct Jump: the target address is directly given in the instruction Indirect Jump: the target address is contained in a register or memorylocation

Loop instruction

LOOP= when a group of instructions may need to be executed over and

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over again until a condition is metThe loop program structure is to implement- REPEAT-UNTIL and WHILE-DO

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