Their memory is always allocated in a sequential order. The CPU never automatically sets or clears the direction flag.
8086 Assembler Tutorial For Beginners
Flags are used among others in the following contexts.
. CMPSB CMPSW or CMPSD is executed. The ability to push and pop FLAGS registers lets a program manipulate information in the FLAGS in ways for which machine-language instructions do not exist. If there are odd number of 1s in the 8-bit result the P flag is reset to 0.
Direction Flag DF Trap Flag TF Interrupt Flag IF Zero Flag ZF Answer. That is only done by software. In certain job situations following instructions is a matter of safety or in some settings its even a matter of life and death.
B the instruction at B has just been executed with the result that the value 0 was stored to R6. ADD SUB CMP AND TEST OR XOR. Which of the following flags cannot be cleared using an assembly instruction.
Clear the direction flag. -1 means decimal negative 1. Flags Affected The DF flag is set.
If directional flag is set 1 then access the string data from higher memory location towards lower memory location. The flag that acts as Borrow flag in the instruction SBB is a direction flag b carry flag c parity flag d trap flag Answer. If the direction flag is clear the CPU increments ESI and EDI after operating upon each string element.
Write a sequence of statements that use only PUSH and POP instructions to exchange the values in the EAX and EBX registers. JZ L10 jump forward to L10 if the zero flag is set to 1 JNZ L1 jump back to L1 if the zero flag is clear. What will be the values of the Carry Overflow Sign and Zero flags after the.
Set carry flag CF to 1. To indicate errors to indicate the sign of the last calculation to enable a carry during arithmetic operations for debugging A number of instructions perform certain tasks based on the current. It is either referred as byte string or word string.
String manipulation instructions in 8086 microprocessor. Directional Flag D This flag is specifically used in string instructions. This instruction resets the interrupt flag IF to 0.
Cld Set Direction Flag std std Operation. CLC Clear Carry Flag. The Flag manipulation instructions directly modify some of the Flags of 8086.
STC Set Carry Flag. DB is used for storing byte and DW is used for storing a word. If borrow exists in the subtraction operation performed then carry flag is set.
There are 3 groups of instructions. Software should set DF before using a string instruction for which SI and DI should be decremented. The CF OF ZF SF AF and PF flags are unaffected.
The CPU uses the direction flag when executing string instructions to determine whether to increment or decrement SI and DI. What will be the values of the Carry Sign and Zero flags after the following instructions have executed. Push content of EAX which is assumed to be Y.
CMC Complement Carry Flag. The MOV instruction does not affect any value in the flag register. This can be achieved with the following assembly code.
B ADD r6 r6 -1. Consider an example to understand the behavior of MOV instruction. Instructions used to manipulate strings are called string manipulation instructions.
Windsocks are used to determine wind speed as well as the direction of the wind. Windsocks can be used as decoration or as a rudimentary reference to wind direction and speed. Represent the hex word xAC9F as a binary word.
Windsocks are commonly used at airports to inform pilots of the winds direction and strength as well as in chemical industries where there is a risk of gaseous leakage. CLD Clear Direction Flag. Which of the following flags gets affected when a compare instruction ie.
In others it can impact opportunities for career advancement. Besides the ESI EDI ECX and ALAXEAX registers one other register controls the 80x86s string instructions - the flags register. Given the following code and given that a the current value in r1 is 72 decimal 72.
Instructions that use the S flag are quite often used in the user programs. Set the direction flag. Interrupt Flag I This flag is for interrupts.
Specifically the direction flag in the flags register controls how the CPU processes strings. For example the cld and std instructions clear and set the direction flag DF respectively. Whenever a compare instruction over strings is executed the status flags are affected.
Assume EBX X and EAX Y here the content of EBX ie. Set direction flag to 1. 1 - DF.
Mov bl-127 dec bl 9. This instruction resets the direction flag DF to 0. String is a series of data byte or word available in memory at consecutive locations.
Sets the direction flag to 1 causing all subsequent string operations to decrement the index registers ESI andor EDI used during the operation. Following instructions at work can have different implications depending on the employment setting. Normally DF is left clear all the time.
1010 1100 1001 1111. This instruction take complement of carry flag CF. FLAGS EFLAGS Instruction Pointer EIP IP 00 000 0 0 00 Status Flags CF Carry Flag PF Parity Flag ZF Zero Flag SF Sign Flag OF Overflow Flag AF Auxiliary Carry Flag System Flags NT Nested Task RF Resume Flag VM Virtual 8086 Mode AC Alignment Check VIP Virtual Interrupt Pending ID ID Flag Control Flags DF Direction Flag TF Trap Flag.
When the DF flag is set to 1 string operations decrement the index registers ESI andor EDI. At the end of servicing an interrupt which of the following is used to inform tha PIC that it is completed. But there is no instruction to complement DF.
This instruction resets the carry flag CF to 0. The zero flag is also used in the JA jump if above JB jump if below and similar conditional jump instructions. Therefore both source and destination operands cannot be memory address.
Operation is the same in all modes. A sequence of statements are. Mov ax0B6D4h add al0B3h 8.
Exceptions All Operating Modes UD If the LOCK prefix is used. If directional flag is reset 0 then access the string data from lower memory location towards higher memory location. Std Arithmetic Logical Instructions Integer Addition add.
What will be the values of the Overflow Sign and Zero flags after the following instructions have executed. In the code given below a and b are the variables. Zero Flag ZF Explanation.
Direction Flag DF - this flag is used by some instructions to process data chains when this flag is set to 0 - the processing is done forward when this flag is set to 1 the processing is done backward. A ADD r1 r1 r2. The zero flag is mainly used with the JZ and JNZ conditional jump instructions for example-.
The final value of EAX will be 5. The P flag is set to 1 if the 8-bit result thus produced against any logical and arithmetic operation has an even number of 1s in it. Operation DF 1.
Which of the following is the highest priority interrupt. Importance of Following Instructions at Work. STD Set Direction Flag.
What Are The Purposes Of Flag Registers Of 8086 Processor Online Class Notes
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